Exteroceptive and interoceptive cue control of hippocampal place cells.

Abstract

Place cells in the hippocampal formation form the cornerstone of the rat’s navigational system and together with head direction cells in the postsubiculum and grid cells in the entorhinal cortex are the key elements of what O’Keefe and Nadel (1978) postulate to be a “cognitive map”. The hippocampal formation is ideally positioned anatomically to receive highly processed inputs from almost all brain regions. Previous research has focused on the cues that determine and contribute to place cell selectivity. Such cues include information about the external world that the rat perceives through its senses (“exteroceptive cues”) as well as cues internal to the body such as proprioception or somatosensation (“interoceptive cues”). This thesis uses a novel experimental paradigm in which the rat runs on a moving-treadmill linear track to investigate the relative contribution of interoceptive and exteroceptive cues for determining place cell spatial selectivity. The major finding is that place fields shift in the direction of the moving treadmill, both when the animal runs along with or against the motion of the treadmill, indicating that self-motion information is a key input to place cells. Furthermore, place fields in the middle of the track shift more than fields closer to the end walls suggesting that exteroceptive information interacts with interoceptive information to assist in accurate navigation. This conclusion is further supported by experiments performed in complete darkness where two populations of cells are observed: the first are cells which become quiescent or remap, presumably under strong exteroceptive control, while the second are cells that maintain similar firing characteristics under both lighting conditions, putatively under the influence of interoceptive inputs.