Evidence against the detectability of a hippocampal place code using functional magnetic resonance imaging

Abstract

Individual hippocampal neurons selectively increase their firing rates in specific spatial locations. As a population these neurons provide a decodable representation of space that is robust against changes to sensory- and path-related cues. This neural code is sparse and distributed, theoretically rendering it undetectable with population recording methods such as functional magnetic resonance imaging (fMRI). Existing studies nonetheless report decoding spatial codes in the human hippocampus using such techniques. Here we present results from a virtual navigation experiment in humans in which we eliminated visual- and path-related confounds and statistical shortcomings present in existing studies, ensuring that any positive decoding results would be only spatial in nature and would represent a true voxel-place code. Consistent with theoretical arguments derived from electrophysiological data and contrary to existing fMRI studies, our results show that although participants were fully oriented during the navigation task, there was no statistical evidence for a place code.