Constructive episodic simulation of the future and the past: Distinct subsystems of a core brain network mediate imagining and remembering

Recent neuroimaging studies demonstrate that remembering the past and imagining the future rely on the same core brain network. However, findings of common core network activity during remembering and imagining events and increased activity during future event simulation could reflect the recasting of past events as future events. We experimentally recombined event details from participants’ own past experiences, thus preventing the recasting of past events as imagined events. Moreover, we instructed participants to imagine both future and past events in order to disambiguate whether future-event-specific activity found in previous studies is related specifically to prospection or a general demand of imagining episodic events. Using spatiotemporal partial-least-squares (PLS), a conjunction contrast confirmed that even when subjects are required to recombine details into imagined events (and prevented from recasting events), significant neural overlap between remembering and imagining events is evident throughout the core network. However, the PLS analysis identified two subsystems within the core network. One extensive subsystem was preferentially associated with imagining both future and past events. This finding suggests that regions previously associated with future events, such as anterior hippocampus, medial prefrontal cortex and inferior frontal gyrus, support processes general to imagining events rather than specific to prospection. This PLS analysis also identified a subsystem, including hippocampus, parahippocampal gyrus and extensive regions of posterior visual cortex that was preferentially engaged when remembering past events rich in contextual and visuospatial detail.     

Hippocampal contributions to the episodic simulation of specific and general future events

Recent studies have demonstrated that remembering past experiences and imagining future scenarios recruits a core network including the hippocampus. Even so, constructing future events engages the hippocampus more than remembering past events. This fMRI study examined whether increased hippocampal activity for future events includes both specific and general events. Participants constructed specific and general past and future events during fMRI scanning. We replicated previous findings of increased activity in the right anterior hippocampus when constructing future relative to past events, and when constructing specific relative to general events. Importantly, both effects were driven by a significant interaction between temporal direction and specificity, with specific future events resulting in more activity than other conditions, including general future events. No regions exhibited greater activity during the construction of past relative to future events, or general relative to specific events. These results suggest that the process of constructing a detailed representation of a novel and specific future event differentially engages the right anterior hippocampus compared with other forms of event simulation and recall. Future work is needed to disambiguate the role of encoding, novelty and detail recombination in engaging the right anterior hippocampus during simulation.     

Remembering the past and imagining the future: common and distinct neural substrates during event construction and elaboration

People can consciously re-experience past events and pre-experience possible future events. This fMRI study examined the neural regions mediating the construction and elaboration of past and future events. Participants were cued with a noun for 20s and instructed to construct a past or future event within a specified time period (week, year, 5-20 years). Once participants had the event in mind, they made a button press and for the remainder of the 20s elaborated on the event. Importantly, all events generated were episodic and did not differ on a number of phenomenological qualities (detail, emotionality, personal significance, field/observer perspective). Conjunction analyses indicated the left hippocampus was commonly engaged by past and future event construction, along with posterior visuospatial regions, but considerable neural differentiation was also observed during the construction phase. Future events recruited regions involved in prospective thinking and generation processes, specifically right frontopolar cortex and left ventrolateral prefrontal cortex, respectively. Furthermore, future event construction uniquely engaged the right hippocampus, possibly as a response to the novelty of these events. In contrast to the construction phase, elaboration was characterized by remarkable overlap in regions comprising the autobiographical memory retrieval network, attributable to the common processes engaged during elaboration, including self-referential processing, contextual and episodic imagery. This striking neural overlap is consistent with findings that amnesic patients exhibit deficits in both past and future thinking, and confirms that the episodic system contributes importantly to imagining the future.     

Self,memory, and imagining the future in a case of psychogenic amnesia

We report a case of psychogenic amnesia and examine the relationships between autobiographical memory impairment, the self, and ability to imagine the future. Case study JH, a 60-year-old male, experienced a 6-year period of pervasive psychogenic amnesia covering all life events from childhood to the age of 53. JH was tested during his amnesic period and again following hypnotherapy and the recovery of his memories. JH’s amnesia corresponded with deficits in self-knowledge and imagining the future. Results are discussed with reference to models of self and memory and processes involving remembering and imagining.     

The Ventral and Dorsal Default Mode Networks Are Dissociably Modulated by the Vividness and Valence of Imagined Events

Recent work has shown that the brain’s default mode network (DMN) is active when people imagine the future. Here, we test in human participants (both sexes) whether future imagination can be decomposed into two dissociable psychological processes linked to different subcomponents of the DMN. While measuring brain activity with fMRI as subjects imagine future events, we manipulate the vividness of these events to modulate the demands for event construction, and we manipulate the valence of these events to modulate the demands for event evaluation. We found that one subcomponent of the DMN, the ventral DMN or medial temporal lobe (MTL) subsystem, responds to the vividness but not the valence of imagined events. In contrast, another subcomponent, the dorsal or core DMN, responds to the valence but not the vividness of imagined events. This separate modifiability of different subcomponents of the DMN by vividness and valence provides strong evidence for a neurocognitive dissociation between (1) the construction of novel, imagined events from individual components from memory and (2) the evaluation of these constructed events as desirable or undesirable.     

