Space and Time in the Hippocampal-Entorhinal System
Bijnen, S. van
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The long tradition of research on hippocampal involvement in spatial memory and spatial properties of episodic memory has often been focused on spatially sensitive neurons called place cells. Together with other spatially sensitive neurons they are proposed to function as an internal representation of the outside world (i.e. a cognitive map). More recently, cells were discovered that encode for a specific moment time. These ‘time cells’ have been found to parse temporally defined environments analogous to how place cells parse spatially defined environments. Similarities between time and place cells have led researchers to propose that they are not distinct cell types, but rather that cells become sensitive to either spatial or temporal information based on the context in which learning occurs. Some evidence supports this notion. However, if time cells, like place cells, are to be an important component of episodic memory, they should be able to code for multiple time domains. The hippocampal-entorhinal system has been proposed to be able to do this using distinct, variable, neuronal representations. Interestingly, input from grid cells in the entorhinal cortex to CA1 cells in the hippocampus have also been suggested to combine linearly to create the place fields, emphasizing the importance of this network. However, the role of the hippocampal-entorhinal system in the creation and stability of place cells have been questioned. Concluding, evidence is beginning to support a role for the temporally sensitive neurons in episodic memory, analogous to the spatial sensitive neurons. However, the importance of the hippocampal-EC connections for the basic properties of both time and place cells needs further investigation.