Autobiographical memory refers to the recollection of specific events from one own's past (episodic autobiographical memory, EAM) and general knowledge about oneself (semantic autobiographical memory, SAM). Whereas SAM is devoid of a specific spatiotemporal context, EAM involves the representation of personal events occurred at a specific time and place, and entails re-experiencing and self-projection in the past (mental time travel). Mental time travel in EAM has been proposed to rely on a mapping of events onto a "mental timeline" (MTL); however, no study has addressed brain correlates of mental travel in EAM along the MTL using real autobiographical events. This study aims to investigate brain correlates of mental time travel in EAM, developing a new functional magnetic resonance imaging (fMRI) paradigm based on the spatiotemporal compatibility effect, for which responses to time-related stimuli are faster when the response direction is compatible with a back-to-front MTL (e.g. forward responses for future-related words; congruent condition), compared to the reverse mapping condition (backward responses for future-related words; incongruent condition). During fMRI, participants will be asked to respond to events (EAMs) and facts (SAMs) from their own life, collected using a memory fluency task. As only EAMs are organized according to a temporal context, the interaction effect between the factors "Category" (EAM vs. SAM) and "Condition" (compatible vs. non-compatible with the MTL) will allow to highlight brain regions selectively supporting mental time travel in EAM.
In the only previous fMRI study investigating brain correlates of mental travel along the MTL, participants were asked to mentally place themselves at a given point along the MTL, and to judge whether different past- or future-related stimuli occurred before or after the imagined self-location in time[1]. However, stimuli presented in this study consisted either in a) pictures of the participant's face and of a famous person's face (actor George Clooney) modified in order to demonstrate different ages, or b) labels referring to common personal life events (e.g. "first child") or nonpersonal events (e.g. "hurricane Katrina"). Thus, which brain networks mediate mental travel along the MTL for real, personal life events, is still completely unknown. Moreover, previous investigations of mental travel along the MTL tested not only past events, but also imagined future ones, and did not assess their vividness[1-4]. The present study could thus provide a significant contribution to the characterization of brain correlates of the spatiotemporal organization of real autobiographical memories, also accounting for differences in the vividness of episodic recall, that has been shown to be a highly relevant factor in the modulation of activation and connectivity patterns during the retrieval of autobiographical events[5].
It also worth noting that a strong theoretical debate still exists on how and where episodic memory traces are stored in the brain, and on their possible transformation over time, with some accounts positing a constant and permanent role of specific brain regions (i.e. the hippocampus) in the storage and retrieval of episodic memories, and others proposing that a reorganization of memory networks occurs as time passes after the events[6,7]. The present study, testing EAMs of events spanning the whole life (i.e. from early childhood to the very recent past), could thus provide important insights on this debated issue, investigating possible differences in brain correlates of mental time travel in EAMs according to the recency of memories.
It is also important to point out that previous studies[1-4] tested mental time travel along the transversal MTL. It has been highlighted, however, that in most natural languages, the flow of time is actually conceived from an egocentric perspective, that is along a sagittal back-to-front axis[8,9]. Whereas the transversal representation of time likely depends on cultural factors such as the direction of reading and writing[10,11], the sagittal MTL has been argued to be grounded in actual sensorimotor experience, such as that related to walking and running[12-15]. The present study thus proposes an innovative approach to the study of EAM, being the first proposing to test brain correlates of the organization of real EAMs along a sagittal MTL, using a spatiotemporal interference task. In this vein, results of the study proposed here could shed light on the contribution of sensorimotor processing to the representation of EAMs, and thus be of great importance considering recent embodied accounts of cognition and memory, that propose a pivotal influence of real-world environment and of perceptual, motor and bodily signals on episodic memory[16].
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