mental time travel to change the past

Mental time travel is a great decision-making tool — this is how to use it

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When the future seems largely unpredictable, is there anything you can do to prepare for it?

“Yes!” says futurist and game designer Jane McGonigal . All you need to do is to tap into your imagination and envision all your potential futures — using what she calls “futures thinking.” 

“Futures thinking isn’t a superpower, and you don’t have to fix everything or save everyone,” McGonigal writes in her new book called Imaginable . “But futures thinking is an incredibly useful, practical tool to prepare your mind to adapt faster to new challenges, build hope and resilience, reduce anxiety and depression, and inspire you to take actions today that set yourself up for future happiness and success.”

Her book draws on the latest research in psychology and neuroscience to show you how to train your mind to think the unthinkable. In this excerpt, you’ll get a taste of how you can start thinking like a futurist — and create a better future for yourself. 

For the next 30 seconds, I want you to imagine yourself waking up tomorrow morning. Try to picture it in your mind or describe it to yourself as clearly as possible. 

These questions may help make your imagined scene clearer. What room or space are you in? What wakes you up — an alarm, the sunlight, someone nudging you or calling you? Is it light out or still dark? Is there anyone with you? What are you wearing? What kind of mood are you in? And what’s the very first thing you do now that you’re awake?

Keep imagining your tomorrow morning until you have a clear answer to all of these questions.

This quick mental time trip you just took is an example of a highly imaginable future — it was likely quite easy for you to envision, with plenty of vivid details.

Now let’s try something more challenging. For the next 30 seconds, I want you to imagine yourself waking up one year from today. 

Again, try to envision this as clearly as possible. Feel free to change as many or as few details as you want from the first scene you imagined. Are you somewhere different? Are you physically changed? What’s your mood? Do you have a different morning habit? What might that new habit be?

Keep imagining your morning one year from now until you have answers to all of these questions — even the harder ones. Notice how easily and automatically ideas came to you, or how hard you had to work to come up with details.

If you’re having difficulty imagining life 10 years from now, write down a description of what you imagine. It can be easier to think about the future with words rather than with mental images.

Now let’s try one more act of imagination. This time, I want you to imagine yourself waking up 10 years from today.

Take as long as you need to come up with a vivid and plausible image — of yourself, of the space that you’re in, and who might be with you. Where are you? What’s around you? What do you see, hear, smell, and feel? What’s the first thing on your mind when you wake up? What do you have planned for the day? How are you physically different?

Try not to make this future scene a total fantasy; stay grounded in what you feel is genuinely realistic and possible for you. If you’re having difficulty, write down a description of what you imagine. Sometimes, it’s easier to think about the future with words rather than with mental images.

You’ll probably find that 10 years is a trickier challenge compared with one year. Why? You’ve never been 10 years older than you are now, so your brain doesn’t know what to expect. Plus, there’s so much opportunity for things to change in a decade — your body, relationships, life circumstances, physical environment.

Your brain intuitively grasps this unknowability, so instead of confidently projecting one possibility, it opens up a blank space for you to consider multiple possibilities. You have to start making intentional choices about what you want to imagine in your future — you have to fill in the blanks.

“Episodic future thinking” or EFT is often described as “mental time travel” — your brain is working to help you see and feel the future as clearly and vividly as if you were already there.

Filling in the blanks takes considerable effort. But that’s precisely why this kind of imagination is so powerful. Instead of simply remembering what it knows, your brain has to invent a new possibility. It draws on past experiences, current hopes and fears, and your intuitions about what might possibly change in 10 years.

Then, after you’ve made this new memory, something amazing happens: What was previously unimaginable to your brain is now imaginable. You can revisit this new memory whenever you want and examine how it makes you feel. Does it spark positive or negative emotions? These pre-feelings can help you decide: Should you change what you’re doing today to make this future more or less likely? And because you invented this memory, you can change it whenever you want.

Scientists call this form of imagination “episodic future thinking,” or EFT. EFT is often described as a kind of “mental time travel” because your brain is working to help you see and feel the future as clearly and vividly as if you were already there.

EFT isn’t an escape from reality. It’s a way of playing with reality, to discover risks and opportunities you might not have considered. EFT is not a daydream in which you fantasize about waking up in a world where your problems are magically solved. It is a way of connecting who you are today with what you might really feel and do in the future.

An important element of imagination training is to fill your brain with what I call “clues to the future,” concrete examples of new ideas that might shape how your future turns out.

Because EFT allows us to pre-feel different possible futures, it’s a powerful decision-making, planning and motivational tool. It helps us decide: Is this a world I want to wake up in? What do I need to do to be ready for it? Should I change what I’m doing today to make this future more or less likely?

According to fMRI studies, EFT involves heightened activity and increased connectivity between 11 distinct brain regions. Compare this to remembering a past event, which activates 6 of the 11 regions of the brain.

There are three major kinds of sense making that happen when you engage in mental time travel to your future. First, your brain has to do what cognitive scientists call “scene construction” — mentally building the future world. Think of this as crafting the stage set, cast and props for a theatrical play.

During EFT, your brain goes on a hunt for realistic details and plausible ideas. To do this, it activates the hippocampus , the seat of memory and learning, and digs through your memories, plus any other facts and ideas you’ve stored away. Depending on what kind of future you’re imagining, the hippocampus identifies the most relevant stuff and retrieves and recombines it into a new scene.

Whatever you see in your future will always come from information your brain has already perceived and processed. Ideally, as you get better at imagining the unimaginable, you’ll incorporate not just obvious ideas and events but also surprising things that could be important in your future.

Another important part of imagination training is to try out new behaviors that could prove useful in the future. I call these micro-actions — taking no more than five minutes to do something today you’ve never done before.

That’s why an important element of imagination training is to fill your brain with what I call “clues to the future,” concrete examples of new ideas that might shape how your future turns out. When you have a hippocampus full of clues, your brain will have better data to draw on, and the scenes you construct will be way more interesting.

After scene construction, your brain starts to do what cognitive scientists call “opportunity detection.” Here, you look for ways to fulfill your needs and achieve your goals. For example: If you predict you will be hungry when you wake up in 10 years, what will future you eat? If you imagine yourself lonely when you wake up, who will future you try to connect with? Opportunity detection is like an actor showing up for rehearsal and asking, “What’s my motivation?” In other words: What do you want in this scene?

To figure this out, your brain fires up the ventromedial prefrontal cortex (vmPFC), a region that’s heavily used whenever you set goals and track your progress. Like the hippocampus, the vmPFC can suggest any goals you’ve had or previously considered. One of the most interesting things about EFT is that the motivations that pop into your mind first are likely to be closely linked to your deepest values and most essential needs, like always learning something new, helping others, pushing yourself to do brave things, taking care of your family, being creative, or  putting new ideas or art into the world.

But you still have to figure out the best way for future you to achieve these future goals. So then the putamen , also part of the motivation and reward system, kicks in. The putamen helps keep track of which specific actions and behaviors typically lead to positive results for you. It’s the part of your brain that knows things like “I feel better when I get some fresh air”; “I make my mom happy when I text her back right away”; “If I’m having a bad day, cooking helps”; or “If I don’t stand up for myself in the moment, I’ll beat myself up about it later.”

There are real benefits to intentionally and carefully imagining futures that frighten you. This can help you do the important work of getting ready for anything — even things you’d rather not experience.

The putamen is like a reality check on your future imagination. Since the putamen is trained on real experiences, the future actions it suggests will be heavily influenced by strategies that worked for you in the past. That’s why another important part of imagination training is to try out new behaviors that could prove useful in the future. I call these micro-actions — taking no more than five minutes to do something today you’ve never done before. When you experiment with micro-actions, you expand what your putamen considers realistic behavior.

Finally, as your brain works to transport you to the future, feelings will kick in. The insula and amygdala , emotion centers in the brain, fire up to give you a preview of how you might feel in the future — excited, disappointed, hopeful, afraid, proud, jealous, joyful, sad, curious, bored, embarrassed, relieved, loved, lonely, awed, confused, stressed out, free or more. These emotions give you important information and help you decide: Is this a future I want to wake up in? Should I take actions today to make this future more or less likely?

Crucially, these are real feelings. Studies show that the emotions you experience during EFT can be just as psychologically powerful as emotions experienced in the present. This is one reason why many of us prefer to imagine best-case-scenario futures and avoid imagining the futures that scare us.

But there are real benefits to intentionally and carefully imagining futures that frighten you. This can help you do the important work of getting ready for anything — even things you’d rather not think about, let alone actually experience, someday.

Excerpted from the new book Imaginable: How to See the Future Coming and Feel Ready for Anything Even Things That Seem Impossible Today by Jane McGonigal. Copyright © 2022 by Jane McGonigal. Used by permission of Spiegel & Grau LLC, New York. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.

Watch her TED Talk here:

About the author

Jane McGonigal is a future forecaster and world-renowned designer of alternate reality games designed to improve real lives and solve real problems. She is the Director of Games Research & Development at the Institute for the Future and currently teaches the course “How to Think Like a Futurist” at Stanford University, as well as serving as the lead instructor for the Institute for the Future’s series on the Coursera platform. She is The New York Times bestselling author of the books "Reality Is Broken" and "SuperBetter" and the new book "Unimaginable." Her innovative games and ideas have been recognized by the World Economic Forum, Harvard Business Review, Fast Company, MIT Technology Review, O Magazine and the New York Times, among others.

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Mental Time Travel

How does the brain zip back and forth in time.

Posted December 23, 2010

Remembering the past and envisioning the future: These are the closest we can get to "time travel," and a new study has investigated how this process works in the human brain.

Results of the study -- performed by Lars Nyberg of Sweden's Umea University, Reza Habib of Southern Illinois University, and Alice S. N. Kim, Brian Levine, and Endel Tulving of the University of Toronto -- appear in a recent issue of the Proceedings of the National Academy of Sciences.

Chronesthesia is the technical term for the brain's ability to maintain simultaneous awareness of past, present and future and to "travel" back and forth between them.

Interviewed by a reporter from PhysOrg.com , Tulving said:

"Mental time travel consists of two independent sets of processes: (1) those that determine the contents of any act of such ‘travel': what happens, who are the 'actors,' where does the action occur; it is similar to the contents of watching a movie - everything that you see on the screen; and (2) those that determine the subjective moment of time in which the action takes place -- past, present, or future.

"In cognitive neuroscience , we know quite a bit (relatively speaking) about perceived, remembered, known, and imagined space," he said. "We know essentially nothing about perceived, remembered, known, and imagined time. When you remember something that you did last night, you are consciously aware not only that the event happened and that you were ‘there,' as an observer or participant ('episodic memory '), but also that it happened yesterday, that is, at a time that is no more. The question we are asking is, how do you know that it happened at a time other than 'now'?"

The study is titled " Consciousness of subjective time in the brain."

The researchers asked participants to repeatedly think about "taking a short walk in a familiar environment in either the imagined past, the real past, the present, or the imagined future." This helped them identify which areas of the brain are linked with thinking about the same event at different times. These are certain regions in the left lateral parietal cortex, left frontal cortex, cerebellum, and thalamus.

Tulving told PhysOrg.com:

"Until now, the processes that determine contents and the processes that determine time have not been separated in functional neuroimaging studies of chronesthesia; especially, there have been no studies in which brain regions involved in time alone, rather than time together with action, have been identified," Tulving said. "The concept of chronesthesia is essentially brand new. ... Therefore, I would say, the most important result of our study is the novel finding that there seem to exist brain regions that are more active in the (imagined) past and the (imagined) future than they are in the (imagined) present. That is, we found some evidence for chronesthesia."

S. Rufus is the author, under the byline Anneli Rufus, of books including Party of One and Stuck.

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Collective Mental Time Travel Can Influence the Future

We’re often told to “be here now.” Yet the mind is rarely tethered in place. We take mental trips to our past, revisiting what happened yesterday or when we were children, or we project into an imagined future: tomorrow’s dinner date, the trajectory of our career at age 50.

Rather than a diversion from the norm of mindful presence, this tendency to internally visit other time lines, called “mental time travel,” is common; young adults, for example, think about their future an average of 59 times a day. Psychologists have suggested that this ability to time travel from the confines of our own heads is a fundamental aspect of what it means to be human.

The past and future are not locations that remain the same regardless of who is visiting and when. The way we envision our past or future is ever-changing, and the construction of these scenarios has an impact on what we do and how we think in the present. Until recently, the study of mental time travel largely focused on individuals and their personal histories. But this doesn’t reflect the social nature of our lives. Identities are comprised of groups that nestle into one another. We are part of our families and friend circles, occupational networks, countries and nations, and ethnic groups. The study of mental time travel is starting to reflect this: When we travel through time, we don’t always go alone.

Research on “collective mental time travel” shows that the way we imagine the collective future or past also impacts the present. It can sway attitudes toward policy decisions and laws, as well as how aligned people feel with their country or existing systems. It can affect a person’s willingness to engage in prosocial behaviors, like voting, donating, or activism. Because of this, collective mental time travel is more than just a neat cognitive trick—it provides an opportunity to be more intentional about how we represent the collective past and future. 

In the 1980s, psychologist Endel Tulving proposed that humans have the ability to relive their past and pre-experience the future, theorizing that the same memory mechanisms were used for both. This was supported by case studies with amnesiacs: One man, “K.C.,” had brain lesions that affected his ability to retain personal memories, like a visit he’d taken to a family lake house. This patient couldn’t imagine going there in the future, despite knowing that his family owned the house.

More recent brain imaging has supported Tulving’s theory by showing that similar networks are activated when remembering the personal past and personal future, said Karl Szpunar, an associate professor of psychology and director of the Memory Lab at Toronto Metropolitan University. Based on this evidence, some scientists think that we imagine the future by recombining past experiences—this is called the “constructed episodic simulation hypothesis.”

For the collective past and future, the story may be more complex. Is our collective future simply made up of fragments of the collective past? Intriguingly, when people with damage to their hippocampus, a brain region involved in personal memory, are asked about collective future events, like “What environmental concerns will the world face over the coming decade?” they are able to come up with answers. Even though their ability to mentally time travel into their personal futures was compromised, the ability to imagine events affecting a group’s future was intact. More work on this is needed, but as Spzunar and his colleague wrote, “The capacity to engage in collective future thought appears to rely on cognitive processes distinct from those involved in individual or personal future thinking.”

The collective past likely has an influence on the collective future, but only to a point, says Meymune Topcu, a visiting scholar at The New School. She’s coauthor of a recent review chapter on collective mental time travel, in which she examined numerous cases of people collectively “visiting” the past and future and looked at whether they felt positively or negatively about their mental representations and how specific the content of their time travel was.

She found that past events can influence what people imagine to happen in the collective future, but there isn’t necessarily a complete overlap. Imagined collective futures can also be less specific than memories of the collective past, Topcu said. Additionally, when we think about our own futures, we tend to have an optimism bias, but when people are asked to think about the future of their countries, they often focus more on potentially worrisome , rather than potentially exciting, possibilities. (These findings have not been found to be culturally universal: Some newer research with Chinese participants has shown that such positive and negative biases are not present in those study groups.)