Increased hippocampus to ventromedial prefrontal connectivity during the construction of episodic future events

Both the hippocampus and ventromedial prefrontal cortex (vmPFC) appear to be critical for episodic future simulation. Damage to either structure affects one's ability to remember the past and imagine the future, and both structures are commonly activated as part of a wider core network during future simulation. However, the precise role played by each of these structures and, indeed, the direction of information flow between them during episodic simulation, is still not well understood. In this study, we scanned participants using functional magnetic resonance imaging while they imagined future events in response to object cues. We then used dynamic causal modeling to examine effective connectivity between the left anterior hippocampus and vmPFC during the initial mental construction of the events. Our results show that while there is strong bidirectional intrinsic connectivity between these regions (i.e., irrespective of task conditions), only the hippocampus to vmPFC connection increases during the construction of episodic future events, suggesting that the hippocampus initiates event simulation in response to retrieval cues, driving activation in the vmPFC where episodic details may be further integrated.     

Autobiographical significance in past and future public semantic memory: A case-study

Refined investigation of infrequent dissociations within remote memory, such as preservation of autobiographical episodic memory and selective impairment of public semantic memory could provide some insight on the interactions of long-term memory systems and their underlying brain correlates. Combining clinical neuropsychological and neuroimaging methods in the present study, we examined a patient surgically treated for temporal lobe epilepsy showing this rare pattern of dissociation. Specifically, we investigated along the two temporal directions, past and future, his autobiographical episodic memory, semantic memory for public events and famous people and their interaction through the concept of autobiographical significance (AS). The results showed impaired ability not only to recall past but also to imagine future public events in a context of preserved past and future personal episodic memory. Remarkably, impersonal future thinking was impaired regardless of AS, while the autobiographical-significant public past knowledge relied exclusively on the patient's spared autobiographical episodic memory. These results were corroborated by neuroimaging data showing the absence of brain activation for public knowledge devoid of personal significance and activation of the autobiographical memory cerebral network for personally significant public knowledge. Our findings suggest that AS did not ‘restore’ the code to access public semantic memory, but bypassed it by using personal memory sources successful only for past public recollections. Therefore, remembering impersonal and imagining public events seems to require the contribution of public semantic knowledge per se. The patient's cognitive profile suggested a reorganization of memory systems.     

Age-related neural changes in autobiographical remembering and imagining

Numerous neuroimaging studies have revealed that in young adults, remembering the past and imagining the future engage a common core network. Although it has been observed that older adults engage a similar network during these tasks, it is unclear whether or not they activate this network in a similar manner to young adults. Young and older participants completed two autobiographical tasks (imagining future events and recalling past events) in addition to a semantic-visuospatial control task. Spatiotemporal Partial Least Squares analyses examined whole brain patterns of activity across both the construction and elaboration of autobiographical events. These analyses revealed that that both age groups activated a similar network during the autobiographical tasks. However, some key age-related differences in the activation of this network emerged. During the construction of autobiographical events, older adults showed less activation relative to younger adults, in regions supporting episodic detail such as the medial temporal lobes and the precuneus. Later in the trial, older adults showed differential recruitment of medial and lateral temporal regions supporting the elaboration of autobiographical events, and possibly reflecting an increased role of conceptual information when older adults describe their pasts and their futures.     

What comes first? Electrophysiological differences in the temporal course of memory and future thinking

It is now widely accepted that remembering the past and imagining the future rely on a number of shared processes and recruit a similar set of brain regions. However, memory and future thinking place different demands on a range of processes. For instance, although remembering should lead to early associative retrieval of event details, event construction may be slower for future events, for which details from different memories are combined. In order to shed light on the question of how the brain distinguishes between memories and future thoughts, we investigated the differences in the electrophysiological correlates of the vivid elaboration of future and past events. In the slow cortical potentials of 24 healthy human participants, differences during early elaboration were observed at temporo-parietal and parieto-occipital electrode sites, presumably reflecting differential recruitment of sensory and semantic detail retrieval. Additional differences emerged over the right pre-frontal cortex during later elaboration, which could be linked to differential retrieval demands. In conclusion, the time course differences, which presumably reflect the varying recruitment of sub-processes engaged during mental time travel, will help to understand the mechanisms with which the brain separates memories from future thoughts.     

