We form cognitive maps of the space around us to navigate and to remember where important events take place. In an immersive VR experiment, participants wore a head-mounted display and navigated different environments using a motion platform to learn object positions in space. Our data show distortions in spatial memory as a function of the geometry of environmental boundaries; in line with deformations of the grid metric of our cognitive maps.
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.