On an individual level, thinking about the future is correlated with specific actions or attitudes. Studies from Hal Hershfield, a psychologist at UCLA who studies the effects of time perception, and his colleagues, have found that people who relate more to their future selves make more future-oriented decisions, like saving money for later, and have higher levels of well-being over a 10-year period. Hershfield has also asked people about their conception of how long the present is. The longer they thought “ right now ” lasted, the fewer emotions they felt about the future. People who said that the present ended sooner were more likely to make future-oriented decisions. Having a future time perspective can also predict pro-environmental attitudes, like favoring and participating in more sustainable behaviors.

If how you think about the future or present can be a guiding influence, it’s a short leap to envisioning how collective pasts and futures might be manipulated for various means. Jeremy Yamashiro, an assistant professor of psychology at UC Santa Cruz, said that rather than creating hard and fast rules for the best way to represent the past and the future, he’s become more sensitive to the ways people use representations in strategic ways . “It’s much more, ‘How are people using that in order to convince you of what they’re trying to convince you of?’” He said. The collective future probably isn’t based only on the building blocks of the past, but also cultural narratives , Yamashiro said.

Those narratives can have immediate and practical policy ramifications. In 2014 , social and cultural psychologist Contance de Saint-Laurent analyzed the parliamentary debates on immigration in France and found two dominant narratives for how left-wing and right-wing politicians thought about the country’s past.

The left saw the past as “a constant struggle between humanists and their adversaries,” while the right saw the central tenet of French history as the “social contract that enables co-existence in society.” Because of the way these groups viewed the past, de Saint-Laurent wrote, people on the left were more willing to see the future as an opportunity to address colonial crimes, while the others would only accept immigrants who adhered to the social contract of the country.

A person’s current reality also affects how much they focus on the future. Johanna Peetz, a social psychologist at Carleton University, has found a link between future thinking and the economic index of countries and their general quality of life. If a country’s economic index was stable or decreasing, and quality of life was declining, people did not want to look toward the future.

We could, however, think of some manipulations being wielded for good. Topcu thinks that collective future thinking could play a role in addressing intergroup conflicts. In one study , people who lived in the European Union were given different descriptions of the EU. One was an excerpt that mentioned the common heritage of European societies, the other was a narrative focusing on how the EU was a project for the future. Then the people played a game in which they had to choose to cooperate with other participants. When people saw the EU as a future-oriented project, rather than one based in the past, they were more likely to play nice.

“I’m just speculating, but if we ask people to imagine a future in a different way, or simply imagining a future where there’s more peace and cooperation between these two groups, it could have an effect on whether they would be more willing to change the present situation,” Topcu said.

This approach could be applied at a more global scale. In 2018, researchers asked people to write about their vision of the best possible overall society. People who imagined utopia-like futures ended up being less satisfied with the status quo and were less likely to justify current systems. People asked to engage in utopian thinking also reported being more willing to participate in individual and collective action to attain that future.

But the type of utopia mattered. In a follow-up study , participants were asked to imagine either a “green utopia” composed of an “ecologically friendly society that champions sustainable efficiency” or a “sci-fi utopia,” where technological advancement and material efficiency dominated. Both utopias were seen as positive, but those who imagined the green utopia were more willing to participate in social change or report that they would donate to a nonprofit. The authors speculated that it had to do with agency—those who invested in a sci-fi future envisioned technology solving every problem and may have felt less able to bring about that positive future.

This suggests that proposing a future on Mars, for example, might unintentionally lead to less action in the present than collectively imagining a different kind of future would. “When we think about techno-fixes, it’s couched in a narrative of progress,” said Piotr Szpunar, a professor in the Communication Department at the University at Albany: “a narrative that technology continuously gets better, and at the same time, that society continuously gets better, or more equitable.” This can happen within nations too, as with the story of American exceptionalism. “There’s this idea that regardless of what happens, we’re still progressing,” he said. William Hirst, professor and cochair of psychology at The New School for Social Research, described the often rigid relationship between memory and history as “mnemonic inertia,” when certain stories become sticky and have outsized weight in terms of how we think about the present and future.

The future can also modify how we view the past, a concept that psychologist Ignacio Brescó de Luna called “prolepsis,” or when “imagined futures are brought into the present by means of particular ways of reconstructing the past.” In 2018, transdisciplinary scholar Séamus A. Power interviewed people engaged in water protests in Ireland. He argued that a driving reason for their collective action was imagining a dystopic future in which water was privatized, an imagined future based on remembering past cases of privatization in Ireland.

“There is a continuous looping from the past to the future and back again, always converging on the focal point of the present,” Power wrote. There’s room for flexibility—the most important lesson currently from collective mental time travel might be how dynamic an interaction there is between our notions of future, present, and past.

No matter how we use it, collective mental time travel ultimately challenges the objective reality of our past and present. The English philosopher C.D. Broad proposed the “growing block theory of time,” which says that only the past and present are real, and the future is not. As the future becomes the present, it is added on to the “growing block of reality.” Collective mental time travel reminds us that all remembrances of the past are reconstructions to some extent, and our present is continuously being informed by the way we imagine the future and conceive of the past.

“When you can change the narrative of the past, it’s going to change the way you conceive of the future too,” Hirst said. We won’t ever be able to escape this relationship, but we can seek a better understanding of how our perceptions are influenced by mental time travel, and how the collective past and future can be tools for building a better present.

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The Complex Role of Mental Time Travel in Depressive and Anxiety Disorders: An Ensemble Perspective

Associated data.

The original contributions presented in the study are included in the article/supplementary material. Further inquiries can be directed to the corresponding author.

The ensemble hypothesis proposes that uniquely human cognitive abilities depend on more than just language. Besides overt language, inner speech, and causal interpretations, executive attention, mental time travel, and theory of mind abilities are essential parts that combine additively and even multiplicatively. In this review, we consider the implications of the ensemble hypothesis for the psychopathologies of anxiety and depression. Generalized anxiety disorder (GAD) and major depressive disorder (MDD) are two of the most common mental disorders worldwide. The mechanisms that differentiate them are difficult to identify, however. Mental time travel has been implicated in models of depressive and anxiety disorders, but here we argue that at least two other ensemble components, namely, interpreter biases and executive attention, must also be considered. Depressive and anxiety disorders have both been found to show impairments in all three of these components, but the precise relationships seem to distinguish the two kinds of disorders. In reviewing the literature, we develop models for depression and anxiety that take into account an ensemble of mental components that are unique for each disorder. We specify how the relations among mental time travel, interpreter biases, and executive attentional control differ in depression and anxiety. We conclude by considering the implications of these models for treating and conceptualizing anxiety and depression.

Introduction

Depressive and anxiety disorders are two major categories of psychopathology, yet they have proven difficult to differentiate in some respects. As will be documented below, both are characterized by dysfunctional executive attention and pessimistic attributional styles, with a high degree of comorbidity. Here we build on the premise of Roepke and Seligman (2016) that the core problem in depression is a difficulty in mental time travel, specifically, an inability to envision positive events in the future. We consider the role of mental time travel in differentiating the two disorders and conclude that this component of human cognition is by itself insufficient. Mental time travel, we suggest, is moderated by problems with executive attention and an interpretive component responsible for causal attributions and inner speech.

In an important paper, Roepke and Seligman (2016) argued that prospection, or the mental representation of future events, plays a major role in depression. Human episodic memory enables mental time travel, that is to say, the ability both to recall past autobiographical events and to imagine possible future events ( Tulving, 2002 ). Roepke and Seligman suggested that the negative beliefs about the future and feelings of hopelessness that characterize depressive disorders ( Beck, 1974 ) can be directly linked to faulty prospection, an inability to envision possible futures and poor evaluation of possible futures. In their view, “.faulty prospection is the core causal process of much depression” (p. 24). A similar proposal was advanced by Miloyan et al. (2014) to account for depression; they also extended the analysis by suggesting that a different form of faulty prospection, centered on worry rather than pessimism and hopelessness, lay at the core of anxiety disorders.

We agree that problems with mental time travel are central to psychopathology, but we question whether this component can be isolated from other cognitive components to ascertain its relative contribution. Instead, we argue that other fundamental components of human cognition are concurrently at work in both depressive and anxiety disorders. In our view, it is important to consider how other components impair or even enhance the functioning of mental time travel. To develop this perspective, we draw on the ensemble hypothesis, which holds that human cognition depends on five core systems or components that interact in non-additive ways ( Kellogg, 2013 ; Kellogg and Evans, 2019 ). Mental time travel is necessary but not sufficient for explaining either the remarkable competencies of human cognition or its breakdowns in disorders such as anxiety and depression. An advanced executive form of working memory, a theory of mind augmenting social cognition, language, the ability to interpret information using inner speech, and causal inference are necessary, as well as an episodic memory capable of mental time travel. Kellogg (2013) introduced the ensemble hypothesis in the context of understanding the exceptional cognitive abilities in the evolution of our species, Homo sapiens . The book provides the reasons for considering the five components and their interactions in normally developing and functioning human beings. Kellogg and Evans (2019) offered further evidence in support of the hypothesis from behavioral studies, lesion studies, and studies involving neuro-atypical populations.

The key claim of the ensemble hypothesis is that two or more mental capacities can interact in a multiplicative fashion to yield competencies in a well-functioning human being that exceed their simple additive effects. For example, delay of gratification is a phenomenon that entails an ability both to prospectively consider the future and to exercise cognitive control using executive attention. In typically developing children, growth in the capacity of executive attention for self-regulation boosts the ability to delay rewards in anticipation of a larger future reward ( Mischel et al., 1989 ). Similarly, planning in problem solving requires future thinking and a normally functioning system of executive attention. Frontal lobe injuries that damage networks of executive attention often impair planning ( Kellogg and Evans, 2019 ). In normally functioning adults, retrospective memory for a list of words presented in a laboratory task requires an intact hippocampus and medial temporal lobe, but it is also boosted by maintenance and elaborative rehearsal strategies that depend on executive attention. Failing to deploy attentional resources to an encoding strategy impairs the recall of a list of words presented in a laboratory task in individuals with depression ( Hertel and Rude, 1991 ). As will be considered in detail later, the normal functioning of mental time travel can be altered by depression because of its effects, in part, on executive attention.

The purpose of the present paper is to consider the implications of the ensemble hypothesis for two broad categories of psychopathology: depression and anxiety. We suggest that much of the phenomenology and symptoms that underlie depressive and anxiety disorders can best be understood as an interaction of components of the hypothesized ensemble. We wish to extend the insights provided by Miloyan et al. (2014) and Roepke and Seligman (2016) by demonstrating how the interpreter and executive attention influence mental time travel. As will be seen, language is considered in the form of inner speech, but the broader concept of language as interpersonal communication falls outside the scope of the current paper. Similarly, as will be addressed in the limitation section of our paper, an extensive literature on theory of mind and social cognition in depression ultimately needs to be accounted for. Even so, our focus on the interpreter, executive attention, and mental time travel documents the importance of the interactions posited by the ensemble hypothesis.

To illustrate, consider the case of depression (see Figure 1 ), as exemplified by major depressive disorder (MDD). As will be discussed in detail later, the interpreter shown in Figure 1 refers to the inner voice and causal inference capacity of the left hemisphere of the human brain that enables attributions about the self and other people ( Gazzaniga, 2000 ; Kellogg, 2013 ). In depression, the interpreter is biased to assign blame to the self for negative experiences. This pessimistic and personally negative explanatory style ( Petersen and Seligman, 1984 ) causes the depressed individual to focus attention on negative past events and have difficulty envisioning anything positive about the future. Further, there is evidence that depression is associated with a concurrent deficit in executive attention ( Ólafsson et al., 2011 ), causing impaired cognitive control over mental time travel resulting in persistent negative rumination. Thus, the influence of both a bias in interpretation and a deficit in executive attention, we propose, could underlie faulty prospection in depressed individuals. The interactive model of Figure 1 differs from the position of Roepke and Seligman (2016) with respect to effective approaches to treatment for depression. They advocate for treatments targeting mental time travel, specifically, the core problem with prospection. Alternately, we contend that efforts to improve executive attention and to correct the pessimistic explanatory style of the interpreter ought not be neglected, because they can alter the functioning of mental time travel.

Model of major depressive disorder.

The plan of the paper is, first, to introduce several components of the ensemble hypothesis that are central to our analysis of depressive and anxiety disorders. Second, we consider evidence on the role of mental time travel in depressive and anxiety disorders. Third, we discuss literature regarding the pessimistic explanatory style in depression and suggest that anxious individuals are characterized by a related but distinct dysfunctional style of explaining events as threatening to the self. The emphasis on loss in depression and threats in anxiety can influence the functioning of mental time travel, we propose. Fourth, we document that both kinds of disorders are associated with impairments in executive attention that may compound problems with mental time travel. Fifth, we discuss how the symptoms of depression versus anxiety can best be understood by considering mental time travel, the interpreter, and executive attention as an integrated ensemble. We conclude by considering the implications of the ensemble perspective regarding effective therapies for depressive and anxiety disorders.

Mental Time Travel, the Interpreter, and Executive Attention

Mental time travel is the unusual form of human episodic memory that allows the mind to recollect the specific time and place of a past event in one’s personal history ( Tulving, 2002 ; Suddendorf and Corballis, 2007 ). It is conceived as mental time travel because the same neural systems are involved in imagining future events as well as recollecting past events. The brain systems involved in mental time travel include the hippocampus and medial temporal lobe structures as well as the default mode network activated in resting state conditions when no external task is presented ( Buckner et al., 2008 ). The ability to construct spatially coherent scenes in which an event takes place is essential in both recollecting the past and imagining the future. It has been suggested that scene construction is a core function of the hippocampus ( Clark and Maguire, 2016 ).

A uniquely human mental ability appears to be the interpretive capacity of the left hemisphere ( Gazzaniga, 2000 ). Over the course of human evolution, our oral language capacity became internalized as inner speech, mediated by language networks in the left hemisphere. Vygotsky (1962) emphasized that speech begins in early childhood as a means for communication, but as speech is internalized, it also becomes a means for planning and problem solving. Self-directed inner speech, then, has long been recognized as an important vehicle for thinking and appraising situations and events. The interpreter constructs a personal narrative that explains why we feel and behave as we do. Inner speech is combined with a specialization of the left hemisphere for a specific kind of thinking. The left hemisphere is not only specialized for the use of language, including self-directed language of inner speech, but it is also specialized for forming hypotheses ( Wolford et al., 2000 ) and making inferences about causal relationships ( Roser et al., 2005 ). Similarly, the ability to reason deductively is known to be impaired in patients with left frontal lesions but not right frontal lesions ( Reverberi et al., 2010 ).

In clinical psychology, the interpreter is important in understanding the role of inner speech and causal inference in how people respond to stressful life events. How an individual cognitively appraises stressors can either attenuate or exacerbate the strain that they cause. This role for causal attributions has long been recognized in understanding depressive and anxiety disorders. For example, Petersen and Seligman (1984) highlighted that depression is characterized by a personalized and pessimistic explanatory style. The individual attributes personal, pervasive, and permanent causes to negative personal experiences, committing what social psychologists call the fundamental attribution error. The role played by the interpreter in explaining why things happen and what significance events have for the self is central to both depression and anxiety, as will be detailed later in the paper.

