Research ReportTime and space in the hippocampus

- An old debate centers on whether the rodent hippocampus is a solely spatial organ.
- Recent evidence has shown both a place code and a time code.
- These firing properties encode relationships between remote stimuli.
- These relationships can then be abstracted to construct a relational memory-a “memory space.”

It has been hypothesized that one of the functions of the hippocampus is to enable the learning of relationships between different stimuli experienced in the environment. These relationships might be spatial (“the bathroom is about 5 m down the hall from the bedroom”) or temporal (“the coffee is ready about 3 min after the button was pressed”). Critically, these spatial and temporal relationships may exist on a variety of scales from a few hundred milliseconds up to minutes. In order to learn consistent relationships between stimuli separated by a variety of spatial and temporal scales using synaptic plasticity that has a fixed temporal window extending at most a few hundred milliseconds, information about the spatial and temporal relationships of distant stimuli must be available to the hippocampus in the present. Hippocampal place cells and time cells seem well suited to represent the spatial and temporal locations of distant stimuli in order to support learning of these relationships. We review a recent computational hypothesis that can be used to construct both spatial and temporal relationships. We suggest that there is a deep computational connection between spatial and temporal coding in the hippocampus and that both serve the overarching function of learning relationships between stimuli-constructing a “memory space.”This article is part of a Special Issue entitled SI: Brain and Memory.