Time Travel in the Brain: The Default Mode Network's Role in Past and Future Simulation

Have you ever wondered what happens in your brain when your mind wanders? Research suggests that humans spend between 30-50% of daily life engaged in thoughts unrelated to the immediate task at hand [1,2]. One might think that our brain enters a resting state during these moments of boredom. However, this could not be further from the truth, since the default mode network (DMN) region of the brain shows increased levels of activity during these phases of mental inactivity. 

In 2001, Marcus Raichle, a neurologist focusing on functional brain imaging techniques, and collaborators formally proposed the concept of a "default mode" of brain activity . This showed that the same regions of the brain exhibited decreased activity when cognition was present and then activated following cessation of those activities [3] . In other words, if we think of a car idling at a red light, the car is not accelerating forward, but it is still on and ready to propel forward when the light switches to green. Similarly, the DMN operates during these intervals of rest even when we are not engaged in any cognitively demanding task, helping to maintain a baseline of activity for more engaging tasks [3]. 

By 2003, Michael Greicius and colleagues had developed strong functional magnetic resonance imaging (fMRI), a scan that tracks the level of activity in different regions of the brain,  results indicating that those deactivated regions constituted a network [4]. Follow-up research published over the past decade has greatly added to our understanding of the subcomponents and functions of the DMN; however, those initial research studies were essential in understanding its basic functions [5].

The DMN’s role extends beyond just activating during rest; it serves as the brain’s internal simulator, allowing us to disengage from our present moment to explore the past and future in a process known as “mental time travel.” [6]. The DMN plays an important role in the brain's capacity for mental time travel, facilitating the recall of past events, simulation of future scenarios, and creation of a mental image of oneself in the present moment [6]. 

The Default Mode Network: Structure and Function

The DMN includes the medial prefrontal cortex (mPFC), posterior cingulate cortex (PCC), the inferior parietal lobule (IPL), the middle temporal lobe, and the precuneus [7]. The major components of the DMN can be explained with the help of a simple analogy involving a movie production crew. The director of this show calls the shots and controls the storyline. This role is similar to the function of the mPFC, which aids in self-reflective thought [8]. The editor ensures that all the scenes have smooth transitions, and they fit together. Similarly, the PCC is responsible for information integration and storage, which involves combining spatial (the location of the body in space) and interoceptive (internal state of the body) information [3]. Combined, these functions of the PCC allow the brain to maintain awareness of its environment. The screenwriter helps add creativity to a movie just like the IPL helps with spatial integration, which combines and merges various sensory inputs to help us better understand our surroundings [8]. The middle temporal lobe helps retrieve and recall past experiences and can be likened to the archivist, who is responsible for maintaining a library of past footage and scripts [9]. Finally, the precuneus helps carry out the functions of the other components of the DMN, and can be likened to a stage manager, who is responsible for making sure that all components of the production work in harmony, including the props, stage setup, and character roles [10].

Techniques such as fMRI have allowed researchers to understand the activity of certain brain regions during a resting state [3]. In a resting state fMRI, participants are advised to remain still and are not given a task. The fMRI detects fluctuations in brain activity, which reveal connections between specific regions in the DMN. Another method is to use a task-based fMRI. In this fMRI, the participants are given a task and researchers observe DMN activity. Since the DMN is not active during attentive states or externally focused tasks, the task-based fMRI shows decreased activity in the DMN [11]. In conclusion, these techniques support the fact that the DMN is active during periods of rest and becomes less active during activities requiring focused attention. 

Mental Time Travel: The Past, Present, and Future

The concept of "mental time travel" refers to the human ability to imagine oneself in the past or the future. This is a fundamental process that allows us to remember events, plan for the future, and maintain our sense of self [12]. Now, some may label this as daydreaming, but mental time travel serves as a tool that allows us to plan and execute future decisions.

From an evolutionary point of view, the ability to picture past events and future possibilities gave early humans significant survival advantages. According to a study published by Suddendorf et al. (2009), early humans who recalled previous encounters and predicted threats or opportunities handled social and environmental challenges better, which boosted their survival and reproductive success [13]. For instance, early humans that could recall an encounter with a predator and avoid that area would have a higher chance of survival. 

In addition, humans often tend to have a positive attitude regarding unfavorable situations. This is because mental time travel enables humans to anticipate and plan for the future, which can act as a beacon of hope during times of uncertainty. An example would be Norway’s seed bank that is designed to protect and preserve crops in the case of global disasters. Just like the seed bank can sustain agriculture during times of crises, the DMN helps us to prepare and plan for future events [3]. So, even though the optimistic view in some situations might be incorrect, it still confers an advantage over a hopeless attitude that may cause the person to give up in that situation [13] . In this view, mental time travel is not simply a thought exercise, but a key tool that aids planning and risk evaluation next to problem resolution when facing uncertainty.

