Contribution of Multisensory Stimuli to the Representation of Space in the Hippocampus and Medial Entorhinal Cortex

How the brain integrates information over multiple sensory modalities to encode highly complex stimuli is a central question of neuroscience. The mammalian brain structures that encode space provide a tractable system in which to explore this question, as there are several well-defined cell types that have spatially selective firing. Place cells in the hippocampus fire when an animal is in a particular location in the environment. Grid cells, found in the upstream medial entorhinal cortex (MEC), have multiple firing fields located on the vertices of a triangular grid. Also found in the MEC are border cells that fire along the boundaries of an environment, and head direction (HD) cells that fire when the animal’s head is facing a particular direction. Using virtual reailty for rats, we explored the contribtion of multiple sensory modalities to the spatial selectivity of these cell types.The hippocampal cognitive map is thought to be driven by distal visual cues and self-motion cues. In VR place cells showed robust spatial selectivity, however a much smaller proportion were track active, compared to the RW. This indicates that distal visual and non-vestibular self-motion cues are indeed sufficient to provide selectivity, but vestibular and other sensory cues present in RW are necessary to fully activate the place cell population. Additionally, bidirectional cells preferentially encoded distance along the track in VR, but encoded absolute position in RW. Taken together these results suggest the differential contributions of sensory cues in shaping the hippocampal population code.Similarly, we also measured the activity of grid cells and head direction cells in open field environments in VR and RW. Here, we found that grid cells lost their spatial selectivity and periodicity, demonstrating that the cues available in the VR were insufficient. Most HD cells, likewise, did not retain their directional tuning in a body-fixed VR, suggesting that visual cues might maintain only a partial ability to update the firing directions of HD cells in the absence of reliable angular vestibular cues. However, a small portion of HD cells found to maintain their selectivity in the VR warrant further investigation.These results provide insight to the contributions of individual sensory modalities to the firing of spatially selective cell types, as well as the mental representation of space