Talks & Conferences

2021

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 environment compared with a square environment—an effect that was particularly pronounced in the narrow part of the trapezoid. Congruent with changes in the spatial frequency of eigenvector grid patterns, distance estimates between remembered positions were persistently biased, revealing distorted memory maps that explained behaviour better than the objective maps. Our findings demonstrate that environmental geometry affects human spatial memory in a similar manner to rodent grid-cell activity and, therefore, strengthen the putative link between grid cells and behaviour along with their cognitive functions beyond navigation.

The hippocampal-entorhinal region supports memory for episodic details, such as temporal relations of sequential events, and mnemonic constructions combining experiences for inferential reasoning. However, it is unclear whether hippocampal event memories reflect temporal relations derived from mnemonic constructions, event order, or elapsing time, and whether these sequence representations generalize temporal relations across similar sequences. Here, participants mnemonically constructed times of events from multiple sequences using infrequent cues and their experience of passing time. After learning, event representations in the anterior hippocampus reflected temporal relations based on constructed times. Temporal relations were generalized across sequences, revealing distinct representational formats for events from the same or different sequences. Structural knowledge about time patterns, abstracted from different sequences, biased the construction of specific event times. These findings demonstrate that mnemonic construction and the generalization of relational knowledge combine in the hippocampus, consistent with the simulation of scenarios from episodic details and structural knowledge.

The hippocampal-entorhinal region supports memory for episodic details, such as temporal relations of sequential events, and mnemonic constructions combining experiences for inferential reasoning. In this talk, I will argue that event representations in the anterior hippocampus reflect temporal relations derived from mnemonic constructions rather than sequence order or elapsing time. Further, I will show that these event representations generalize temporal relations across similar sequences. Participants mnemonically constructed times of events from multiple sequences using infrequent cues and their experience of passing time. After learning, event representations in the anterior hippocampus reflected sequence relations based on constructed times. These event representations generalized across sequences, revealing distinct representational formats for events from the same or different sequences. Structural knowledge about time patterns, abstracted from different sequences, biased the construction of specific event times. These findings demonstrate that the hippocampus reconciles representations of specific relations with the generalization across different episodes, consistent with memory-based constructions combining episodic details and general knowledge to simulate scenarios.

Details to be added

2020

The hippocampal-entorhinal region forms cognitive maps of space and stimulus relations in general. In this talk, I will address how cognitive maps underlie human spatial and temporal memory. First, I will show that boundary geometry distorts spatial memory in ways consistent with deformations of grid-cell firing patterns. In a highly-immersive virtual reality experiment, mnemonic distortions through environmental geometry followed predictions from a model of grid-pattern deformations based on the successor representation.These findings strengthen the putative link between the grid metric of cognitive maps and spatial memory. Turning to cognitive maps of temporal relations in the hippocampal-entorhinal region, I will argue that hippocampal sequence representations do not merely reflect the sequential order of events or elapsing time, but are flexibly scaled to an experimentally-defined temporal reference frame. Further, the hippocampus and entorhinal cortex generalize temporal relations across multiple sequences. Together, these findings show that spatial and temporal reference frames anchor cognitive maps and shed new light onto how cognitive maps underlie episodic memory.

The hippocampal-entorhinal region forms cognitive maps of learned relations. Environmental boundaries are central to spatial orientation. However, trapezoidal boundary geometry distorts the regular firing patterns of entorhinal grid cells that proposedly provide a metric for cognitive maps. In this talk, I will first show that environmental geometry affects spatial memory in ways consistent with a grid-cell model based on the successor representation. Positional memory was degraded in a trapezoid compared to a square control environment and distance estimates between remembered positions were persistently biased. The reference frame provided by environmental geometry affected human spatial memory similarly to rodent grid-cell activity — thus strengthening the putative link between the grid metric of cognitive maps and behavior. Second, I will turn to cognitive maps of temporal relations in the hippocampus and anterior-lateral entorhinal cortex. I will address the question whether we flexibly reference sequence memories to a continuous clock as opposed to merely storing the sequential order of events or representing time as a passive “absolute”, akin to the readout of a stopwatch. In a sequence learning task, participants inferred when individual events took place based on infrequent umaskings of a hidden clock. We manipulated the clock’s speed between sequences to partially dissociate event times from their sequential order and the time objectively elapsing between them. After learning, multi-voxel patterns reflected the temporal relationships of event pairs in both hippocampus and entorhinal cortex. Hippocampal sequence memories were anchored to the reference frame of the hidden clock. Further, hippocampal and entorhinal sequence representations were organized in a way that generalized temporal relations across multiple sequences. Together, these findings suggest that cognitive maps built in service of episodic memory are tied to the external world by anchoring them to spatial and temporal reference frames.

Time is the fundamental dimension along which we organize our experience. For example, sequences of events shape our episodic memory. The hippocampus and, more recently, the anterior-lateral entorhinal cortex have been implicated in memory for event sequences. Specifically, multi-voxel pattern similarity in these regions scales with learned temporal relationships of events encountered in a sequence. However, the precise nature of these representations remains to be understood: Do we merely store the sequential order of events or do we learn temporal relations along a continuous dimension? Is time represented as a passive “absolute”, akin to the readout of a stopwatch, or can temporal relationships be adjusted flexibly depending on the task at hand? Here, we combined a novel learning task with fMRI to investigate the level of complexity at which temporal relations are represented in the hippocampal-entorhinal memory system. Participants encountered four event sequences, which we refer to as virtual days, and were to infer when individual events occurred during the virtual days based on infrequent unmaskings of an otherwise hidden clock. Importantly, we manipulated the speed of this clock between virtual days to (partially) dissociate event times from their sequential order and the time objectively elapsing between them. Participants successfully learned event times and their mnemonic responses reflected information beyond sequence order and objectively elapsed time, indicating that knowledge of temporal relations was referenced to the hidden clock. We further investigated how learning changed multi-voxel pattern representations in the anterior hippocampus and the anterior-lateral entorhinal cortex. Representational change relative to a pre-learning baseline scan reflected the temporal relationships of events in both the hippocampus and entorhinal cortex. Interestingly, our findings suggest a dissociation between representations of temporal relations of events from the same or a different virtual day. Together, we show that participants’ memory scaled with the temporal structure of the virtual days with respect to the hidden clock and thereby demonstrate that event representations in the hippocampus and entorhinal cortex are shaped by learned temporal relationships within and across sequences. Thereby, our findings provide novel insights into how multiple event sequences are organized in the hippocampal-entorhinal memory system.

2019

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.

2018

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.