As previously mentioned, the key regions involved in mental time travel include the DMN network, the hippocampus, and medial temporal lobe structures. The DMN contributes to self-referential or self-focused thinking [8]. The hippocampus and the medial temporal lobe are particularly important for storing autobiographical memories of personal events [3]. For example, the hippocampus and medial temporal lobes are activated when remembering the location of an object that has just been mislocated [13]. Since mental time travel involves the simulation of future events and the retrieval of past events, the DMN provides the ability to reflect while the hippocampus and medial temporal lobe regions offer a collection of memories  [3]. Combined, these regions are responsible for the various aspects of mental time travel [14].

The Connection Between the DMN and Mental Time Travel 

The DMN is active during both remembering the past and imagining the future. Studies using fMRI and other neuroimaging techniques show that the same brain regions within the DMN are activated when people engage in mental time travel, whether they are thinking about past experiences or future possibilities [3].

In a recent study by Casadio et al. (2024), participants were asked to imagine themselves in different periods of time–ranging from the past, present, and the foreseeable future [15]. Then, they were asked to judge whether these events happened before or after their imagined point in time. The researchers aimed to understand how perceived temporal distance (PTD)—how near or far events feel in time—affects the way those events are mentally represented. After analyzing and recording the data, the fMRI scans showed that when events were perceived as closer in time, brain regions typically active during self-focused, internal thinking were more active [15]. In other words, the brain regions involved in creating a sense of self were more active when events were thought to have taken place closer in time. These brain regions are part of the DMN, which as previously mentioned, are known to be involved in recalling memories and imagining future scenarios.

Another finding from the study is that it took more effort for the brain to recall events that seemingly occurred in the past few weeks compared to events that seemingly occurred a few years ago [15]. Although it may seem counterintuitive at first glance, this finding is reasonable since the brain tries to create a more detailed account of events that have occurred recently. On the other hand, events that seemingly occurred more distant in the past or future are processed more abstractly. Put simply, recalling seemingly distant events is like sketching a rough outline, while recalling seemingly closer events is like painting a detailed portrait; and the portrait is naturally expected to take more effort [15].

Implications and Future Directions

Abnormalities in DMN function and the regions governing mental time travel have a significant link with many psychiatric and neurological disorders such as depression, anxiety, attention-deficit/hyperactivity disorder (ADHD), and Alzheimer's disease [11]. In depression and anxiety, overactivation in the DMN can be linked with excessive worrying about past and future events. Posttraumatic stress disorder (PTSD) also involves the DMN and time travel systems as vivid and intrusive memories of stressful events from the past are associated with disruptions in the DMN [12]. For individuals with schizophrenia, DMN dysfunction can contribute to hallucinations and delusions as the boundaries between reality and imagination are blurred [11]. 

So, why is the DMN involved in so many of these disorders? One reason is because the DMN is interconnected with many other parts of the brain and it also has high metabolic activity. In the context of Alzheimer’s disease, high levels of DMN activity can lead to damage in one of the many interconnected networks, explaining why the DMN is affected in these diseases. Another reason is that the DMN is a relatively new evolutionary system in the brain compared to more established and stable systems in the brain [11]. For comparison, other animals such as rodents also have structures that are broadly similar to the DMN, which indicates that other species also have these capabilities. Even though structures resembling the DMN can be found in other species, this brain region is better developed and understood in humans, and it is distinct from older brain systems that are responsible for basic sensory or motor functions [16].

Because the DMN is so interconnected, it has been difficult to study its various components in isolation. A study by Paquola et al. (2025) has used innovative imaging methods and cytoarchitectural studies, which involve studying the arrangement of cells in tissue, to address this challenge [17]. They utilized a high-field 7-T MRI, which has a significantly greater magnetic field strength than regular MRI machines. This allowed the researchers to produce clearer images of the specific brain regions involved in the DMN. Additionally, they studied the brain tissue of deceased individuals using cytoarchitectural mapping to investigate the various structures involved in the DMN. Combined, both approaches allowed the researchers to develop a more comprehensive map of the DMN. 

In conclusion, the DMN is an essential component of the active brain network that becomes active during periods of cognitive inactivity. Mental time travel is the process that includes recalling past experiences, imagining the future, and introspection. Thus, the brain regions involved in the DMN enable the process of mental time travel. The DMN also holds clinical relevance as it underscores many psychiatric disorders such as depression and PTSD [18]. This connectivity between the DMN and mental time travel not only forms a cornerstone of human cognition, but also enables us to navigate through the many challenges in life with purpose and foresight. 

