Human and animal brains can represent events in time and space in fascinating ways, for instance, by accessing a chronology of events that happened in the past via stimuli perceived by the senses.
In the human brain, spatial and temporal dimensions of events are combined and manifest in what is known as episodic memory, which can be elicited at any point while a person is navigating the world.
Episodic memory refers to the human ability to recall and play out specific experiences or events from the past. Past neuroscience studies suggest that the mapping of time-space associated with episodic memory is rooted in a specific region of the brain: the hippocampus.
Based on research findings gathered so far, humans are the only species on Earth also able to imagine chronological sequences that never happened but that might be in some ways connected to real events.
This ability lies at the basis of what is known as causal reasoning, which entails identifying cause-and-effect relationships, such as ‘if this thing happens in the future, then that other thing could happen.”
Researchers at École Polytechnique Fédérale de Lausanne, Manipal Institute of Technology and Université Paris-Saclay have recently carried out a study investigating the role of brain regions within the hippocampus formation in the human ability to navigate through imagined events rooted in a different time or space.
Their paper, published in theMIT Press Journal of Cognitive Neuroscience, offers valuable new insight about the neural underpinnings of what is known as ordinal psychological time in the human brain.
“Herein, we asked whether the hippocampal formation is involved in mental navigation in time (and space), which requires internal manipulations of events in time and space from an egocentric perspective,” the researchers wrote in their paper.
“To address this question, we reanalyzed a magnetoencephalography data set collected while participants self-projected in time or in space and ordered historical events as occurring before/after or west/east of the mental self.”
Baptiste Gauthier, Pooja Prabhu, Karunakar A. Kotegar and Virginie van Wassenhove, the team of researchers behind the study, carried out an in-depth analysis of a dataset that they collected in one of their previous works.
In their past study, they used time-resolved neuroimaging techniques to characterize the brain activity of a group of people that was asked to imagine historical events chronologically from different mental perspectives over time (e.g., nine years into the future) or in different spaces (e.g., while in a specific place).
In their new paper, the researchers review the data they previously collected using a different method. As the method they used in their past study had significant limitations that prevented them from investigating the implication of the hippocampus, they replaced it with a technique that explicitly accounts for the volume of the hippocampus.
The new source reconstruction method they used allowed them to clarify how deep structures within the hippocampal formation are involved in the human ability to mentally order imagined events in time and space from an egocentric perspective.
The hippocampal formation is composed of several brain structures, including the bilateral hippocampi, entorhinal cortices and parahippocampal cortex.
“We found selective involvement of the medial temporal lobes (MTLs) with a notable lateralization of the main effects: Whereas temporal ordinality engaged mostly the left MTL, spatial ordinality engaged mostly the right MTL,” the researchers wrote.
The results of the analyses shed some light on the contribution of brain regions within the hippocampal formation in ordinal psychological time, particularly that of the MTLs and MTL.
While the exact function these regions is still unclear, their work suggests that they play a key role in mentally ordering hypothetical or imagined events occurring in different places and at different times.
In their paper, Gauthier, Prabhu, Kotegar and Wassenhove also introduce an hypothesis regarding how mental time and space travel might be processed by the hippocampal formation.
More specifically, they suggest that the human ability to imagine traveling to different places or times could be guided by top-down control of neural activity within this particular brain formation.
Interestingly, the selective patterns of neural activity reported by the researchers occur when humans are mentally ordering events that they never experienced in real life (i.e., non-episodic events), but not when they mentally revisit actual events from their past. In the future, their findings could pave the way for new studies investigating ordinal psychological time in the human brain and focusing on the hippocampal formation, perhaps using new methods or newly compiled datasets.
We predominantly stand in the present facing the future rather than looking back to the past —Suddendorf and Corballis (2007)
Although not immediately intuitive, the idea that memory, imagination, and predicting what might happen in the future are intimately linked is not new. Throughout the centuries this notion has consistently reemerged within philosophical, psychological, and contemporary work, along with the belief that the role of recollection is to serve imagination and prediction of the future.
For instance, in 1798, Immanuel Kant noted that “Recalling the past (remembering) occurs only with the intention of making it possible to foresee the future” (p. 77); in 1871 the White Queen in Lewis Carroll’s Through the Looking Glass astutely observed, “It’s a poor sort of memory that only works backwards” (Carroll 1871, chap. 5); whereas in 2006, Suddendorf argued, “It is accurate prediction of the future, more so than accurate memory of the past per se, that conveys adaptive advantage” (p. 1007).
There is behavioral evidence supporting the connection between memory and imagination of the future. For instance, D’Argembeau and Van der Linden (2004) asked participants to mentally “re-experience” personal past events (episodic memory) or to “pre-experience” (episodic future thinking; Atance and O’Neill 2001) possible future events that had/would occur in the close or distant past/future.