The executive attention component of working memory enables the coordination and regulation of representations held in verbal, visual, and spatial stores of short-term memory. Working memory, planning, cognitive control, self-regulation, and response inhibition have all been referred to as executive functions that have traditionally been viewed as dependent on the frontal lobe ( Alvarez and Emory, 2006 ; Posner and Rothbart, 2007 ; Diamond, 2013 ; Ajilchi and Nejati, 2017 ). A more complex understanding has emerged in the literature with two distinct brain networks involved in executive attention; these include but are not limited to regions in the frontal lobe ( Posner and Peterson, 1990 ; Petersen and Posner, 2012 ).

By studying a battery of executive functioning tasks, Miyake et al. (2000) identified three correlated but distinctive processes underlying performance. Updating the contents of working memory, shifting goals as required in multitasking, and inhibiting irrelevant information are considered three essential and irreducible functions of executive attention. A widely used test of individual differences in working memory capacity, called the Operation Span (OSPAN) test, indicates that the ability to inhibit irrelevant information is especially important and shows a strong correlation with general fluid intelligence or the ability to solve novel problems ( Engle et al., 1999 ).

Mental time travel, the interpreter, and executive attention are three fundamental components of human cognition. Kellogg (2013) proposed that these components, together with theory of mind and language, comprise an ensemble that renders human cognition unique and qualitatively different from non-human cognition. Importantly, his hypothesis suggests that it is the interaction of these components that yields the unique properties of human cognition. If that is so, then it stands to reason that common forms of psychopathology should reveal such interactions, too. In persons experiencing anxiety or depression, a deficit in one component can cascade to degrade the functioning of another component, despite that the latter component is not necessarily dysfunctional.

Mental Time Travel Impairments

Roepke and Seligman (2016) reviewed a variety of evidence that faulty prospection lies at the heart of depression. First, persons experiencing depression can envision negative future scenarios more readily, compared to non-depressed persons ( MacLeod and Byrne, 1996 ). This characteristic is also shared with those experiencing anxiety, indicating it is not a unique dysfunction of mental time travel associated with depression. Miloyan et al. (2014) suggested that anxious as well as depressed individuals anticipate negative future events but that each disorder shows a unique profile of faulty prospection. Individuals with anxiety anticipate more negative experiences, but not fewer positive experiences, relative to control participants without a history of psychiatric diagnosis, according to some studies ( MacLeod and Byrne, 1996 ; MacLeod et al., 1997b ). Depression, on the other hand, is associated with a failure to anticipate positive future events ( Miranda and Mennin, 2006 ; Pomerantz and Rose, 2014 ). When depressed psychiatric outpatients were asked to describe a distressing personal problem and to imagine and rate the likelihood of both the worst and best possible outcomes, they rated the worst outcome as being more likely and the best outcome as being less likely, relative to generalized anxiety disorder (GAD) and control groups ( Beck et al., 2006 ).

Thus, it is possible that a faulty form of prospection found in depression results in a diminished ability to envision positive future events ( MacLeod and Salaminiou, 2001 ). However, both this finding and the finding that individuals with depression envision more negative future events than do controls can also be linked to a pessimistic explanatory style. MacLeod et al. (1997a) found that both depressed and anxious patients not only judged future negative events to be more likely, relative to controls; they also provided more supportive as opposed to contradictory reasons for their occurrence. As MacLeod et al. (p. 22) concluded, “…mood-disturbed subjects were pessimistic about what would happen to them in the future, and this was supported by their causal thinking about those events.” Thus, the pessimistic explanatory style of the interpreter rather than a malfunction in mental time travel per se could explain the findings. They could also be linked to the deficits in executive attention that are associated with depression ( Ólafsson et al., 2011 ). As will be argued in later sections of the paper, problems with mental time travel may arise because of the moderating influences of the interpreter and executive attention.

An important exception regarding memory impairment in depression is the tendency to focus and elaborate upon sad events ( Williams et al., 1997 ). A case can be made for mood congruent memory in depression ( Mineka and Nugent, 1995 ). For example, in a study by Derry and Kuiper (1981) , a list of depression-related adjectives (e.g., bleak, dismal, helpless) and non-depression-related adjectives (e.g., amiable, curious, loyal) were presented in an incidental learning task. The nature of the orienting task was manipulated, with one way being whether the adjective applied to the self. On a subsequent recall test, this self-reference orienting task resulted in a greater proportion of depressed-content words recalled (41%) than non-depressed content (16%) for depressed patients. Strikingly, this pattern was completely reversed for normal controls, who recalled more non-depressed content (43%) compared with depressed content (8%). Even a group of psychiatric controls showed a reversal with more non-depressed content (36%) relative to depressed content (18%). None of these effects were observed for structural (small letters?) and semantic (means the same?) orienting tasks, indicating that they are contingent on judging the word as relevant to the self.

Similarly, in another study, after being shown a list of words including pleasant, unpleasant, and neutral words, individuals with depression recalled more unpleasant words compared with pleasant words ( McDowall, 1984 ). A non-depressed control group as well as another control group made up of psychiatric patients with a diagnosis other than depression did not show this bias toward improved memory for unpleasant words. The depressed patients’ free recall of unpleasant words was at the same level as that for the two control groups, whereas they showed a memory impairment for pleasant words. This indicates that the mood congruent benefit of remembering unpleasant words can offset the usual memory impairment found in depression.

Clark and Teasdale (1982) found that autobiographical experiences also reveal mood congruency even within the same group of individuals with depression. The investigators compared the recall of personal memories at two different times of day to capitalize on diurnal variations in mood among psychiatric patients experiencing depression. The percentage of unhappy memories (52.3%) was reliably greater when the individual reported being more depressed compared with less depressed (36.7%). Happy memories (37.7 versus 51.1%) showed exactly the reverse pattern.

The above studies show that depression can bias retrospection in the direction of remembering sad events more readily than happy events. Would such findings also hold for prospection? MacLeod et al. (1997b) measured the recall of past experiences and the anticipation of future experiences in anxious, depressed, and control individuals. The study prompted the participants to remember or anticipate either positive experiences or negative experiences. This prompt variable allowed the comparison of the number of positive events versus negative events produced under conditions of both retrospection and prospection. Their findings showed no difference between the retrospection and prospection conditions for either disorder. Of importance, individuals with depression produced fewer events compared with controls—both positive and negative—both in recalling their past and in anticipating their future.

An analogous outcome has been found in laboratory studies of the retrospective recall of word lists versus prospective memory for future actions. Hertel and Rude (1991) found poorer free recall of a list of words presented earlier for currently depressed patients compared with recovered patients and control individuals with no history of depression in a retrospective task. Rude et al. (1999) similarly reported that depressed individuals perform poorly on a prospective memory task requiring the ability to self-initiate an action in the future. Their difficulties with “remembering to remember” to act in the future were parallel to impairments found in retrospective tasks, according to the authors. Of course, these tasks are different from the autobiographical reports examined by MacLeod et al., but the conclusions reached are consistent. MacLeod et al. (1997a) also found that anxious individuals did not differ from controls either in remembering or in anticipating positive events. However, they generated more negative events compared with controls regardless of whether they were engaged in retrospection or prospection. Their findings thus confirm that anxiety is primarily a disorder of worrying about negative outcomes ( Barlow, 1988 ). Whereas MacLeod et al.’s control participants both recalled and anticipated about 44% more positive life events than negative ones, the anxious participants only recalled 15% more positive events. Compared to participants with depression, the participants with anxiety recalled and anticipated about 67% more negative events.

Finally, MacLeod et al. expected that individuals with depression would show a mood congruent effect by remembering or anticipating more negative events compared with positive events. In contrast to prior studies reviewed earlier, this outcome did not occur. Rather, negative events were remembered by patients with depression at about the same rate as found in the controls. This rate was equivalent to the number of positive events remembered by those with depression, who were 75% less likely to remember positive events than were patients with anxiety and controls. This is reminiscent of the findings with the free recall of word lists reported by McDowall (1984) . Unpleasant words were remembered as well by patients with depression as by controls, but recall for pleasant words showed a marked impairment.

The above findings on memory could depend on the severity of the depressive disorder. It is important to note in that regard that MacLeod et al. (1997b) examined patients who met the diagnostic criteria for panic disorder and MDD. Similarly, the studies by Derry and Kuiper (1981) , Clark and Teasdale (1982) , McDowall (1984) , Hertel and Rude (1991) , and Rude et al. (1999) examined psychiatric inpatients or patients with depression in the community with screening done to insure they met the diagnostic criterion for depression. By contrast, in a non-clinical student population, neither trait anxiety nor trait depression was associated with difficulties in a measure of prospective memory ( Arnold et al., 2014 ). Thus, the severity of the disorder probably plays a role in the effects of depression and anxiety on mental time travel.

In contrast to the picture for clinical depression, the findings on retrospective memory for anxiety disorders are mixed. MacLeod and McLaughlin (1995) found that individuals currently receiving treatment for GAD performed worse than those in a control group on an explicit recognition test for words presented in a laboratory setting. By contrast, on explicit memory tests of cued recall ( Mathews et al., 1989 ) and free recall ( Becker et al., 1999 ), anxious individuals performed at the same level as control participants. For threatening words included among the lists presented in the laboratory, GAD patients showed no advantage in recall or recognition, but they did show superior performance on various implicit memory tests compared with controls ( Mathews et al., 1989 ; MacLeod and McLaughlin, 1995 ). A similar heightened explicit memory for threatening words was found by Becker et al. (1999) for individuals diagnosed with panic disorder but not with social phobia or GAD. In a review of the literature on memory and anxiety disorders, Mineka and Nugent (1995) concluded that the evidence for an explicit memory bias for threatening events is weak, difficult to replicate, and unconvincing, at least with respect to persons experiencing GAD.

We conclude from this sample of findings in the literature that while depression impairs mental time travel ability, it does not seem to be a selective difficulty with prospection. Judging from the findings of MacLeod et al. (1997a) , at least for positive events, anxious individuals do not appear to show any impairment in mental time travel, either in its prospective or in its retrospective form. In fact, they appear to recollect past negative events and envision future negative events more often than is found in both non-anxious controls and depressed patients. However, other studies indicate that such memory bias for threatening events is tenuous at best in anxiety. Patients with depression, on the other hand, forget positive events more readily than is found in non-depressed controls. A central question is what accounts for these differences in the functioning of the mental time travel component. We propose that considering the role played by the interpreter and executive attention helps to understand the pattern of results found for mental time travel.

Interpreter Biases

As noted earlier, the interpreter in individuals with depression employs a pessimistic explanatory style ( Petersen and Seligman, 1984 ). An inability to envision a positive future and a facility with envisioning a negative future could be understood as a dysfunction of the explanatory style found in individuals with depression rather than a fault with mental time travel per se ( MacLeod et al., 1997a ). A negative style of explaining why things happen as they do is a prime reason for feelings of hopelessness in depressed people ( Alloy et al., 1988 ). Individuals with depression tend to attribute the reasons for events in life to internalized causes about the self that are stable over time and that are global or pervasive in multiple situations. Another compounding factor is a negative attributional style that attributes negative events to uncontrollable causes ( Sanjuán and Magallares, 2009 ). As a consequence, persons with depression might be able to recollect or imagine an event that most people would regard as positive (e.g., getting a job promotion) but then interpret it as negative. Individuals with depression might appraise the promotion as full of pitfalls—more responsibility, longer working hours, and greater stress. Remembering or anticipating a job promotion may not be the problem but, rather, its pessimistic interpretation.

The interpreter, therefore, has a prominent, if not central, role in depression. Indeed, Beck (1974) designed cognitive behavioral therapy (CBT) to confront and modify a depressed person’s inner speech of hopelessness and self-deprecation. By altering the person’s cognitive appraisals of situations and causal explanations of events, mood improves as a result. Beck’s concept of the cognitive triad included a negative view of the self, negative interpretations of ongoing experiences, and a negative view of the future. Although the latter could be caused by faulty prospection, the first two stem from the distorted explanations of the interpreter.

Comparisons of the interpretative style of depressed versus anxious individuals have yielded conflicting results, however. For example, Heimberg et al. (1989) , by contrast, found that the attributional style found in the learned helplessness of individuals with depression was also characteristic of multiple anxiety disorders, such as social phobia, agoraphobia, and panic disorder. In their study, the two disorders differed only in that depression produced global and unstable attributions regarding the causes of positive events, whereas anxiety was associated with the same kind of attributions for negative events. Still other findings indicate that attributing internal, stable, and global causes to negative events is in fact found in currently depressed individuals, but especially in those with comorbid anxiety ( Fresco et al., 2006 ). Similarly, Luten et al. (1997) concluded that a pessimistic attributional style is not specific to depression but, rather, is correlated with high levels of negative affect as is also found in in persons with anxiety disorders. Ahrens and Haaga (1993) even reported that a negative event attributional style was only found with anxiety disorders rather than with depression.

Thus, it seems that pessimistic forms of causal inference about life’s events are a non-specific risk factor for anxiety and depression. This commonality with respect to the dysfunction of the interpreter is a likely reason why depressive and anxiety disorders share a high degree of comorbidity ( Gotlib, 1984 ; Kessler et al., 2007 ; Grisanzio et al., 2018 ).

Despite their similarities and high rates of comorbidity, there may be some unique aspects to the interpreter’s dysfunction in anxiety disorders, however. Riskind and Williams (2005) identified a looming cognitive style in which individuals overestimate the progression of a potential threat in terms of both spatial and temporal dimensions. Individuals with a high score on their looming cognitive scale misinterpret potential threats as catastrophic threats. A study by Reardon and Williams (2007) showed that this looming cognitive style is uniquely associated with anxiety disorders. A pessimistic cognitive style contributed to both anxiety disorders and depressive disorders, but individuals predisposed to anxiety disorders also were prone to a looming cognitive style that magnifies potential threats. Anxiety disorders also feature highly persistent negative self-talk. The excessive worry that characterizes anxiety is largely verbal in nature ( Borkovec et al., 1998 ). Instead of imagining a threat in a visual–spatial context, anxious individuals talk to themselves about it. Finally, it has long been recognized that the causal inferences made in depression are associated with personal failures and self-deprecation ( Beck, 1974 ). This contrasts with worries about uncertainties and potential dangers in the case of anxiety disorders ( Beck et al., 1987 ; Clark et al., 1990 ).

As shown in Figure 2 , the anxious interpreter views events as threatening to the self rather than as a negative reflection of the self as in depression ( Figure 1 ). Kendall and Ingram (1989) differentiated the two disorders precisely in terms of their characteristic attributions. The interpretations of the depressive person often are “self-referent, definitive, past-oriented cognitions of sadness, failure, degradation, and loss,” in contrast to the “future oriented ‘questioning’ cognitions” found in anxiety disorders ( Kendall and Ingram, 1989 ; p. 36).

Model of generalized anxiety disorder.

In our view, the interpreter biases play a role in the problems observed in mental time travel with depressed and anxious individuals ( Eysenck et al., 2006 ). For depression ( Figure 1 ), the pessimism of the interpreter causes individuals with depression to remember and ruminate about negative life experiences that reinforce feelings of loss and self-blame. Perhaps the difficulty with imagining a positive future is a direct consequence of depressed individuals focusing on negative past events. Roepke and Seligman (2016 , p. 27), in fact, suggest the possibility that persons experiencing depression “struggle to recall a good past,” with few positive memories ( Williams and Scott, 1988 ).

