On the Integration of Space, Time, and Memory

Abstract

The hippocampus is famous for mapping locations in spatially organized environments, and several recent studies have shown that hippocampal networks also map moments in temporally organized experiences. Here I consider how space and time are integrated in the representation of memories. The brain pathways for spatial and temporal cognition involve overlapping and interacting systems that converge on the hippocampal region. There is evidence that spatial and temporal aspects of memory are processed somewhat differently in the circuitry of hippocampal subregions but become fully integrated within CA1 neuronal networks as independent, multiplexed representations of space and time. Hippocampal networks also map memories across a broad range of abstract relations among events, suggesting that the findings on spatial and temporal organization reflect a generalized mechanism for organizing memories.

Keywords: CA1; CA3; entorhinal cortex; hippocampus; mixed selectivity; parahippocampal cortex; place cells; temporal context model; time cells.

Figure 1

Schematic outline of the spatial and temporal information processing systems in the brain. Spatial and temporal perception and cognition are performed in widespread overlapping cortical networks that connect with the hippocampus via the parahippocampal cortex (PHC), which is a major cortical input to the medial entorhinal cortex (MEC). MEC then sends merged spatial-temporal information to hippocampal areas CA3 and CA1. While some studies suggest differential representation of space and time within hippocampal circuitry that could be separately supported by subcortical and cortical pathways, respectively, information about events in space and time are fully integrated in CA1.

Figure 2

Idealized CA1 mappings of spatial and temporal information by mixed selectivity in different behavioral paradigms. a. Place cell maps during foraging behavior in two environments (distinguished by solid and dashed outlines). Some neurons (dark red) code for space only and some (dark blue) for time only. Other neurons signal both space and time by variations in firing rates in the two environments or on different days, respectively. b. Place cell maps during T-maze alternation. Some neurons (dark blue) fire at specific locations only on left turn trials and others (dark red) only on right-turn trials. Other cells have higher (blue/red) or lower (light blue/pink) firing rates that differentiate left and right trials, respectively. Yet other cells (black) fire equivalently on both trajectories. c. Time and distance maps during treadmill running. Some cells (dark blue) fire strongly associated with time and not distance, and others (dark red) fire strongly associated with distance and not time. Other cells (dark & light blue/pink and red) differentially signal time and distance by firing rates, and yet other cells (black) equivalently signal time and distance. Dark red = spatial; dark blue = temporal; lighter blue = rate coding of time > space; lighter red = rate coding of space > time. Black = equal coding of space and time.