Periodic Time Cells in Human Entorhinal Cortex

Abstract

Grid cells in the entorhinal cortex demonstrate spatially periodic firing patterns, which are thought to provide a map of space on behaviorally relevant length scales. It is unknown, however, whether such periodicity exists for behaviorally relevant time scales in the human brain. Here we investigated neuronal firing during a temporally continuous uninterrupted experience by presenting fourteen neurosurgical patients with an audiovisual video while recording single neuron activity from multiple brain regions. We report on a set of units that modulate their activity in a strikingly periodic manner across different timescales--from seconds to many minutes. These cells were most prevalent in the entorhinal cortex. Time within the video could be decoded from their population activity. Furthermore, these cells remapped their dominant periodicity to shorter timescales during a subsequent recognition memory task. When the audiovisual sequence was presented at two different speeds (regular and faster), a significant percentage of these temporally periodic cells (TPCs) maintained their timescales, suggesting a degree of invariance with respect to the narrative content. The temporal periodicity of TPCs may complement the spatial periodicity of grid cells Whether these cells provide scalable spatiotemporal metrics for encoding and retrieval of human experience warrants future investigations.