For GAD ( Figure 2 ), the interpreter is biased to detect threats to the self. This might cause one to recall, see, and foresee dangers rather than losses. Instead of mental time travel being impaired relative to control individuals who are neither depressed nor anxious, there is, if anything, an excessive prospection and retrospection of negative events. This outcome can be seen in the study by MacLeod et al. (1997b) , who reported that anxious individuals both retrospectively and prospectively generated more negative events than did controls and even individuals with depression. Similarly, when asked to recall recent life events, 77% of anxious participants remembered danger events compared to 44% of depressed participants ( Finlay-Jones and Brown, 1981 ). Loss events, on the other hand, were more frequently remembered by participants with depression (65%) compared with anxiety (15%). Comparable findings for autobiographical recall have been reported by Witheridge et al. (2010) .

Thus, in our view, the interpreter plays a key role in the functioning of the mental time travel component. The content of the events that are remembered in depression is more likely to deal with loss rather than danger. This, we suggest, occurs because of the bias of the interpreter on mental time travel. In anxiety disorders, retrospection is not impaired; if anything, there is excessive rather than impaired prospection and retrospection about threatening events. In addition to mental time travel being moderated by the interpreter, we further consider in the next section the possible influence of executive attention.

Executive Attention Deficits in Depression and Anxiety

A number of studies have shown that anxiety and depression are associated with impaired performance on a variety of neuropsychological tests that measure for executive control functions ( Reinholdt-Dunne et al., 2013 ; Devito et al., 2018 ). This supports the notion that both disorders are associated with impairments in executive attentional control. In this article, we refer to executive attention as executive attentional control and attentional control, interchangeably. In accordance with Stefanopoulou et al. (2014 , p. 330), attentional control can be defined as “the ability to sustain focus on tasks in the face of competing activities or to shift attention from one task to another.” However, depression and anxiety do not show the same pattern of executive attention deficits.

The Attentional Control Scale (ACS) is a self-reported attention control measure that is comprised of two components: focusing and shifting ( Reinholdt-Dunne et al., 2013 ). Ólafsson et al. (2011 , p.77) define attentional focusing as “the capacity to intentionally hold the attentional focus on desired channels and thereby resist unintentional shifting to irrelevant or distracting channels” and define attentional shifting as “the capacity to intentionally shift the attentional focus to desired channels, thereby avoiding unintentional focusing on particular channels.” Because it has been noted that those with anxiety show attentional impairment in relation to shifting and focusing ( Devito et al., 2018 ), the ACS has been used to compare the relationship between attentional focusing, attentional shifting, and levels of anxiety and depression in adults. Ólafsson et al. (2011) found that when controlled for depression, the focusing ACS subscale significantly predicted anxiety ratings, whereas when anxiety ratings were controlled for, the shifting subscale significantly predicted depression ratings. Reinholdt-Dunne et al. (2013) supported these findings when they found ACS focusing to be associated with lower anxiety and ACS shifting to be associated with fewer depression symptoms. These findings support the claim that anxiety is more associated with attentional focusing and depression is more associated with attentional shifting.

Shi et al. (2019) performed a meta-analysis to investigate the size and nature of attentional control deficits in participants with anxiety versus non-anxious participants. They found that anxiety-producing deficits were supported in processing efficiency, rather than effectiveness, on a variety of behavioral tasks. However, they also found that when looking at task switching studies alone, both efficiency and effectiveness produced anxiety-related deficits in attentional control. Their results also showed that studies requiring participants to operate under high cognitive load conditions showed greater anxiety-related attentional control deficits compared to studies where participants were under normal cognitive load conditions ( Shi et al., 2019 ).

Although attentional control deficits have been related to anxiety disorders, these deficits are prominently seen in individuals diagnosed with GAD, characterized by uncontrollable worry. This uncontrollable worry has been connected to deficits of the central executive function of working memory, which includes attentional control as a key component of working memory ( Stefanopoulou et al., 2014 ). Uncontrollable worrying can be attention-demanding and, consequently, consumes voluntary attentional resources required ( Eysenck et al., 2007 ). This links uncontrollable worry to impairments in attentional control.

Stefanopoulou et al. (2014) used the key-pressing task to assess the extent to which attentional resources were depleted by worry in individuals with GAD. Stefanopoulou et al. (2014) found that GAD individuals were less random on the key-pressing task while worrying compared to when thinking of a positive topic, indicating that fewer residual attentional control resources were available during the worrying process. However, the performance of the healthy participants did not differ between conditions. GAD participants also reported having more negative thoughts and anxiety during this task compared to healthy participants. This same study also used the N-back task, which “varies in difficulty and is sensitive to subtle difference in ability to handle increasing demands on attentional control” ( Stefanopoulou et al., 2014 , p. 330). During this task, GAD participants exhibited longer reaction times compared to healthy participants for the higher load conditions. These results together indicate a greater difficulty in sustaining focus in conditions requiring a higher degree of attentional control, suggesting that poor attentional control may partially explain the excessive worry seen in individuals with GAD.

Further, there appears to be a bidirectional relationship between attentional control and anxiety ( Devito et al., 2018 ). Impairments in attentional control may increase one’s risk for developing anxiety, and anxiety symptoms may prevent executive components of attention from being recruited. We indicate this bidirectional relationship between the interpreter and executive attention in Figure 2 . The pessimistic explanatory style and negative self-talk of the interpreter consume limited attentional resources. The resulting deficit in executive attention weakens the ability to inhibit the dysfunctional thinking of the interpreter in anxiety disorders.

Whether a similar bidirectional relationship occurs in depression is unclear. An argument against this takes into account the speech and inner speech of depressed versus anxious individuals based on the symptoms outlined in the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition ( DSM-V ). The hallmark of GAD is excessive worry in the form of inner speech. By contrast, in MDD, fatigue and tiredness occur on nearly a daily basis, and this can be accompanied by slowed speech, long pauses before responding, and a decrease in the amount and variety of speech content ( American Psychiatric Association, 2013 , p. 132). These suggest that inner speech in MDD is more likely to be inhibited or overly regulated rather than exaggerated, as is apparent in GAD. Moreover, in a review of the literature on inner speech, Alderson-Day and Fernyhough (2015) noted that the evidence for inner speech playing a central role in anxiety disorders is stronger and more specific than it is with depression. The verbalized worry of anxiety is, in their words (p. 948), “…repetitive thinking that is.negative, uncontrollable, and aimed at some ill-defined problem solving, such as a problem with a clear solution.” We propose that the bidirectional links between executive attention and the interpreter produce worry in GAD that is indeed out of control (see Figure 2 ). A positive feedback loop ensues in which worry depletes attention, which in turn worsens worry. In depression, the negative impact of depleting attention does not appear to feed back on the interpreter. Instead, we suggest, it feeds forward to impact mental time travel. Specifically, the deficit in executive attention found in depression results in a loss of control in mental time travel (see Figure 1 ). The arrows shown in Figures 1 , ​ ,2 2 are intended to reflect the major pathways of influence from one component to another. From the perspective of the ensemble hypothesis, all possible links among components are potentially relevant, including bidirectional relationships. In a normally developed and well-functioning adult human being, each of these components influences the others. Our aim in these figures is to take a minimalist approach by highlighting only strong interactions that differ from normal under a diagnosis of psychopathology. The purpose is to differentiate as clearly as possible how MDD and GAD differ from each other. For example, we intentionally omit an influence of executive attention on mental time travel in GAD. Although it is known that the availability of executive attention affects the functioning of mental time travel even in healthy individuals, we only indicate interactions that are unique to GAD or MDD.

Memory Impairment From an Ensemble Perspective

As shown in Figure 1 , we suggest that both retrospection and prospection will be impaired as a result of a deficit in executive attention ( Hertel and Rude, 1991 ; Rude et al., 1999 ). Evidence for a causal role played by attention comes from an intervention designed by Hertel and Rude (1991) to remediate the attentional deficits. Hertel and Rude studied three groups of individuals who were currently depressed, recovered from depression, or without a history of depression in an incidental learning and memory task. The participants’ ability to recall a list of target words that they had viewed in the first phase of the experiment was markedly impaired in the individuals with depression compared with recovered and healthy controls. But this outcome only occurred when their attention to the words during learning was unconstrained by the demands of the task. For half of the participants, the investigators required the participants to repeat the target words aloud on each trial, as a means of focusing their attention. Strikingly, this manipulation eliminated the memory impairment of the depressed patients entirely. This result suggests that retrospection per se is not necessarily deficient in depression, but a memory deficit can be observed as a result of the influence of executive attention not being appropriately allocated to the task at hand.

A comparable finding was reported by McDowall (1984) . On a free recall test, inpatients with depression performed markedly worse than did a control group consisting of non-depressed psychiatric inpatients in remembering pleasant words. However, when given an orienting task of rating each word for pleasantness as was shown during the study phase, patients with depression showed no difference in recall between pleasant and unpleasant words and performed no worse than did the psychiatric control group doing the same task. Their mean recall of 5.6 words out of 12 was only slightly less than was found for a non-psychiatric control group (6.8 words), again with no difference between pleasant versus unpleasant words. As with the word repetition technique used by Hertel and Rude (1991) , the orienting task directed attention to the words in a way that eliminated most, if not all, of the memory impairment for individuals with depression.

Ruminating on negative life experiences is part and parcel of the sense of loss, hopelessness, and self-deprecation frequently seen in persons experiencing depression. In our view, these phenomena are the direct result of the interpreter bias found in depression. It is the influence of the interpreter with mental time travel that contributes to the inability of individuals with depression to think about positive life experiences, whether they lie in the past, the present, or the future. Further, the persistence and intrusiveness of negative memories in depression could reflect an inability to inhibit them because of executive attention deficits (see Figure 1 ). Poor cognitive control may combine with the loss bias of the interpreter to produce the profile of memory problems found in depression.

As shown in Figure 2 , for GAD, the mental time travel system is biased to focus on the uncertainties and threats of life experiences. Instead of loss and self-blame, the content of memories predominately concerns threats to the self in anxiety disorders to the extent that they are biased at all. This can account for why negative events are, at times, better remembered or anticipated by anxious individuals (e.g., MacLeod et al., 1997b ). However, in contrast to the memory bias effects for losses observed in depression, similar effects for threatening events in anxiety disorders are harder to detect reliably ( Mineka and Nugent, 1995 ). They might be found in panic disorder but not GAD ( Becker et al., 1999 ). Or they can be observed with implicit memory tests but not explicit tests of recall or recognition ( Mathews et al., 1989 ; MacLeod and McLaughlin, 1995 ). They might also be observed when people are asked to recall autobiographical events of personal relevance ( Finlay-Jones and Brown, 1981 ; Witheridge et al., 2010 ) but not when they are asked to remember word lists that contain some threatening versus neutral words ( Levy and Mineka, 1998 ).

We suggest that the mixed picture for memory bias in anxiety disorders occurs because executive attention deficits do not generally disrupt mental time travel in persons experiencing GAD, which is not the case for MDD (see Figure 2 ). The deficit in executive attention causes a loss of control with the interpreter but not with mental time travel. Without both a loss of cognitive control and a threat bias from the interpreter, the mental time travel system functions relatively normally in GAD. That implicit tests of memory reveal bias effects for negative information implies that a threat bias from the interpreter is at work. But for the declarative memory system of episodic memory to show such effects, it requires both the threat bias and a loss of cognitive control over mental time travel. Perhaps only in severe cases of anxiety disorders, such as panic disorder, does the loss of cognitive control from deficits in executive attention spill over to affect mental time travel, much as it does in depression. This could account for the results of Becker et al. (1999) for panic disorder in contrast with other forms of anxiety disorder. It is worth noting that MacLeod et al. (1997b) studied anxious participants who all met the criteria for panic disorder. Thus, the characteristics of their sample might have explained why they observed a bias for negative events when so many other studies have been unable to do so, as they noted in their discussion section.

In summary, accounting for the consistent memory bias for losses or a lack of positive memories in MDD seems to depend on distorting inputs from both executive attention and the interpreter (see Figure 1 ). For persons experiencing GAD without panic disorder, the input from executive attention is weak or non-existent. Without this concomitant symptomatology, the bias of the interpreter for threatening events does not distort either retrospective or prospective memory, although it shows up on implicit, non-declarative forms of memory.

Limitations, Implications, and Future Directions

As noted previously, our explication of the complex role of mental time travel in explaining the phenomenology and research findings related to MDD and GAD has focused on interrelationships between three of the five components of the ensemble hypothesis. In focusing on these three constructs, we acknowledge the limited attention we have given to the importance of the two remaining ensemble components—overt use of language and social cognition—in accounting for differences and similarities in MDD and GAD. Reviewing the broader concept of language as interpersonal communication falls outside the scope of the current paper. Similarly, the extensive literature on theory of mind and social cognition in disorders such as MDD and GAD merits careful consideration that is not undertaken by our current analysis. Research indicates that theory of mind, a specialized aspect of social cognition ( Frith and Frith, 2007 ), plays a complex role in presentations of depression and anxiety where aspects of social cognition are prominent ( Bora and Berk, 2016 ; Washburn et al., 2016 ). Examples would include depression in the context of discordant relationships or bereavement, and social anxiety disorder. Exploring the interrelationships between social cognition and other components of the ensemble hypothesis is a fruitful direction for further theorizing and research.

Also, our paper is limited in scope, in that we focused on accounting for differences between disorders such as MDD and GAD, rather than examining similarities in their phenomenology and accounting for the high comorbidity of these conditions. We believe that further analysis of the interrelationships among the ensemble of mental components in MDD and GAD may help account for the comorbidity of these two disorders. For example, the high incidence of comorbidity might be accounted for by the reciprocal relations between the cognitive ensemble components and symptoms that constitute pathways that connect the disorders ( Borsboom and Cramer, 2013 ). It is worth noting the strong similarities of MDD and GAD as portrayed in Figures 1 , ​ ,2. 2 . Both disorders involve several components of the ensemble hypothesis, including executive attention and the interpreter, in addition to mental time travel. The specific characteristics of memory functioning seem to depend on these interrelated cognitive components of the ensemble perspective. Thus, future theorizing and research should explore the interrelated components of the ensemble hypothesis as they relate to comorbid presentations of MDD and GAD.

Regarding one final limitation of our paper, we acknowledge that the ensemble component of “mental time travel” as it pertains to episodic foresight involves multiple constructs, each with substantive theoretical and empirical literatures that lie beyond the scope of our paper. Examples would include the role of mental time travel in future decision making involving delayed rewards ( Boyer, 2008 ) and the literature on “affective forecasting” ( Wilson and Gilbert, 2005 ) as it relates to the ensemble components in persons experiencing depression or anxiety. Once again, future theorizing and research should explore the interrelationships of such constructs with the ensemble components as they pertain to the etiology and phenomenology of MDD and GAD.

In review, we believe that similarities and differences between MDD and GAD are best conceptualized by considering an ensemble of mental components. Although mental time travel plays a role in both disorders, this component is influenced by the interpreter that assigns causal attributions to events and a dysfunction in executive attention.

If depression is primarily a problem with faulty prospection, then it is reasonable to target future thinking as perhaps the most effective form of treatment. Roepke and Seligman (2016) reviewed four variations of CBT that emphasize positive expectancies, hopeful thinking, a focus on future-oriented solutions to problems, and goal setting and planning. Initial results with each of these approaches have been positive and are worthy of additional study in randomized trials. Further, Roepke and Seligman (2016) suggest several new future-oriented interventions that might be considered (e.g., using visual imagery to imagine a route to future success).

