Talks & Conferences

Environmental boundaries anchor cognitive maps that support memory. However, trapezoidal boundary geometry distorts the regular firing patterns of entorhinal grid cells proposedly providing a metric for cognitive maps. Here, we test the impact of trapezoidal boundary geometry on human spatial memory using immersive virtual reality. Consistent with reduced regularity of grid patterns in rodents and a grid-cell model based on the eigenvectors of the successor representation, human positional memory was degraded in a trapezoid compared to a square environment; an effect particularly pronounced in the trapezoid’s narrow part. Congruent with spatial frequency changes of eigenvector grid patterns, distance estimates between remembered positions were persistently biased; revealing distorted memory maps that explained behavior better than the objective maps. Our findings demonstrate that environmental geometry affects human spatial memory similarly to rodent grid cell activity - thus strengthening the putative link between grid cells and behavior along with their cognitive functions beyond navigation.

The temporal organization of our experience is central to episodic memory, thought to be supported by the hippocampal-entorhinal region. The lateral entorhinal cortex (EC) carries temporal information in navigating rodents and its human homologue, the anterior-lateral EC, is activated during accurate temporal memory retrieval. However, how learning a temporal structure shapes entorhinal mnemonic representations remains unclear. In a first study, participants learned temporal and spatial relationships of object positions - dissociated via teleporters - along a fixed route through a virtual city. After learning, multi-voxel representations in the anterior-lateral EC specifically reflected the temporal structure of events. Holistic representations of this temporal structure related to memory recall behavior and we reconstructed the temporal structure of object relationships from entorhinal multi-voxel patterns. In a second study, we investigate the nature of temporal information underlying temporal mapping in the hippocampal-entorhinal region. Based on infrequent unmaskings of a hidden clock, participants learned when events occurred during virtual days, partially dissociating the event times from their sequential order and the time objectively elapsed between them. We show representations of temporal relationships based on time within virtual days in the hippocampus and anterior-lateral EC. Together, our findings demonstrate that temporal relationships between events are represented in the anterior-lateral subregion of the EC specifically. Further, temporal maps in the hippocampal-entorhinal region are shaped by temporal information above and beyond sequence order and elapsed time. These findings provide novel evidence for the way in which time organizes episodic memories in the hippocampal-entorhinal region.

Episodic memories are thought to be formed by binding events to their spatial as well as temporal context and medial temporal lobe structures have been implicated in the formation of such integrated memories. But which neural mechanisms allow us to remember when a specific event occurred? Theoretical accounts and recent empirical evidence suggest that the entorhinal cortex provides a slowly varying temporal signal, which might enable the tagging of individual events in time; thereby allowing the formation of a temporal mnemonic map. Here, we probe the development of temporal representations in the entorhinal cortex through learning. We relate changes in multi-voxel pattern similarity in entorhinal cortex to temporal distances between events participants encountered through repeated navigation along a fixed route in a large-scale urban environment. FMRI data were acquired during isolated presentation of events in random order, indicating that the observed similarity structure reflects the reactivation of a learned temporal mnemonic map rather than time per se. Our findings speak to how the entorhinal cortex might provide temporal context information for episodic memory and elucidate the mechanisms underlying the mapping of time in the medial temporal lobe in general.

Entorhinal grid cells are characterized by spatially periodic patterns of activity and have been suggested to provide a metric of space. However, environmental geometry distorts grid-cell firing patterns in highly polarized trapezoidal compared to less polarized square environments. Here, we address the question whether human spatial cognition is influenced by these distortions of grid-cell firing patterns. Participants navigated a trapezoidal and a square arena using immersive virtual reality while learning the positions of objects. Positional memory was degraded in the trapezoid compared to the square. Following the distortions of grid-cell firing patterns in rodents, this effect was more pronounced in the narrow compared to the broad part of the trapezoid. Further, identical distances between object pairs were estimated to be different between the two parts of the trapezoid. These findings suggest that distortions of grid-cell firing patterns impact cognitive functions beyond spatial navigation.