Our new preprint on temporal mapping in the human medial temporal lobe is online at bioRxiv!
In this project, we investigated how the human entorhinal cortex and the hippocampus map time for episodic memory. Using fMRI and virtual reality, we show that multi-voxel patterns change through memory formation to resemble the objectively elapsed time between events. In the hippocampus on the other hand, activity patterns resembled the subjectively remembered time between these events.
Abstract
Episodic memories consist of event information linked to spatio-temporal context. Notably, the hippocampus is involved in the encoding, representation and retrieval of temporal relations that comprise a context [1-11], but it remains largely unclear how coding for elapsed time arises in the hippocampal-entorhinal region. The entorhinal cortex (EC), the main cortical input structure of the hippocampus, has been hypothesized to provide temporal tags for memories via contextual drift [12,13] and recent evidence demonstrates that time can be decoded from population activity in the rodent lateral EC [14]. Here, we use fMRI to show that the anterior-lateral EC (alEC), the human homologue region of rodent lateral EC [15,16], maps the temporal structure of events. Participants acquired knowledge about temporal and spatial relationships between object positions-dissociated via teleporters-along a fixed route through a virtual city. Multi-voxel pattern similarity in alEC changed through learning to reflect elapsed time between event memories. Furthermore, we reconstructed the temporal structure of object relationships from alEC pattern similarity change. In contrast to the hippocampus, which maps the subjective time between event memories in this task [1], the temporal map in alEC reflected the objective time elapsed between events. Our findings provide evidence for the notion that alEC represents the temporal structure of memories, putatively derived from slowly-varying population signals during learning. Further, our findings suggest a dissociation between objective and subjective temporal maps in EC and hippocampus; thereby providing novel evidence for the role of the hippocampal-entorhinal region in representing time for episodic memory.