While new approaches certainly merit exploration, we note that the premise underlying these—namely, that faulty prospection is the core causal process in depression—is open to debate. We believe that the effects of the interpreter and executive attention, in conjunction with mental time travel, should be considered to better understand both MDD ( Figure 1 ) and GAD ( Figure 2 ). From this ensemble perspective, therapies should target all three components rather than focusing only on mental time travel.

For example, mindfulness-based therapies including short-term meditation explicitly address deficits in executive attention. A short-term program (5 days of training for 20 min per day) has been shown to improve attention and self-regulation in a sample of healthy young adults ( Tang et al., 2007 ). Such mindfulness-based interventions have been shown to minimize relapse and offer promise in the treatment of acute symptoms of depression and anxiety, although more research is needed to clearly establish their clinical efficacy ( Edenfield and Saeed, 2012 ). In a different approach, training attention using computer-based tasks has been found beneficial in treating generalized social anxiety disorder ( Schmidt et al., 2009 ). If anxiety disorders as well as depression primarily are influenced by the mental time travel component ( Miloyan et al., 2014 ), then it is difficult to explain why treatments targeting the executive attention deficit would be effective. Yet, it is known that executive functioning matters. Although neurocognitive abilities can improve with CBT treatment for anxiety and depression, individuals with poor attentional control show decreased benefit from such treatment compared to those with adequate executive skills ( Devito et al., 2018 ).

Many techniques in traditional CBT build on the premise of altering the pessimistic explanatory styles employed by depressive and anxious individuals. These techniques are based on the premise that the symptoms and dysfunctional behaviors of these disorders are mediated by cognitive factors. The therapeutic goal, then, is to restructure the dysfunctional thinking and beliefs underlying the disorder. Cognitive distortions must be identified and refuted in restructuring the functions of the interpreter. The evidence supporting CBT as an effective treatment of both anxiety and depression is solid ( Butler et al., 2006 ). As Roepke and Seligman (2016) pointed out, CBT interventions already include a number of techniques that improve future thinking. Even so, the aim of CBT is to alter thinking patterns in general, including past and present thinking as well as future thinking. It is not clear that new approaches that emphasize future-oriented thinking only would be, or even should be, superior to standard CBT.

In terms of future directions, transdiagnostic psychotherapies for depression and anxiety ( Clark, 2009 ) could potentially be understood within and informed by the aspects of the ensemble hypothesis. The ensemble models shown in Figures 1 , ​ ,2 2 suggest that a unified approach to CBT plus mindfulness/attention training might well be plausible for treating both depressive and anxiety disorders. Finally, in recent years, network approaches to psychopathology have emphasized the interplay of symptoms across a variety of traditionally defined, yet comorbid, disorders ( Borsboom and Cramer, 2013 ). The psychopathology network approach contends that such emotional disorders arise from interactions among symptoms, as well as their reciprocally reinforcing relationships ( Borsboom, 2017 ). It may be possible to conceptualize these networks of psychopathology within the context of the ensemble hypothesis of human cognition considered here.

Data Availability Statement

Author contributions.

RK developed the concept of the paper. RK and CC wrote the first draft. JG contributed with advice and revisions to subsequent drafts. All authors reviewed the final manuscript.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Funding. The funds required for open access fees required to publish this article were provided through Faculty Development accounts available to the first and third authors from the Department of Psychology at Saint Louis University.

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Mental Time Travel: Episodic Memory and Our Knowledge of the Personal Past

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Matthew Frise, Mental Time Travel: Episodic Memory and Our Knowledge of the Personal Past, The Philosophical Quarterly , Volume 68, Issue 271, April 2018, Pages 414–416, https://doi.org/10.1093/pq/pqx010

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Episodic remembering returns us to events in our personal past. This crisp and impressive book addresses three main questions about it: What exactly is it? How does it enable human knowledge? Why did we evolve to have it? Chapter 1 previews the book's intertwined answers: episodic remembering is a reliable and evolutionarily adaptive kind of imagining.

The next two chapters stage-build. Chapter 2 argues that, given the human memory systems that science recognizes, memory is not a natural kind. So, naturalistic philosophy best approaches memory piecemeal. Michaelian focuses on episodic remembering in particular because, he claims, epistemologists have neglected it, yet it specially distinguishes the human mind.

Chapter 3 plots the book's naturalistic, pluralistic foray in applied epistemology. Michaelian aims to explain how episodic remembering is reliable, but reliability is one among many epistemically valuable properties. ‘Knowledge’ is just a term of approbation, flagging beliefs with many of these properties. Even to value reliability, however, is to court the generality problem. Any belief is formed by multiple process types, varying in reliability. How then are we to evaluate beliefs? Here Michaelian fires both the naturalist and the pluralist pistols. The sciences reveal which memory process types are natural. Naturalists will appeal only to those in evaluating beliefs, and pluralists will not privilege any one over the others. Consequently, belief has no unqualified epistemic status. We may evaluate belief only relative to an operative process and contextual interest. The process of interest in this book is ‘episodic remembering in cognitively normal adults’ (p. 50).

The next two chapters detail influential accounts of episodically remembering. Chapter 4 carefully explains the Common Sense Theory, which very roughly says that remembering is having, and having had , a decent representation of an experienced episode. Unfortunately, the Common Sense Theory counts some imagining as remembering—when, for example, your imagination now delivers by chance a representation of an experienced, previously represented, but forgotten episode.

Chapter 5 lays out the Causal Theory, designed to improve on the Common Sense Theory. The Causal Theory adds that remembering requires a suitable causal connection between your current and past representations, something imagination does not provide. The connection must be via a properly functioning memory system, thereby ruling out remembering from deviant causal chains. And the current representation mustn't be confabulatory, containing more information than the past representation. But even these additions disappoint. Michaelian argues at length that normal human memory is radically constructive. As a result, everyday remembering involves representations with content beyond that of past representations.

Chapter 6 is the heart of the book. It presents Michaelian's favoured theory of remembering, the Simulation Theory. Research suggests that our ability to remember episodically is due to a general episodic construction system responsible for mental time travel—remembering the past, imagining the future and imagining alternate pasts. Remembering is simulating , a kind of imagining that blends past experiences and semantic information, delivering a new representation of an episode. But not all imagining is remembering. A subject remembers an episode just in case she represents it as a result of a properly functioning episodic construction system aiming to produce a representation of an episode in the subject's personal past. The subject's original experience need not play any causal role.

Many streams feed into remembering. Even learning via testimony shapes the content of episodic remembering; what you have heard about an event can affect your memory of it. Chapter 7, as a case study, checks whether this testimonial stream epistemically corrupts remembering. Michaelian sees no evidence that it compromises remembering's reliability. And it is not blocking knowledge, yielding at best a kind of luckily true belief. Testifiers tend to tell the truth and hearers tend to believe it. So, accurate remembering that incorporates testimony is accurate in nearby worlds and therefore not relevantly lucky in ours.

Still, the streams feeding into episodic memory vary in quality. If remembering is to be reliable, agents shouldn't typically endorse recollected information that originates, for example, in mere episodic imagination. Yet memories generally don't explicitly tag the origins of their information. Chapter 8 explains how, fortunately, through metacognition we identify origins using ‘implicit marks’ as heuristics; memories originating in imagination tend to be informationally impoverished in certain regards. Subpersonally we infer origins accurately enough, generally preventing us from endorsing recollected episodes that we had merely imagined in the past.

Michaelian thinks the reliability of remembering is not yet shown, however. If a subject cannot typically distinguish current remembering from other potentially active episodic imagining processes, she will often form or withhold belief in the wrong cases. Michaelian proposes in Chapter 9 that distinct metacognitive feelings fallibly but reliably signpost different forms of current episodic imagining. The feelings arise due to unconscious inference from various heuristics Michaelian reviews at length, allowing us to fallibly distinguish episodic imagining processes and form belief just when appropriate.

Chapters 10 argues that episodic remembering should be characterized phenomenally, making information available in a special way that involves autonoesis (consciousness of the self in subjective time) and chronesthesia (consciousness of the subjective time in mental time travel). Episodic remembering appears uniquely human, and the complexity of our general episodic construction system suggests it is evolutionarily adaptive. But it is unclear what makes it adaptive. Chapter 11 rejects the available explanations. Michaelian then proposes that several aspects of the episodic memory system together ensure remembering's reliability despite its simulational nature, and reliable episodic remembering is adaptive.

Chapter 12 concludes with a refreshing flouting of sensationalism. Science impugns common sense thinking about how memory works. But common sense is nonetheless right to trust memory.

Unsurprisingly, philosophers who do not naturalize their epistemology or who shy from pluralism will not take Michaelian's exact route to his conclusions. Even so, this outstanding book is a new pillar in the philosophy of memory. Philosophers in and beyond graduate school will find that it helpfully organizes and integrates heaps of empirical findings into long-standing philosophical debates, debates Michaelian moves forward. Sceptics of the Simulation Theory will find Michaelian's case for it uncomfortably compelling. Michaelian makes clear that epistemology has, to its detriment, neglected metacognition in human memory.

Michaelian could slightly further align parts of the book. If ‘knowledge’ is just a term of approbation, then accounting for our episodic memory knowledge loses charm. Pluralism limits the urgency of showing that episodic remembering forms belief reliably; on pluralism, episodic remembering is just one among many unprivileged natural processes responsible for the formation of belief, and other responsible natural processes will be reliable. Also, it's unclear how confusing remembering with other potentially active imagining processes could threaten the reliability of remembering. If a subject mistakes an ongoing process of merely imagining for remembering, she might falsely believe what she is imagining. This tarnishes the track record of merely imagining, but not of remembering — by stipulation it is inoperative. If she mistakes remembering for merely imagining, she might needlessly withhold belief. That hurts remembering's power (tendency to produce many true beliefs) but not its reliability (tendency to produce true rather than false beliefs).

But these are just specks on a still clean achievement.

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The Science of Mental Time Travel and Why Our Ability to Imagine the Future Is Essential to Our Humanity

By maria popova.

Lewis Carroll’s Alice in Wonderland remains one of my all-time favorite books , largely because Carroll taps his training as a logician to imbue the whimsical story with an allegorical dimension that blends the poetic with the philosophical. To wit: The Red Queen remembers the future instead of the past — an absurd proposition so long as we think of time as linear and memory as beholden to the past, and yet a prescient one given how quantum physics (coincidentally, a perfect allegorical exploration of Wonderland ) conceives of time and what modern cognitive science tells us about how elastic our experience of time is . As it turns out, the Red Queen is far more representative of how human memory actually works than we dare believe.

“To be human,” writes Dan Falk in In Search of Time: The History, Physics, and Philosophy of Time ( public library ), “is to be aware of the passage of time; no concept lies closer to the core of our consciousness” — something evidenced by our millennia-old quest to map this invisible dimension . One of the most remarkable and evolutionarily essential elements of experiencing time through human consciousness is something psychologists and cognitive scientists call mental time travel — a potent bi-directional projection that combines episodic memory , which allows us to draw on our autobiographical experience and call up events, experiences, and emotions that occurred in the past, with the ability to imagine and anticipate future events. Falk puts it unambiguously:

Without it, there would be no planning, no building, no culture; without an imagined picture of the future, our civilization would not exist.

As it turns out, episodic memory — a term coined in the early 1970s by Canadian neuroscientist Endel Tulving, author of the seminal book Elements of Episodic Memory — is central to our capacity for mental time travel and, according to many scientists, fairly unique to humans. Unlike other facets of memory, such as the acquisition of new skills, which are rooted in the here-and-now, Falk points out that episodic memory allows us “to peer back across time, using our imagination to revisit just about any event that we choose.” This mental reliving of the past may be the root of some distinct human maladies — take the wistful reminiscence over a lost love, for instance — but it is also central to our evolutionary survival, allowing us to anticipate future outcomes based on past ones and thus to plan better and be more prepared for what tomorrow may bring. (The dark side of this evolutionarily beneficial faculty is that our over-planning often ends up shortchanging our happiness .)

And yet the benefits outweigh the costs, in evolutionary terms. Falk explains:

The capacity for mental time travel gave our ancestors an invaluable edge in the struggle for survival. They believe there is a profound link between remembering the past and imagining the future. The very act of remembering, they argue, gives one the “raw material” needed to construct plausible scenarios of future events and act accordingly. Mental time travel “provides increased behavioral flexibility to act in the present to increase future survival chances.” If this argument is correct, then mental time travel into the past — remembering — “is subsidiary to our ability to imagine future scenarios.” Tulving agrees: “What is the benefit of knowing what has happened in the past? Why do you care? The importance is that you’ve learned a lesson,” he says. “Perhaps the evolutionary advantage has to do with the future rather than the past.” Modern neuroscience appears to confirm that line of reasoning: as far as your brain is concerned, the act of remembering is indeed very similar to the act of imagining the future.

Though we might not be able to “remember” the future, as the Red Queen does, we do envision it in ways strikingly similar to how we picture events from the past — Falk notes that fMRI studies indicate we use similar regions in the brain’s frontal and temporal lobes when thinking about events in either direction of time. What’s more, psychologists have found that much like it’s harder for us to remember an event in the distant past than a recent one, it’s harder for us to imagine an event in the distant future than one expected to take place soon. This hints at the massively misguided way in which we think of and evaluate memory, which we falsely depict as a recording device , versus foresight. Falk writes:

When we imagine the future, we know what we picture is really just an educated guess; we may be right in the broad brushstrokes, but we are almost certainly wrong in the details. We hold memory to a higher standard. We feel — most of the time — that our memories are more than guesses, that they reflect what really happened. When confronted with a conflicting account of how last week’s party unfolded, we cling to our beliefs: He must be mistaken; I know what I saw.

Falk cites the Harvard psychologist Daniel Schacter:

[The brain is] a fundamentally prospective organ that is designed to use information from the past and the present to generate predictions about the future. Memory can be thought of as a tool used by the prospective brain to generate simulations of possible future events… We tend to think of memory as having primarily to do with the past… And maybe one reason we have it is so that we can have a warm feeling when we reminisce, and so on. But I think the thing that has been neglected is its role in allowing us to predict and simulate the future.

In order to mentally time-travel into the future, the brain has to accomplish a couple of things at once — we activate our “semantic memory,” which encompasses our basic knowledge of facts about the world and thus helps paint a backdrop for the imagined scene, and we call on our episodic memory, which pulls on our autobiographical library of remembered experiences to fill in specific details for this general scene. Curiously, episodic memory tends to be rather flawed but, according to two scientists Falk quotes, that’s okay since its core purpose is to provide “a more general toolbox that allowed us to escape from the present and develop foresight, and perhaps create a sense of personal identity.”

To be sure, just like elsewhere in cognitive science , human exceptionalism may be misplaced here — scientists have found that other species are also capable of varying degrees of mental time travel. Falk cites one of the most intriguing experiments, involving scrub jays. He writes:

Psychologist Nicola Clayton and her colleagues housed the birds on alternate days in two different compartments — one in which the jays always received “breakfast,” and one in which they did not. Then the birds were unexpectedly given extra food in the evening, at a location where they could access either compartment. The jays promptly cached their surplus — and they preferentially cached it in the “no breakfast” compartment. Because the birds were not hungry at the time of the caching, the researchers claim that the birds truly anticipated the hunger they would experience the next morning.