References

1. A Nghiem, T.-A. E., A Lee, B., A Chao, T.-H. H., Branigan, N. K., Mistry, P. K., B Shih, Y.-Y. I., & Menon, V. (2024). Space wandering in the rodent default mode network. PNAS, 121(15), e2315167121. https://med.stanford.edu/content/dam/sm/scsnl/documents/nghiem-et-al-2024-space-wandering-in-the-rodent-default-mode-network.pdf

2. Andrews-Hanna, J. R. (2011). The brain’s default network and its adaptive role in internal mentation. The Neuroscientist, 18(3), 251–270. https://doi.org/10.1177/1073858411403316 

3. Brocas, I., & Carrillo, J. D. (2018). A Neuroeconomic Theory of mental Time travel. Frontiers in Neuroscience, 12. https://doi.org/10.3389/fnins.2018.00658

4. Buckner, R. L. (2013). The brain’s default network: origins and implications for the study of psychosis. Dialogues in Clinical Neuroscience, 15(3), 351–358. https://doi.org/10.31887/dcns.2013.15.3/rbuckner

5. Casadio, C., Patané, I., Candini, M., Lui, F., Frassinetti, F., & Benuzzi, F. (2024). Effects of the perceived temporal distance of events on mental time travel and on its underlying brain circuits. Experimental Brain Research, 242(5), 1161–1174. https://doi.org/10.1007/s00221-024-06806-x

6. Cleveland Clinic. (2023, May 27). Functional MRI – Seeing Brain Activity as it Happens. Cleveland Clinic. https://my.clevelandclinic.org/health/diagnostics/25034-functional-mri-fmri

7. Davey, C. G., & Harrison, B. J. (2018). The brain’s center of gravity: how the default mode network helps us to understand the self. World Psychiatry, 17(3), 278–279. https://doi.org/10.1002/wps.20553

8. Kellogg, R. T., Chirino, C. A., & Gfeller, J. D. (2020). The complex Role of Mental time travel in depressive and Anxiety Disorders: An Ensemble perspective. Frontiers in Psychology, 11. https://doi.org/10.3389/fpsyg.2020.01465

9. Killingsworth, M. A., & Gilbert, D. T. (2010). A wandering mind is an unhappy mind. Science, 330(6006), 932. https://doi.org/10.1126/science.1192439

10. Klinger, E., & Cox, W. M. (1987). Dimensions of thought flow in everyday life. Imagination Cognition and Personality, 7(2), 105–128. https://doi.org/10.2190/7k24-g343-mtqw-115v

11. Marc Dingman. (n.d.). Know Your Brain: Default Mode Network. Neuroscientifically Challenged. https://neuroscientificallychallenged.com/posts/know-your-brain-default-mode-network

12. Menon, V. (2023). 20 years of the default mode network: A review and synthesis. In Neuron. Elsevier Inc. https://doi.org/10.1016/j.neuron.2023.04.023

13. Paquola, C., Garber, M., Frässle, S., Royer, J., Zhou, Y., Tavakol, S., Rodriguez-Cruces, R., Cabalo, D. G., Valk, S., Eickhoff, S., Margulies, D. S., Evans, A., Amunts, K., Jefferies, E., Smallwood, J., & Bernhardt, B. C. (2025). The architecture of the human default mode network explored through cytoarchitecture, wiring and signal flow. Nature Neuroscience. https://doi.org/10.1038/s41593-024-01868-0

14. Raichle, M. E., MacLeod, A. M., Snyder, A. Z., Powers, W. J., Gusnard, D. A., & Shulman, G. L. (2001). A default mode of brain function. Proceedings of the National Academy of Sciences, 98(2), 676–682. https://doi.org/10.1073/pnas.98.2.676

15. Suddendorf, T., Addis, D. R., & Corballis, M. C. (2009). Mental time travel and the shaping of the human mind. Philosophical Transactions of the Royal Society B Biological Sciences, 364(1521), 1317–1324. https://doi.org/10.1098/rstb.2008.0301

16. Wagelmans, A.M.A, Wassenhove, V.V. (2023, September 28). Mental time travel: when our brain is our time machine [Video]. YouTube. https://www.youtube.com/watch?v=a3ac42WG1R8

17. Greicius, M. D., Supekar, K., Menon, V., & Dougherty, R. F. (2008). Resting-State functional connectivity reflects structural connectivity in the default mode network. Cerebral Cortex, 19(1), 72–78. https://doi.org/10.1093/cercor/bhn059

18. Østby, Y., Walhovd, K. B., Tamnes, C. K., Grydeland, H., Westlye, L. T., & Fjell, A. M. (2012). Mental time travel and default-mode network functional connectivity in the developing brain. Proceedings of the National Academy of Sciences, 109(42), 16800–16804. https://doi.org/10.1073/pnas.1210627109