Differential engagement of brain regions within a ‘core’ network during scene construction

Reliving past events and imagining potential future events engages a well-established “core” network of brain areas. How the brain constructs, or reconstructs, these experiences or scenes has been debated extensively in the literature, but remains poorly understood. Here we designed a novel task to investigate this (re)constructive process by directly exploring how naturalistic scenes are built up from their individual elements. We “slowed-down” the construction process through the use of auditorily presented phrases describing single scene elements in a serial manner. Participants were required to integrate these elements (ranging from three to six in number) together in their imagination to form a naturalistic scene. We identified three distinct sub-networks of brain areas, each with different fMRI BOLD response profiles, favouring specific points in the scene construction process. Areas including the hippocampus and retrosplenial cortex had a biphasic profile, activating when a single scene element was imagined and when 3 elements were combined together; regions including the intra-parietal sulcus and angular gyrus steadily increased activity from 1 to 3 elements; while activity in areas such as lateral prefrontal cortex was observed from the second element onwards. Activity in these sub-networks did not increase further when integrating more than three elements. Participants confirmed that three elements were sufficient to construct a coherent and vivid scene, and once this was achieved, the addition of further elements only involved maintenance or small changes to that established scene. This task offers a potentially useful tool for breaking down scene construction, a process that may be key to a range of cognitive functions such as episodic memory, future thinking and navigation.     

Construction of Past and Future Events in Children and Adolescents with ASD: Role of Self-relatedness and Relevance to Decision-Making

We studied episodic memory and future thinking for self-relevant and other-relevant events at different levels of retrieval support, theory of mind, and delay discounting in ASD children and adolescents (ASDs). Compared to typically developing controls, ASDs produced fewer internal (episodic) but a similar number of external (semantic) details while remembering past events, imagining future events, and imagining future events happening to others, indicating a general impairment of event construction. This deficit was driven by group differences under high retrieval support, and therefore unlikely to depend on self-initiated retrieval/construction deficits. ASDs’ event construction impairment related to the severity of ASD symptoms, and to theory of mind deficits. ASDs, however, showed normal delay discounting, highlighting preserved forms of future-based decision-making in ASD.     

Constructive episodic simulation: temporal distance and detail of past and future events modulate hippocampal engagement

Behavioral, lesion and neuroimaging evidence show striking commonalities between remembering past events and imagining future events. In a recent event-related fMRI study, we instructed participants to construct a past or future event in response to a cue. Once an event was in mind, participants made a button press, then generated details (elaboration) and rated them. The elaboration of past and future events recruited a common neural network. However, regions within this network may respond differentially to event characteristics, such as the amount of detail generated and temporal distance, depending on whether the event is in the past or future. To investigate this further, we conducted parametric modulation analyses, with temporal distance and detail as covariates, and focused on the medial temporal lobes and frontopolar cortex. The analysis of detail (independent of temporal distance) showed that the left posterior hippocampus was responsive to the amount of detail comprising both past and future events. In contrast, the left anterior hippocampus responded differentially to the amount of detail comprising future events, possibly reflecting the recombination of details into a novel future event. The analysis of temporal distance revealed that the increasing recency of past events correlated with activity in the right parahippocampus gyrus (Brodmann area (BA) 35/36), while activity in the bilateral hippocampus was significantly correlated with the increasing remoteness of future events. We propose that the hippocampal response to the distance of future events reflects the increasing disparateness of details likely included in remote future events, and the intensive relational processing required for integrating such details into a coherent episodic simulation of the future. These findings provide further support for the constructive episodic simulation hypothesis (Schacter and Addis (2007) Philos Trans R Soc Lond B Biol Sci 362:773-786) and highlight the involvement of the hippocampus in relational processing during elaboration of future events.     

What about the mental time travel and age-related effects?

According to Tulving, episodic memory allows humans to travel mentally through subjective time into either the past or the future, this ability being at the origin of adaptation, organization and planning of future behavior. The main aim of this review is to present a state of art of episodic mental time travel and a lifespan perspective from children to elderly people. We examine the numerous similarities between remembering the past and envisioning the future which have been highlighted in cognitive, neuroimaging, and neuropsychological studies. We also present studies that have given evidence that remembering the past and imagining the future differ somewhat. We focus on demonstrating that hippocampal dysfunction is associated with disturbances in the recall of episodic autobiographical details in past memories, but also in the imagining of episodic detailed future events. More specifically, we discuss that the future seems to involve higher semantic processes mediated by the inferior frontal and lateral temporal gyri. We propose that the study of mental travel in personal time could be undertaken in line with the distinction between the memory of (episodic) experiences and (semantic) personal knowledge of one's life, which constitutes a major part of the self and constraints what we have been, what we are now, and what we might yet become.     