Still, the fact that humans are capable of remarkably elaborate and detailed mental time travel reveals something unique about our evolution and the development of such hallmarks of humanity as language and theory of mind. Falk writes:

In all likelihood, the capacity for mental time travel did not develop in isolation but rather alongside other crucial cognitive abilities. “To entertain a future event one needs some kind of imagination,” [the prominent psychologists Thomas] Suddendorf and [Michael] Corballis write, “some kind of representational space in our mind for the imaginary performance.” Language could also play an important role. Our language skills embrace mental time travel by the use of tenses and recursive thinking; when we say “A year from now, he will have retired,” we’re imagining a future time in which some event — which has not yet happened — will lie in the past… Mental time travel may have been “a pre-requisite to the evolution of language itself.” If mental time travel is indeed unique to humans, it may help us understand why complex language is also, apparently, unique.

In fact, the development of mental time travel may even be how the concept of time itself came into existence — according to Suddendorf and Corballis, our species emerged victorious in “an extraordinary evolutionary arms race” largely due to our growing capacity for foresight and sophisticated language, which not only gave us culture and “coordinated aggression” but also, for the first time in evolutionary history, enabled us to understand the concepts of “past” and “future.” The mental reconstruction of what has been and the imagining of what could be, they argue, created the concept of time and enabled us to understand the continuity between the past and the future. Falk, once again, puts it succinctly:

Mental time travel may indeed be the cognitive rudder that allows our brains to navigate the river of time.

In Search of Time is a fantastic read in its entirety, covering such facets of life’s most intricate dimension as how the calendar was born, why illusion and reality aren’t always so discernible from one another, and what the ultimate fate of the universe might be. Complement it with these seven excellent books on time and a fascinating read on how our memory works .

— Published July 1, 2014 — https://www.themarginalian.org/2014/07/01/mental-time-travel-dan-falk/ —

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REVIEW article

Wandering tales: evolutionary origins of mental time travel and language.

• Department of Psychology, University of Auckland, Auckland, New Zealand

A central component of mind wandering is mental time travel, the calling to mind of remembered past events and of imagined future ones. Mental time travel may also be critical to the evolution of language, which enables us to communicate about the non-present, sharing memories, plans, and ideas. Mental time travel is indexed in humans by hippocampal activity, and studies also suggest that the hippocampus in rats is active when the animals replay or pre play activity in a spatial environment, such as a maze. Mental time travel may have ancient origins, contrary to the view that it is unique to humans. Since mental time travel is also thought to underlie language, these findings suggest that language evolved gradually from pre-existing cognitive capacities, contrary to the view of Chomsky and others that language and symbolic thought emerged abruptly, in a single step, within the past 100,000 years.

Introduction

If people are left to think for themselves undisturbed, without focusing on the immediate environment or on a particular task, their minds wander. Brain-imaging studies show that mind-wandering activates a widespread network in the brain, first identified and described by Raichle et al. (2001) as the default mode network , in which the frontal and parietal lobes play a major role. Rather paradoxically, this network is revealed by reverse subtraction; that is, the activation during involvement in some designated task is subtracted from that under passive conditions in which subjects were given no explicit instructions, and were free to let their minds wander ( Buckner and Vincent, 2007 ). Indeed, blood flow to the brain under passive conditions is only about 5–10 percent lower than to the engaged brain, and covers wider regions of the brain. It has been estimated that people spend just under half their waking hours in mind wandering ( Killingsworth and Gilbert, 2010 ).

A critical component of mind wandering is memory, which provides the basic elements from which our mind wanderings are constructed. Memory itself can be divided into declarative memory, which can be made explicit or conscious, and non-declarative memory, which comprises the non-conscious products of learning, such as habits or learned skills like driving or playing the piano. Declarative memory, in turn, can be divided into episodic memory, which is personal memory for past episodes, and semantic memory, which is basic knowledge about the world ( Squire, 2004 ). According to Tulving (1972) , episodic memory is unique to humans.

Memory, both episodic and semantic ( Klein, 2013 ), provides the ingredients for imagining possible future events. What has been termed episodic foresight ( Suddendorf, 2010 ), along with autobiographic memory and theory of mind, also makes up much of our mind wandering ( Spreng and Grady, 2009 ), as we preview some future activity or consider possible future options in order to select appropriate action. The capacity to mentally relive past events and imagine possible future ones comprises has been termed mental time travel ( Suddendorf and Corballis, 1997 , 2007 ), taking us into an imagined future as well as into an imagined past. Both are essentially constructive processes. Brain imaging shows considerable overlap in brain activation between the two, with slightly more frontal-lobe activity in imagining the future (e.g., Addis et al., 2007 ). Critical to both is the hippocampus, whose role is discussed in more detail below.

Is Mental Time Travel Unique to Humans?

He said “What's time? Now is for dogs and apes! Man has Forever!”

—Robert Browning, A grammarian's funeral

Extending Tulving's conjecture, Suddendorf and Corballis (1997 , 2007) suggested that mental time travel, like episodic memory, is uniquely human. This suggestion, though, has proven contentious. A serious challenge has come from studies of a number of non-human species, including birds. For instance, scrub jays can recover cached food on the basis not only of where it was cached, but also of when it was cached, which might be taken to imply episodic memory of the caching episode itself (e.g., Clayton et al., 2003 ). Jays also appear to select food to cache based not on present hunger, but on the basis of what they expect to have access to on the following day ( Correia et al., 2007 ). Chimpanzees have been shown to select tools for future use ( Osvath and Osvath, 2008 ) or to collect and conceal stones to be later thrown at visitors to the zoo ( Osvath, 2011 ). In these and other studies there are methodological issues, and questions as to whether the results can be interpreted in terms of associative learning rather than the imagining of past or future events (see Suddendorf and Corballis, 2007 for a critique).

One problem in documenting mental time travel in non-human species is their lack of language. In humans, we have immediate evidence for both episodic memory and future thinking by simply asking for verbal report. Indeed, language itself may have evolved precisely to allow communication about the non-present ( Corballis, 2009 ; Gärdenfors and Osvath, 2010 ), so we can share our mental travels to other places and other times. The absence of articulate language in non-human species may therefore be considered evidence of incapacity for mental time travel itself. Recent evidence from neurophysiology, though, suggests that non-human animals may indeed have the capacity for at least limited mental time travel, even though they do not have the means to communicate it. A default mode network homologous to that in humans has been identified in the monkey ( Vincent et al., 2007 ), and does suggest a basis for mind wandering, if not for mental time travel itself. More critical, though, may be the hippocampus, which performs two important roles in mammals, as well as in birds.

First, the hippocampus contains so-called “place cells” that encode where an animal is located in space, and so constitute what O'Keefe and Nadel (1978) called a “cognitive map.” This role appears to apply to humans as well as to other mammalian species ( Maguire et al., 1998 ). For example, London taxi drivers, who are required to memorize the streets of London in sufficient detail to navigate without referring to a map or GPS, have enlarged hippocampi relative to controls—although not all trainees manage to finish the course and these show no structural change ( Woollett and Maguire, 2011 ). The taxi drivers also have larger hippocampi than do London bus drivers, who drive on designated routes that impose relatively small demands on memory ( Maguire et al., 2006 ). Similarly, birds that cache items of food in multiple locations, and later retrieve them, have larger hippocampi than birds that do not cache ( Macphail, 2002 ).

Second, the hippocampus appears to be critically involved in declarative memory systems and, in humans at least, in mental time travel generally. Loss of hippocampal function in humans results in severe amnesia, including an apparent inability to imagine possible future events as well as failure to recall past ones ( Hassabis et al., 2007a , b ; Andelman et al., 2010 ; Race et al., 2011 ). Conversely, the hippocampus is activated in neurologically intact individuals when they bring to mind past episodes and imagine possible ones As suggested earlier, the hippocampus appears to be the hub of the system, drawing detailed information from other regions of the brain, including the default-mode network ( Addis et al., 2007 ), for the reconstruction of past or future events. There is some differentiation along the long axis of the hippocampus, with the posterior hippocampus more involved in storage and the retrieval of past episodes and the anterior hippocampus more activated by the imagining of future ones ( Szpunar et al., 2007 ; Martin et al., 2011 ).

Micro-electrode recordings suggest that the hippocampus may play a similar role in rats. Place cells in the rat hippocampus, which encode specific locations in a structured environment, such as a maze, also fire when the animal is outside that environment, sometimes when the animal is asleep ( Wilson and McNaughton, 1994 ) and sometimes when it is awake but immobile ( Karlsson and Frank, 2009 ). Recordings show that this firing occurs in what have been termed sharp-wave ripples , sweeping out trajectories corresponding to earlier locations in the environment. These ripples are accompanied by widespread activation in the cerebral cortex, along with inhibition of activity in the diencephalon, limbic system, and brain stem, suggesting an interaction between hippocampus and cortex in the consolidation of acquired awake experience ( Logothetis et al., 2012 ). It might also be interpreted as representing the experiencing of trajectories, either previously experienced or planned ( Corballis, 2013 )—in other words, mental time travel.

This is further suggested by evidence that the trajectories need not correspond to actual trajectories that the animal took while it was in the environment. Sometimes they correspond to a previously taken path in a maze, but sometimes to the reverse of such paths, or even to paths through regions the rat did not actually visit ( Gupta et al., 2010 ). This might be taken as evidence for mental time travel along not only past trajectories, but also along imagined future ones. More direct evidence that hippocampal activity signals future behavior comes from rats trained to alternate left and right turns at a particular location in a maze. Between trials, they were introduced to a running wheel, and while they were running differential activity in the hippocampus signaled which turn they would take next. Based on this and other findings, the authors concluded that self-organized activity in the hippocampus, “having evolved for the computation of distances, can also support the episodic recall of events and the planning of action sequences and goals” ( Pastalkova et al., 2008 , p. 1327).

A similar conclusion is suggested by a more recent study in which rats were given experience with 36 locations in an open-field environment, and learned that a particular goal location contained a reward. When located in randomly chosen locations, the rats were able to determine routes leading back to the goal, even though these routes had not been previously traversed. Sharp-wave ripples pre-played these routes prior to the animal actually setting out ( Pfeiffer and Foster, 2013 ). The authors suggest that the hippocampus “function in multiple conceptual contexts: as a cognitive map in which routes to goals might be explored flexibly before behavior, as an episodic memory system engaging in what has been termed ‘mental time travel’ … ” (p. 5).

The trajectories implied by the hippocampal sharp-wave ripples are much more rapid than those actually taken by the animal. Diba and Buzsáki (2007) recorded hippocampal firing while rats ran back and forth along a straight track. Before each run, the ripples indicated a forward “preplay” of the next run, and after each run a second bout of ripples indicated a “replay” of the run in reverse order. These events were an order of magnitude faster than the sequence recorded during the run itself. While this may suggest that the ripples are not evidence of mental time travel, I suspect our own mental time travels are also speeded up. It takes me an hour to walk from my home to where I work, but mentally it takes less than a minute. Diba and Buzsáki suggest that “preplay events may have a role in ‘planning’ upcoming trajectories” (p. 1242).

These findings, together with the role of the hippocampus is human mental time travel, suggest a strong thread of continuity between rat and human. Indeed, mental time travel in the sense of imagined journeys through space may well have evolved very early as a consequence of movement, and the need to be aware of location and to remember and plan movement through space. It is understandable, too, that the hippocampus should play a role in time as well as in space, since movement in time necessarily involved trajectories in space. That is, the hippocampus operates in 4D space-time. Mental travel, moreover, has one property denied actual travel, in that it can reverse time. We can mentally relive the past, and also imagine events in the reverse order of their actual occurrence, and it seems that rats can do so too.

Nevertheless, there can be little doubt that mental time travel in humans is more complex than that in the rat. Darwin (1871) famously wrote that “The difference in mind between man and the higher animals, great as it is, certainly is one of degree and not of kind” (p. 126). Our own recollections of past events and imaginings of future ones are populated by more than just locations. We remember individual people, actions, objects, emotions, and so forth, and these are present in different combinations in different episodes. Of course we do not yet know whether ripples in the rat hippocampus can signal more than location, and in future research it might be useful to add features or other distinctive characters to the environments in which animals are located, and seek markers in later hippocampal recordings. But for the time being, it seems reasonable to suppose that our imaginings of past and future carry a complexity far greater than that experienced by other species.

As suggested earlier, language may be considered to have evolved so that we can share our mental time travels, and indeed any experience or knowledge not tied to the immediate environment. If mental time travel is indeed unique to our species, this might well explain why language itself is also confined to Homo sapiens . But if the origins of mental time travel reach far back in mammalian evolution, and perhaps even to our joint ancestry with birds, then language itself may be considered to have precursors that long preceded the emergence of our species. This notion, though, is sharply contradicted by a contemporary view that language and the thought processes underlying it emerged de novo well within the time span of Homo sapiens .

Chomsky (2010) , for instance, argues that language evolved in a single step, perhaps as a mutation in a single individual, within the past 100,000 years, long after the emergence of Homo sapiens some 200,000 years ago in Africa. He writes

Within some small group from which we are all descended, a rewiring of the brain took place in some individual, call him Prometheus , yielding the operation of unbounded Merge, applying to concepts with intricate (and little understood) properties ( Chomsky, 2010 , p. 59).

Chomsky is also clear that the outcome of this rewiring was a new mode of thought, called internal language (I-language), which was not primarily concerned with communication itself. The mapping of I-language onto external language (E-language) was in effect a secondary outcome. The 7000 or so languages of the present-day world are then considered to have their basis in a shared I-language.

The paleoanthropologist Ian Tattersall reaches a similar conclusion as to the abruptness with which language and symbolic thought emerged:

Our ancestors made an almost unimaginable transition from a non-symbolic, nonlinguistic way of processing information and communicating information about the world to the symbolic and linguistic condition we enjoy today. It is a qualitative leap in cognitive state unparalleled in history. Indeed, as I've said, the only reason we have for believing that such a leap could ever have been made, is that it was made. And it seems to have been made well after the acquisition by our species of its distinctive modern form ( Tattersall, 2012 , p. 199).

Such views are profoundly at odds with Darwin's theory of evolution by natural selection, and smack of the miraculous. According to Pinker and Bloom (1990) , evolution proceeds in small increments rather than in a single “unimaginable” leap. Indeed Darwin himself wrote:

If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down. But I can find no such case ( Darwin, 1859 , p. 158).

Chomsky (e.g., 2011 , p. 6) has frequently referred to language as “an organ of the body,” so language might indeed be the case that Darwin feared.

The idea that mental time travel has more ancient roots raises the possibility of a more gradual scenario, and one more consistent with Darwinian theory. Mental time travel itself may well have undergone progressive refinement and extension before reaching a level that might support language. Gärdenfors and Osvath (2010) suggest that the critical period was the Oldowan, dating from some 2.6 to about 1.6 million years ago ( Plummer, 2004 ), and defined by the emergence of stone tools. They describe the Oldowan as a “long ranging culture,” characterized by an extension in time and space. The Oldowan hominins ranged over large distances to gain raw materials or to scavenge or slaughter for food, and long time intervals intervened between the manufacture and use of tools. Gärdenfors and Osvath suggest that this heightened the reliance on prospective cognition, which they consider the basis for the subsequent emergence of symbolic communication. The emergence of tools may have added complexity to the activities of these early hominins, and indeed to their mental time travels, creating further selective pressure toward more effective communication.

