Institute of Behavioural Neuroscience, Division of Psychology and Language Sciences, University College
The neural encoding of place in mammals is supported by a network of structures in the limbic system including hippocampus and entorhinal cortex. Hippocampal neurons, called place cells, exhibit spatially localized firing fields and are collectively thought to form a map-like representation, the so-called “ognitive map.” More recently, it has been found that neurons in entorhinal cortex, afferent to the place cells, have multiple firing fields arranged in a spatially regular grid-like array. These “grid cells” may provide metric information to the place cells, and thus study of the properties of grids can shed light on the underlying metric structure of the cognitive map. In particular, the grid cells open up the opportunity to explore the encoding of complex, three dimensional spaces.
There are various forms of three-dimensional encoding that the cognitive map could theoretically incorporate, ranging from a completely flat representation containing no height information at all, through intermediates (e.g. contour map, slice map) to a fully three-dimensional, volumetric map. I will present data from our preliminary investigations into encoding of a partially three-dimensional space, the helical maze. On the helical maze, place cells show a sensitivity to height but grid cells, surprisingly, do not. This dissociation between place and grid cells will be explained in terms of a hypothesized modulation (by height) of the grid cell inputs to place cells. It suggests that the vertical dimension may be encoded differently from the horizontal ones, a possibility that is currently under investigation in a homogeneous, volumetric maze.