Deficits in past remembering extend to future imagining in a case of developmental amnesia

Patient and neuroimaging studies report that the ability to remember past personal experiences and the ability to envision future personal experiences are interconnected. Loss of episodic memory is typically accompanied by loss of future imagining, and engaging in either activity recruits common brain areas. The relationship between episodic memory and future imagining is also suggested by their co-emergence in ontogenetic development. However, it is unknown whether a failure of one ability to emerge in early development precludes the development of the other ability. To investigate this possibility, we tested H.C., a young woman with amnesia of developmental origin associated with bilateral hippocampal loss, and demographically matched controls on an adapted version of the Autobiographical Interview using Galton-Crovitz cueing. In response to cue words, participants described both past personal events and imagined future personal events that occurred, or could occur, in near and distant time periods. Results indicated a parallel pattern of impairment for both past and future event generation in H.C., such that her narratives of both types of events were similarly deficient. These results indicate that mental time travel can be compromised in hippocampal amnesia, whether acquired in early or later life, possibly as a result of a deficit in reassembling and binding together details of stored information from earlier episodes.     

Counterfactual thinking: an fMRI study on changing the past for a better future

Recent studies suggest that a brain network mainly associated with episodic memory has a more general function in imagining oneself in another time, place or perspective (e.g. episodic future thought, theory of mind, default mode). If this is true, counterfactual thinking (e.g. ‘If I had left the office earlier, I wouldn’t have missed my train.’) should also activate this network. Present functional magnetic resonance imaging (fMRI) study explores the common and distinct neural activity of counterfactual and episodic thinking by directly comparing the imagining of upward counterfactuals (creating better outcomes for negative past events) with the re-experiencing of negative past events and the imagining of positive future events. Results confirm that episodic and counterfactual thinking share a common brain network, involving a core memory network (hippocampal area, temporal lobes, midline, and lateral parietal lobes) and prefrontal areas that might be related to mentalizing (medial prefrontal cortex) and performance monitoring (right prefrontal cortex). In contrast to episodic past and future thinking, counterfactual thinking recruits some of these areas more strongly and extensively, and additionally activates the bilateral inferior parietal lobe and posterior medial frontal cortex. We discuss these findings in view of recent fMRI evidence on the working of episodic memory and theory of mind.     

Effects of prospective thinking on intertemporal choice: The role of familiarity

Imagining future events while performing an intertemporal choice task can attenuate the devaluation of future rewards. Here, we investigated whether this effect and its neural basis depend on the degree of personal prior experience associated with the simulated future scenarios. Functional magnetic resonance imaging was combined with a modified intertemporal choice task in which the delayed options were either purely monetary, or linked with a social event. Subject‐specific events differed regarding familiarity, that is, meeting a close, familiar person or a celebrity in a café. In line with recent hypotheses on episodic construction, the simulation of future familiar and unfamiliar events equally attenuated delay discounting behavior in comparison with the control condition and both were imagined with similar richness. Imaging data, however, indicate that these results rely on differential neural activation patterns. The hippocampus was particularly involved in the simulation of unfamiliar future scenarios, probably reflecting enhanced construction processes when personal experience with similar past events is lacking. Consequently, functional coupling of the hippocampus with neural valuation signals in the anterior cingulate cortex predicted the subjective value only of rewards offered in the unfamiliar context. In contrast, valuation of rewards in a familiar context was predicted by activation in key nodes of emotional and autobiographical memory retrieval and dynamically modulated by frontal‐striatal connectivity. The present data emphasize that the mechanisms underlying neural valuation of prospective rewards largely depend on the pre‐experience with the context in which they are offered. Hum Brain Mapp 36:4210–4221, 2015. © 2015 Wiley Periodicals, Inc.     

Imagining the future in complicated grief

Background: Complicated Grief (CG) is a chronic and debilitating consequence of bereavement. Although sharing features with depression and anxiety, CG is associated with independent negative health outcomes. Despite these significant health costs, relatively little is known about the cognitive mechanisms that contribute to the maintenance of CG. The ability to envisage the future is important for adaptive functioning. This study investigates future‐related thinking in CG. Method: Twenty‐one individuals with CG and 24 bereaved individuals without CG were asked to imagine specific events that may take place in their future and recall specific autobiographical memories in response to cue words, and complete a personal goals task. Results: CG participants were less specific in their imagining of future positive events and were more likely to imagine future events relating to their loss. The extent to which individuals were able to imagine a specific future event was significantly correlated with recalling specific memories. The tendency to imagine loss‐related events in the future was associated with holding grief‐related goals. Conclusions: These results are consistent with propositions of the self‐memory system model of autobiographical memory and shed light on factors that may maintain grieving in people affected by CG. Depression and Anxiety, 2011. © 2011 Wiley‐Liss, Inc.