Perhaps more critical than mental time travel per se , though, were the adaptive advantages to be gained by sharing memories and plans with others. Mental time travel, including memory and prospective cognition, has probably long served to the benefit of the individual, but the ability to share has vast potential to enhance experience and increase survival, at both individual and societal levels. The Pleistocene, the epoch that began with the Oldowan, is widely recognized as the period in which hominins came to occupy what has been termed the “cognitive niche” ( Tooby and DeVore, 1987 ), depending on social bonding and enhanced communication for survival in the more exposed and dangerous environment of the African savanna. Social sharing seems to be ingrained in humans in a manner not evident in our closest non-human relatives. Tomasello (2008) notes, for example, that infants point to interesting objects in their environments, not to request them, but to share the experience with those around them. This may be a precursor to language, in phylogeny as well as ontogeny. Chimpanzees, in contrast, rarely point, and when they do the aim is usually to request something out of their reach.

Underpinning social cognition is theory of mind, the capacity to understand what others think or believe. Some 35 years ago Premack and Woodruff (1978) raised the question, “Does the chimpanzee have a theory of mind?” They were themselves equivocal as to the answer, and their question has led to a long and at times bitter controversy. In a review, Call and Tomasello (2008) conclude that the years of subsequent research have shown chimpanzees to have some understanding of the goals, intentions, perceptions, and knowledge of others, but no understanding of the beliefs and desires of others. True theory of mind, then appears to be limited to humans, at least among extant species, and may well have emerged as a critical aspect of what has also been called the “social mind” as it evolved during the Pleistocene (e.g., Forgas et al., 2007 ).

The incorporation of theory of mind adds a further dimension to mind wandering; as Buckner et al. (2008) put it, “the default network is active when individuals are engaged in internally focused tasks including autobiographical memory retrieval, envisioning the future, and conceiving the perspectives of others” (p. 1). That is, we can wander mentally not only into past and future, but also into the minds of others. This is well illustrated by the human predilection for story-telling, whether through gossip, fiction, or TV soaps.

Indeed, theory of mind can be regarded as a prerequisite for language itself. Grice (1975) pointed out that language depends on inference rather than explicit decoding. In this respect it contrasts with animal communication, which is generally unambiguous, whereas human language, despite its apparent richness, is characteristically ambiguous and imprecise. In order to converse, individuals must understand what is going on in each other's minds, so that each can infer what the other means. As an example of the ambiguity of language, Sperber and Origgi (2010) give the sentence “It was too slow.” This could mean anything from a chemical reaction being too slow, to the decrease in unemployment in France being too slow, to a car being too slow for an anticipated journey—or a sluggish movement in a symphonic production. In uttering such a sentence, the speaker knows what is in the listener's mind, and has no need to elaborate further. She also knows that the listener knows what's in her mind. In this sense, conversational language, at least, serves as a series of prompts to guide shared thought.

Disambiguation also involves projection into the future. Through theory of mind, listeners can create an emulation of what a speaker has just said, and use this to predict upcoming words, meanings, and even grammatical categories. This can not only disambiguate upcoming utterances, but also facilitate rapid comprehension and help the listener deal with noisy input ( Pickering and Garrod, 2007 ). In these respects, then, language draws on both theory of mind and mental time travel.

Contrary to the Chomskyan view of language, another ingredient of language that may go well back in primate and even mammalian evolution is symbolic understanding. Great apes and even dogs are easily taught to understand symbols in terms of what they represent. The bonobo Kanzi communicates by pointing at nonrepresentational symbols on a keyboard, and can even obey simple requests conveyed through spoken English ( Savage-Rumbaugh et al., 1998 ). The gorilla Koko is said to use over 1000 signs and to understand and express signed requests, and he too can respond meaningfully to simple requests spoken in English ( Patterson and Gordon, 2001 ). A border collie called Rico has been shown to rapidly acquire the meanings of some 200 spoken English words ( Kaminsky et al., 2004 ). Rico has since been trumped by another border collie called Chaser, who understands over 1000 proper names as verbal referents ( Pilley and Reid, 2011 ). These accomplishments might also be taken to reflect mental time travel, since they often involve reference to non-present objects or actions. For instance, Kanzi might point to a symbol to request a banana, or ask to be tickled, or invite play, and Rico and Chaser demonstrate their linguistic skills by going on request into another room to fetch a designated object.

Some birds, too, may have the capacity to understand symbols. For instance Alex, a gray parrot, evidently understood number symbols as abstract representation of assemblages of real-world objects in much the same way as apes and small children do. According to Pepperberg (2013) , moreover, he learned them more in a human-like than an ape-like fashion. Unlike apes and dogs Alex, like other parrots, was able to produce reasonable simulations of human speech.

Gestural Origins, and the Switch to Speech

The main impediment to language-like communication in apes and dogs is a deficiency not so much in symbolic understanding as in the means to produce symbols intentionally. Kanzi needs an artificially contrived keyboard to communicate his requests, and Rico and Chaser can understand words but have no means of producing them, or of acquiring surrogates such as signed gestures. Non-human species of course do communicate through calls and cries, but these are for the most part outside of intentional control. Even chimpanzees, according to Premack (2007) , “lack voluntary control of their voice” (p. 13866). The more likely option for intentional communication in our primate precursors lay in the hands. Premack goes on to write that chimpanzees “could not have speech. But sign language is a possibility, for they do have voluntary control of their hands” (p. 13866). As the examples of Kanzi and Koko illustrate, apes can learn to communicate through gestures, whether based on sign language or on pointing to visual symbols, but their production of symbols is far less proficient than their ability to understand them—as indeed it is also in human infants.

The idea that language evolved from manual gestures has a long history, going back at least to Condillac (1746/1971) in the 18th century, and restated in modern form by Hewes (1973) . The gestural theory was boosted with the discovery in monkeys of mirror neurons, so called because they respond both when the monkey makes a grasping movement and when it observed the same movement performed by another individual ( Rizzolatti et al., 1988 ). Mirror neurons are now considered part of a more extensive mirror system, involving regions in the ventral prefrontal cortex, parietal cortex, and superior temporal sulcus ( Rizzolatti and Sinigaglia, 2010 ), and in fact overlapping extensively with the default network. The idea that mirror neurons may underlie the evolution of language has been elaborated by a number of authors (e.g., Corballis, 2002 ; Arbib, 2005 ; Rizzolatti and Sinigaglia, 2008 ). The gestural theory was also boosted by the realization that the signed languages of the deaf are true languages, with full syntactic and semantic properties, albeit based entirely on visible movements of the hands and face ( Armstrong et al., 1995 ; Armstrong, 1999 ).

The question then is why authors like Chomsky and Tattersall are so insistent that language emerged as a single package within the past 100,000 years. Their reasoning appears to be based at least in part on archeological evidence for what had been termed a “cultural revolution” within the past 100,000 years, characterized by the seemingly abrupt appearance of bodily ornamentation derived from shells, beads, or animal teeth, of sophisticated cave art, and improved technology in tool making. Summarizing this evidence, Mellars (2005) writes:

To describe the Upper Paleolithic revolution in Europe as reflecting preeminently an explosion in explicitly symbolic behavior and expression is in no sense an exaggeration, as most prehistorians would now agree. We are probably on safe ground in assuming that symbolic behavior and expression of this level of complexity would be inconceivable in the absence of highly structured language systems and brains closely similar, if not identical to, our own (p. 12).

Nevertheless, not all prehistorians are in agreement. McBrearty and Brooks (2000) write of the “revolution that wasn't,” suggesting a more gradual rise in technological sophistication from the Middle Stone Age around 250,000–300,000 years ago, and Shea (2011) similarly argues that human technology over the past 200,000 years is characterized by a variability that persists today, rather than by the abrupt appearance of “modern behavior.” Given that our species is estimated to have emerged some 200,000 years ago, it seems unlikely that there was a dramatic rewiring of the brain within the past 100,000 years.

One possibility is that any change in behavioral patterns in our species was the outcome, not of a rewiring of the brain, nor of the “unimaginable transition” declared by Tattersall (2012) , but was the outcome of a change in the manner of communication. I suggested above that language may have originated in manual gestures, and of course it persists in this form in the signed languages of the deaf. If this scenario is correct, then, it must have switched to the vocal form that we call speech at some point. Some have argued against the gestural theory on the grounds that it must have required an unlikely transition from a visuo-manual format to an auditory-vocal one (e.g., Burling, 2005 ; MacNeilage, 2012 ). However, in my view the transition is better viewed not as a switch of modalities, but rather as a switch in gestural format. In a recent analysis of the neural mechanisms of speech articulation, Bouchard et al. (2011) conclude that their findings “support gestural theories of speech control over alternative acoustic … or vocal-tract geometry theories” (p. 331).

The switch from manual to voiced language was probably gradual, with facial gestures playing an intermediary role. Signed languages include silent movements of the face as well as of the hands. Facial expressions and head movements can turn an affirmative sentence into a negation, or a question. Mouth gestures are especially important, and have been linked to the equivalent of phonology, especially in European signed languages. Explicit schemes for the phonological composition of mouth movements have been proposed for a number of European Sign languages, including Swedish, English, and Italian ( Sutton-Spence and Boyes-Braem, 2001 ). Mouth gestures can serve to disambiguate hand gestures, and as part of more general facial gestures provide the equivalent of prosody in speech ( Emmorey, 2002 ). Gestures of the face and head also accompany normal speech, including raised eyebrows, winking, down-turning the mouth, tilting or shaking the head.

Speech itself can be regarded as a gestural system, comprising movements of the lips, the larynx, the velum, and the blade, body, and root of the tongue ( Studdert-Kennedy, 2005 ). In the course of evolution, then, intentional communication may have evolved from manual gestures, to overt facial gestures, and finally to the largely hidden gestures that comprise speech—although all three forms of gesture remain present in conversation. Speech gestures, although largely contained within the mouth, retain a visible component, as illustrated by the McGurk effect: A syllable (such as da ) is dubbed onto a mouth saying another syllable (such as ba ), and people tend to “hear” what they see rather than what was actually voiced ( McGurk and MacDonald, 1976 ). Other studies show the parts of the brain involved in producing speech are activated when people simply watch silent videos of people speaking ( Calvert and Campbell, 2003 ; Watkins et al., 2003 ). Rhesus monkeys also show dynamic interactions between perceptions of face movements and voicing, mediated by connections between the superior temporal sulcus and auditory cortex ( Ghazanfar et al., 2008 ).

The transition was probably not a dramatic one, since movements of the hand and mouth are coordinated in activities such as eating, and hand movements and mouth movements mutually interact ( Bernardis and Gentilucci, 2006 ; Gentilucci and Corballis, 2006 ). The introduction of voicing to the gestural repertoire probably also involved modification of neural mechanisms, including a direct connection between the motor cortex and the nucleus ambiguus (a midbrain vocalization center) that seems to be unique to humans ( Jürgens, 2002 ), and that may explain why vocalization is under precise voluntary control in humans but not in chimpanzees. The continuing link between speech and gesture is further illustrated by the fact that people habitually gesture with their hands as they speak. Moreover gestures are in strict synchrony with speaking, implying a common underlying source ( McNeill, 1985 ).

The switch from manual gesture to speech may well have been the change that led to the dominance of Homo sapiens , perhaps even leading to the demise of the other large-brained hominins, including the Neanderthals and the recently identified Denisovans. Although we habitually gesture manually while speaking, manual movements can be disengaged, and play no role in communicating by phone or on radio. Speech, then, may be regarded as an early example of miniaturization , tucking language output neatly into the mouth. This resulted in increased energy efficiency. Manual language is effortful, requiring considerable expenditure of energy, while the physiological costs of speech are so low as to be nearly unmeasurable ( Russell et al., 1998 ). Speech adds little to the cost of breathing, which we must do anyway to sustain life. Speech also allows communication at night, or when speaker and audience are not in visual contact. More importantly, perhaps, the transition to speech freed the rest of the body for other activities, including the use and manufacture of tools. This compartmentalization and increase in communicative efficiency may well explain the survival and dominance of our species, whether it occurred as part of the cultural revolution or as a more gradual change over the past 200,000 years.

The emergence of speech as the dominant mode may well have enhanced story-telling, and the sharing of cultural myths and legends that do much to bind societies together. Boyd (2009) points out that religious ideas derive their power less from doctrine than from stories, and stories told orally were passed down the generations with remarkable fidelity before the invention of writing. But the invention of writing has also had a profound effect on human culture, as have more recent inventions such as the Internet and cellphone. The story of human progress may well be in large part the story of advances in communication, and the switch from gestural to vocal communication was an early example. The switch, moreover, may have been more a blend from one to the other, and is still arguably incomplete—especially in Italy.

Conclusions

It is widely held that humans evolved a distinctive mode of thinking, which included language, in a single step within the past 100,000 years. One important aspect of language is that it permits intentional communication about the non-present, allowing people to share their mental time travels—their experiences, plans, and ideas. It has also been argued that the capacity for mental time travel itself is uniquely human, perhaps evolving in concert with language.

Neurophysiological recordings from rat hippocampus raise the possibility that mental time travel may have ancient origins. This casts a different perspective on the nature of cognitive evolution, and suggests an incremental approach more consistent with Darwinian theory. The cognitive underpinnings of language include not only mental time travel, but also theory of mind and the capacity to attach symbols to real-world entities. The evolution of productive language also required an intentional system with sufficient flexibility to produce the requisite variety of outputs to serve as meaningful symbols. In our primate predecessors, such a system was more likely to have been found in the hands rather than the voice, supporting the idea that language evolved from manual gestures.

The critical period for the evolution of language, then, was likely to have been the Pleistocene, when the transition from a forested environment to the more open savanna placed a survival premium on social bonding and the sharing of experiences. The capacity for mental time travel may have been extended to enable longer-term plans and deeper access to the past. As evidence for cognitive enhancement, brain size approximately tripled during the Pleistocene ( Wood and Collard, 1999 ). These developments would have underpinned more effective communication, with the emergence of obligate bipedalism adding to the communicative power of gesture, initially by freeing the hands but also exposing the rest of the body, including the face, as communicative systems.

The emergence of Homo sapiens from around 200,000 years ago seems to have marked further cognitive advances. Our species radiated out of Africa to eventually populate most of the globe, while other equally large-brained hominins were driven to extinction. But rather than suppose that this was the result of some unexplained event, perhaps a mutation, a more parsimonious possibility, and one more consistent with Darwinian theory, is that the emergence of Homo sapiens saw a gradual shift from a predominantly manual language to a predominantly vocal one. It may have been this shift that freed the hands for the remarkable and ever increasing technological advances since the dawn of our species ( Corballis, 2004 ). Changes in the mode of communication can have profound effects, the most recent example being the invention of the Internet, and as communication and ensuing technology grow more complex they progress in ratchet like fashion, with each advance building on the previous ones.

