Investigating the Janus hypothesis : an fMRI study of the neural substrates of mental time travel

Abstract

‘Mental time travel’ (MTT) refers to the general neurocognitive capability to re-experience past events (past MTT) and imagine future ones (future MTT). In recent years, several studies have indicated that past and future MTT are closely linked in brain and mind, a claim referred to as the ‘Janus hypothesis’. Questions pertaining to the nature of this link and its neurocognitive basis have been raised. The present study employed functional magnetic resonance imaging (fMRI) to delineate the neurocognitive processes involved in past and future MTT. Participants were scanned as they performed tasks requiring them to remember past events and imagine future ones. Whole-brain contrast analyses revealed that there were no differences in brain activity during the past and future tasks. Relative to a control task, these tasks activated regions in medial frontal, medial temporal, medial parietal (including posterior cingulate) and lateral temporoparietal areas. Analyses of functional connectivity identified four functional brain networks active during both the past and future tasks. These networks were equally modulated by both tasks, and largely replicated the regions identified in the whole-brain contrast analyses. The networks can tentatively be ascribed to processes related to the retrieval and integration of information from episodic memory. The author suggests that the observed link between past and future MTT reflect the simple fact that when imagining future events, one must draw upon past experiences. Therefore, future MTT involves the same set of processes traditionally reserved for episodic memory retrieval.