For the past and future, temporally close events were associated with more sensorial and contextual details and evoked stronger feelings of re-experiencing (or pre-experiencing) than the temporally distant equivalents. Similarly, D’Argembeau and Van der Linden (2006) showed that individual differences, such as capacity for visual imagery, affect the phenomenological experience of episodic memory and episodic future thinking. Notably, specific errors made when recollecting the past are also evident when people engage in predicting the future (for a review, see Gilbert and Wilson 2007).
If memory and imagination are intimately linked, it is natural to ask whether they are supported by the same neural structures. This has been examined in two ways, one with a focus on the hippocampus (Fig. 1) and the other with an eye to an extended set of brain areas—including medial and lateral prefrontal, posterior cingulate and retrosplenial cortices, lateral temporal cortex, and the medial temporal lobes (MTL)—often called the “core network” for episodic memory and imagination (Fig. 2; Buckner and Carroll 2007; Spreng and others 2009).
Considering first the hippocampus, since the seminal work of Scoville and Milner (1957), the MTL and in particular the hippocampus have been recognized as playing a pivotal role in our ability to recollect past experiences. Their article described the case of HM who underwent bilateral temporal lobectomy for the relief of intractable epilepsy, rendering him amnesic, unable to acquire new episodic memories.
They noted “after the operation this young man could no longer recognize the hospital staff nor find his way to the bathroom, and he seemed to recall nothing of the day-to-day events of his hospital life” (Scoville and Milner 1957, p. 14). The case of HM precipitated 50 years of subsequent work examining the role of the hippocampus in memory (for reviews, see Corkin 2002; Squire 2004).
Memory, however, is not the only function that has been ascribed to the hippocampus. In the 1970s, O’Keefe and Dostrovsky (1971) discovered cells in the rat hippocampus that displayed location-specific firing (so-called “place cells”; Fig. 3A), and damage to the hippocampus was found to severely disrupt spatial navigation ability (Morris and others 1982).
This evidence prompted O’Keefe and Nadel (1978) to suggest the hippocampus plays a key role in both memory and spatial navigation. Although this idea has been debated (Cohen and Eichenbaum 1991), the onus remains on theoretical accounts of hippocampal function to explain the mnemonic (Spiers and others 2001) and navigation (Fig. 3B; Maguire and others 2006) deficits observed in patients following bilateral hippocampal damage (Burgess and others 2002).
But it seems that even explaining memory and navigation is not sufficient; as the links between memory, imagination, and thinking about the future have crystallized, evidence has started to accrue implicating the hippocampus and the core network in these latter functions also.
In fact, there has been an explosion of interest in this domain, with Klein (2013) noting a 10-fold increase in investigative activity in the last five years. So what is the evidence that a common neural system, which includes the hippocampus, underpins memory, imagination, and prediction of the future?
Conclusions and Future Directions
In conclusion, metacognitive, cognitive, neuropsychological, and neuroimaging evidence clearly illustrate the close ties between episodic memory, imagination and predicting the future.
In general, we believe that future studies in humans could greatly benefit from de-conflating two issues—how do we learn and remember our past experiences, and what does the hippocampus do.
Despite the hippocampus being widely regarded as the quintessential episodic memory device, as outlined in this article, memory and the hippocampus are not simply interchangeable. By releasing the hippocampus from strictly mnemonic accounts of its function, we believe that a theoretically enriched understanding of its fundamental role and its breakdown in pathology can emerge.
To truly ascertain, then, what it is the hippocampus does, a useful strategy going forward, as proposed in the SCT, may be to consider the range of disparate cognitive functions that have been linked to the hippocampus, including memory, imagination, and predicting the future, and deduce from this what common underlying processes or mechanisms may be hippocampally mediated (Maguire and Mullally 2013).
In particular, we need to know more about precisely how the hippocampus supports the construction of scenes, and how this interacts with known computations, such as pattern separation and pattern completion, that occur in its subfields.
Does BE occur in non-humans, and if so, what can we learn about the mechanisms involved from electrophysiological studies?
Looking beyond the hippocampus, how does hippocampal-dependent scene construction relate to the operation of other cortical areas in the core network such as the parahippocampal and retrosplenial cortices, that are often labeled as “scene-selective” (Auger and others 2012; Mullally and Maguire 2011).
What are the precise functions of each area within the core network, and the connectivity between them? There is much yet to learn, and an understanding of the relationship between episodic memory, imagination and predicting the future is still in its infancy. Nevertheless, we are confident that the next five years will hasten important new insights into this question that has intrigued down the ages.
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More information: Baptiste Gauthier et al. Hippocampal Contribution to Ordinal Psychological Time in the Human Brain, Journal of Cognitive Neuroscience (2020). DOI: 10.1162/jocn_a_01586
Baptiste Gauthier et al. Building the Arrow of Time… Over Time: A Sequence of Brain Activity Mapping Imagined Events in Time and Space, Cerebral Cortex (2018). DOI: 10.1093/cercor/bhy320