Language is characterized by what Chomsky (e.g., 2011 ) has termed “discrete infinity,” the construction of potentially unlimited meanings from finite elements. In his view, this capacity arose in the singular event that created I-language and universal grammar. The alternative is that the generative aspect of language is provided by mind wandering, a capacity that may have ancient origins. The rat that envisages a past or future trajectory may not have infinite options, but does seem able to imagine trajectories not actually experienced. Mental life no doubt gathered more furniture in the course of evolution, as life itself imposed more challenges. The pace changed with the emergence of the bipedal hominins who were our forebears, as they adapted to new habitats, began to develop tools, and formed more complex social structures. Our mental travels are populated for the most part by familiar elements, such as people, things, places, and actions, which combine in different ways to make up our memories, plans, and fantasies. The combinations are more or less unlimited. With this elaboration, it would have been adaptive to share, so that mental as well as physical resources could be distributed, and our hominin forebears could act and plan in groups. Language itself, then, probably did emerge fairly late in hominin evolution. The best guess, I think, is that language, along with other aspects of social life, emerged in the Pleistocene, and not as a sudden cataclysmic event within the time period of our own species.

Conflict of Interest Statement

The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Keywords: default mode network, evolution, gesture, hippocampus, language, mental time travel, mind wandering, Pleistocene

Citation: Corballis MC (2013) Wandering tales: evolutionary origins of mental time travel and language. Front. Psychol . 4 :485. doi: 10.3389/fpsyg.2013.00485

Received: 30 April 2013; Accepted: 11 July 2013; Published online: 29 July 2013.

Reviewed by:

Copyright © 2013 Corballis. This is an open-access article distributed under the terms of the Creative Commons Attribution License , which permits use, distribution and reproduction in other forums, provided the original authors and source are credited and subject to any copyright notices concerning any third-party graphics etc.

*Correspondence: Michael C. Corballis, Department of Psychology, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand e-mail: [email protected]

Disclaimer: All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.

Involuntary (spontaneous) mental time travel into the past and future

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• 1 Department of Psychology, University of Aarhus, Nobelparken, Jens Chr. Skousvej 4, 8000 Aarhus C., Denmark. [email protected]
• PMID: 18424178
• DOI: 10.1016/j.concog.2008.03.001

Mental time travel (MTT) is the ability to mentally project oneself backward in time to relive past experiences and forward in time to pre-live possible future experiences. Previous work has focused on MTT in its voluntary (controlled) form. Here, we introduce the notion of involuntary (spontaneous) MTT. We examined involuntary versus voluntary and past versus future MTT in a diary study. We found that involuntary future event representations-defined as representations of possible personal future events that come to mind with no preceding search attempts-were as common as involuntary autobiographical memories and similar to them regarding cuing and subjective qualities. Future MTT involved more positive and idyllic representations than past MTT. MTT into the distant future/past involved more representations of cultural life script events than MTT into the immediate past/future. The findings are discussed in relation to cultural learning and MTT considered as a higher mental process.

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Paradox-Free Time Travel Is Theoretically Possible, Researchers Say

Matthew S. Schwartz

A dog dressed as Marty McFly from Back to the Future attends the Tompkins Square Halloween Dog Parade in 2015. New research says time travel might be possible without the problems McFly encountered. Timothy A. Clary/AFP via Getty Images hide caption

A dog dressed as Marty McFly from Back to the Future attends the Tompkins Square Halloween Dog Parade in 2015. New research says time travel might be possible without the problems McFly encountered.

"The past is obdurate," Stephen King wrote in his book about a man who goes back in time to prevent the Kennedy assassination. "It doesn't want to be changed."

Turns out, King might have been on to something.

Countless science fiction tales have explored the paradox of what would happen if you went back in time and did something in the past that endangered the future. Perhaps one of the most famous pop culture examples is in Back to the Future , when Marty McFly goes back in time and accidentally stops his parents from meeting, putting his own existence in jeopardy.

But maybe McFly wasn't in much danger after all. According a new paper from researchers at the University of Queensland, even if time travel were possible, the paradox couldn't actually exist.

Researchers ran the numbers and determined that even if you made a change in the past, the timeline would essentially self-correct, ensuring that whatever happened to send you back in time would still happen.

"Say you traveled in time in an attempt to stop COVID-19's patient zero from being exposed to the virus," University of Queensland scientist Fabio Costa told the university's news service .

"However, if you stopped that individual from becoming infected, that would eliminate the motivation for you to go back and stop the pandemic in the first place," said Costa, who co-authored the paper with honors undergraduate student Germain Tobar.

"This is a paradox — an inconsistency that often leads people to think that time travel cannot occur in our universe."

A variation is known as the "grandfather paradox" — in which a time traveler kills their own grandfather, in the process preventing the time traveler's birth.

The logical paradox has given researchers a headache, in part because according to Einstein's theory of general relativity, "closed timelike curves" are possible, theoretically allowing an observer to travel back in time and interact with their past self — potentially endangering their own existence.

But these researchers say that such a paradox wouldn't necessarily exist, because events would adjust themselves.

Take the coronavirus patient zero example. "You might try and stop patient zero from becoming infected, but in doing so, you would catch the virus and become patient zero, or someone else would," Tobar told the university's news service.

In other words, a time traveler could make changes, but the original outcome would still find a way to happen — maybe not the same way it happened in the first timeline but close enough so that the time traveler would still exist and would still be motivated to go back in time.

"No matter what you did, the salient events would just recalibrate around you," Tobar said.

The paper, "Reversible dynamics with closed time-like curves and freedom of choice," was published last week in the peer-reviewed journal Classical and Quantum Gravity . The findings seem consistent with another time travel study published this summer in the peer-reviewed journal Physical Review Letters. That study found that changes made in the past won't drastically alter the future.

Bestselling science fiction author Blake Crouch, who has written extensively about time travel, said the new study seems to support what certain time travel tropes have posited all along.

"The universe is deterministic and attempts to alter Past Event X are destined to be the forces which bring Past Event X into being," Crouch told NPR via email. "So the future can affect the past. Or maybe time is just an illusion. But I guess it's cool that the math checks out."

• time travel
• grandfather paradox

‘Mental Time Travel’: Remembering the Past, Imagining the Future, and the Particularity of Events

• Published: 26 April 2014
• Volume 5 , pages 333–350, ( 2014 )

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• Dorothea Debus 1  

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The present paper offers a philosophical discussion of phenomena which in the empirical literature have recently been subsumed under the concept of ‘mental time travel’. More precisely, the paper considers differences and similarities between two cases of ‘mental time travel’, recollective memories (‘R-memories’) of past events on the one hand, and sensory imaginations (‘S-imaginations’) of future events on the other. It develops and defends the claim that, because a subject who R-remembers a past event is experientially aware of a past particular event, while a subject who S-imagines a future event could not possibly be experientially aware of a future particular event, R-memories of past events and S-imaginations of future events are ultimately mental occurrences of two different kinds.

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Self-Referential Memory and Mental Time Travel

Mental Time Travel

Hidden Duration: Time-Lag in the World and Mind

Suddendorf and Corballis ( 2007 ) 299. Published in the same year, Schacter et al. ( 2007 ) offer another very useful review of research on ‘mental time travel’ up to 2007.

For recent empirical work on relevant neurophysiological issues see e.g. Addis et al. ( 2007 ), Schacter et al. ( 2008 ), and Viard et al. ( 2011 ).

For recent work on relevant developmental themes see e.g. Perner et al. ( 2010 ).

For example, it has been suggested that people who suffer from amnesia—that is, people whose ‘mental time travel’ into the past is impaired—also have serious difficulties with engaging in ‘mental time travel’ in the other direction, i.e., with imagining the future (see e.g. Klein and Loftus ( 2002 )). Similarly, people who suffer from depression have a tendency to remember the past in over-general ways, and it has been suggested that they might be imagining the future in over-general ways also (see e.g. Williams et al. ( 1996 )).

See e.g. Suddendorf and Corballis ( 1997 ) and Suddendorf and Corballis ( 2007 ) who argue that ‘mental time travel’ is unique to humans; however, Corballis ( 2013a ) now argues for the opposing view, while Suddendorf ( 2013 ), in a reply to Corballis ( 2013a ), continues to defend their earlier joint position and insists that ‘on current evidence, it still appears that human mental time travel is profoundly special’ (151). Corballis ( 2013b ) responds in turn, concluding that ‘from a Darwinian perspective, it might sometimes be more prudent to assume differences in degree rather than kind as the default position, and seek evidence that this is not the case’ (152). The debate is clearly ongoing.

Matthen ( 2010 ) 8f.

Byrne ( 2010 ) 25.

Martin ( 2002 ) 403.

As far as recollective memories of past events and sensory imaginations of future events are concerned, one might try to capture this similarity with the help of the suggestion that both recollective memories of past events and sensory imaginations of future events ‘concern the self’s experiences in subjective space and time’ (Tulving ( 1993 ) 67). Tulving refers to mental states which display this characteristic as cases of ‘autonoetic consciousness’, and one might accordingly hold that both recollective memories of past events and sensory imaginations of future events are, when considered in isolation and from the experiencing subject’s own point of view, similar because they are both cases of ‘autonoetic consciousness’. For example, some such suggestion has recently been made by Szpunar ( 2010 ). (Thanks to the editor for suggesting that a reference to Tulving’s and Szpunar’s work might be relevant in the present context.)

For relevant empirical work see e.g. Addis et al. ( 2007 ).

Without awaiting any such further discussion, an opponent might object immediately that the Main Argument could not possibly be defended successfully because neither the Memory-Claim nor the Imagination-Claim could possibly be true. As far as the Memory-Claim is concerned, the opponent might suggest that ‘when recollecting a past event I am not experiencing the event, I am experiencing […] a recollection (a re-experience or mental simulation of reliving this event). So remembering [a past event does not] involve [any] experiential awareness [of the relevant past event]’ (referee’s report), and thus the Memory-Claim must be false. - Clearly, this is one possible view which one might take on memory in general and R-memory more specifically; however, it is not the only possible view. Quite on the contrary, at least at first sight it might seem possible to endorse the alternative view that when, for example, Anna recollectively remembers yesterday night’s dinner, she is in some way experientially aware of yesterday night’s dinner. Thus, I think the Memory-Claim does at least deserve a fair hearing, and I therefore hope the opponent is willing to suspend judgement on the tenability of the Memory-Claim until Section  4 below, where I will offer a more detailed discussion of that claim. (A further, even more detailed (paper-length) discussion of the Memory-Claim is offered in the Debus ( 2008 ).) We might hope that our opponent is prepared to grant us this much; nevertheless, the opponent might then move on to point out that the Imagination-Claim could not possibly be true, and that the Main Argument is therefore bound to fail regardless of the present concession. The opponent might suggest that the Imagination-Claim seems to presuppose that a future event could not possibly be a particular event. But then, ‘[w]hether a future event can be a particular event or not depends on how we individuate the event’ (referee’s report), and it might well be possible to individuate future events in such a way that future events can be thought of as particular events.—In response to the present objection, we should grant immediately that the individuation of future events is of great importance in the present context. However, even if it was possible to individuate future events in such a way that they should count as particulars, as the opponent holds it is possible to do, this by itself would not entail that the Imagination-Claim is false. For the Imagination-Claim talks of a subject’s experiential awareness of future events, and it holds that a subject could not possibly be experientially aware of a future particular event. Even if it was possible to individuate future events in such a way that they should count as particulars, it might still be impossible to be experientially aware of such future particular events. Thus, while the present suggestion rightly brings out the importance of the individuation of events for our discussion, we should wait with an assessment of the Imagination-Claim until the end of the next section, in which the Imagination-Claim will be discussed in greater detail. (Thanks to an anonymous referee for prompting the present set of comments.)

However, an opponent might hold that in some sense, we can and should say that Jo is aware that Bill is approaching, even though it is Bob who is coming towards her. And indeed, everybody will all agree that, in the situation as described, Jo (wrongly) takes the person coming towards her to be Bill; we might also say that Jo has a visual experience as of Bill approaching (even though it actually is Bob). However, the suggestion that we should say that Jo is aware that Bill is approaching (even though it is Bob who is coming towards her) does strike me as idiomatically problematic. Indeed, I think it would be idiomatically inappropriate to say this, because I take it that the verb ‘to be aware of’ is, in this respect, used in ways which are analogous to our usage of the verb ‘to know’: We might be very happy to say that Jo believes that Bill is coming towards her, but we certainly would not want to say that Jo knows that Bill is coming towards her (given that it is Bob who is); analogously, while we might be happy to say that Jo ‘takes it’ that Bill is coming towards her, we would not want to say that Jo is aware that Bill is coming towards her. This observation in turn does seem to give us reason to accept that the verb ‘to be aware of’ is factive , just as the verb ‘to know’ is. (Thanks to the editor for prompting the present comment.)

Davidson develops the view in his classic paper on ‘Events as Particulars’ (Davidson ( 1980a )). For a survey of alternative views, cf. Simons ( 2003 ). More generally, as Simons ( 2003 ) 357 points out, ‘philosophical discussion of the ontological category of events is relatively young. […] Most philosophically mooted categories are old, but events come to full prominence only in the twentieth century.’—I think it will be important to keep the category of events firmly on the list of core ontological and wider philosophical concerns during the twenty-first century, because it seems an important concept in various areas of contemporary philosophical interest. In any case, the argument presently under consideration clearly does rely on one particular (and in my view accurate) conception of what events are.

More precisely, we have meanwhile seen that we have good reason to accept the Memory-Claim and the Imagination-Claim, and I had so far assumed that the third premise of the Main Argument is uncontroversial. However, an opponent might hold that being experientially aware of a particular event and being experientially aware of a general type of event are not, as the third premise of the Main Argument has it, two different kinds of mental states, but are ultimately mental states of the same kind which just happen to be directed at different kinds of objects. Hence, so the opponent concludes, the third premise of the Main Argument is false.—In response, we might point out that at least at first sight, it seems plausible to assume that being experientially aware of a particular event and being experientially aware of a general type of event are two different kinds of mental states precisely because, as the opponent emphasizes, the two kinds of mental states have different kinds of objects. It seems plausible to assume that what kind a mental state is of is in part determined by what kind of object it is directed at. Indeed, it seems that anybody who wants to insist in the face of this intuitive idea that ‘ultimately’, mental states which have objects of different kinds nevertheless are mental states of the same kind, as our present opponent wants to do, will have to offer an explanation as to why relevant differences should be disregarded rather than taken into account. Thus, it seems plausible to conclude that as long as the opponent has not provivded any such reasons, we can continue to endorse the third premise of the Main Argument. (Thanks to the editor for prompting the present set of comments.)

Classic points of reference for this suggestion are Block ( 1980 ) and Shoemaker ( 1975 ). For an interesting more recent development of relevant ideas see also Shoemaker ( 2007 ).

This thought does, for example, motivate Davidson ( 1980b ) to develop his non-reductive materialist (or, in his own terminology, ‘anomalous monist’) account of mental events—see especially the introductory paragraphs of the relevant paper (Davidson ( 1980b ) 207).

Suddendorf and Corballis ( 2007 ) 299, my emphasis.

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Debus, D. ‘Mental Time Travel’: Remembering the Past, Imagining the Future, and the Particularity of Events. Rev.Phil.Psych. 5 , 333–350 (2014). https://doi.org/10.1007/s13164-014-0182-7

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DOI : https://doi.org/10.1007/s13164-014-0182-7

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