Abstract
The spatial mapping function of the hippocampal formation is likely derived from two sets of information: one based on the external environment and the other based on self-motion. Here, we further characterize ‘boundary vector cells’ (BVCs) in the rat subiculum, which code space relative to one type of cue in the external environment: boundaries. We find that the majority of cells with fields near the perimeter of a walled environment exhibit an additional firing field when an upright barrier is inserted into the walled environment in a manner predicted by the BVC model. We use this property of field repetition as a heuristic measure to define BVCs, and characterize their spatial and temporal properties. In further tests, we find that subicular BVCs typically treat drop edges similarly to walls, including exhibiting field repetition when additional drop-type boundaries are added to the testing environment. In other words, BVCs treat both kinds of edge as environmental boundaries, despite their dissimilar sensory properties. Finally, we also report the existence of ‘boundary-off cells’, a new class of boundary-coding cells. These cells fire everywhere except where a given BVC might fire.
Highlights
The spatial mapping function of the hippocampal formation [1,2] is likely derived from two sets of information: one based on the external environment and the other based on self-motion
We find that the majority of cells with fields near the perimeter of a walled environment exhibit an additional firing field when an upright barrier is inserted into the walled environment in a manner predicted by the boundary vector cells’ (BVCs) model
The subiculum is typically regarded as an output region of the hippocampal formation [18,19,20], constraining views on how boundary cells might input to place cells in the hippocampus proper. (We suggest a more complex view of the subiculum by noting, e.g. physiological evidence that subicular output can enter the hippocampus proper indirectly [21] and that CA1 place cells are not obviously influential on subicular firing in many situations; full discussion of this is beyond the scope of this brief report.) Additional support for the BVC model was provided by the discovery of border/boundary cells in the medial entorhinal cortex [22,23], which receives a prominent input from the subiculum, and which projects monosynaptically to hippocampal place cells
Summary
The spatial mapping function of the hippocampal formation [1,2] is likely derived from two sets of information: one based on the external environment and the other based on self-motion. We focus on how a certain type of spatial cell responds to changes in environmental boundaries. A BVC would fire whenever an environmental boundary intersected a receptive field located at a specific distance from the rat in a specific allocentric direction, with breadth of tuning to distance that increases with the preferred distance. The firing of model BVCs depends solely on the rat’s location relative to environmental boundaries and is independent of the rat’s heading direction. The firing of a place cell can be modelled as a thresholded sum of the firing of the BVCs synapsing onto it, and the BVC model captures several features of place fields in different environmental configurations [5]
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