Episodic boundary cells in human medial temporal lobe during the free recall task.

A necessary condition for forming episodic memories is the construction of specific episodes demarcated from other episodes in space and time. Evidence from studies of episodic memory formation using rodent models suggest that the medial temporal lobe (MTL) supports the representation of boundary information. Building on recent work using human microelectrode recordings as well, we hypothesized of human MTL neurons with firing rates sensitive to episodic boundary information. We identified 27 episodic boundary neurons out of 736 single neurons recorded across 27 subjects. Firing of these neurons increased at the beginning and end of mnemonically relevant episodes in the free recall task. We distinguish episodic boundary neurons from a population of ramping neurons (n = 58), which are time-sensitive neurons whose activity provides complementary information during episodic representation. Episodic boundary neurons exhibited a U-shaped activity pattern demonstrating increased activity after both beginning and end boundaries of encoding and retrieval epochs. We also describe evidence that the firing of boundary neurons within episodic boundaries is organized by hippocampal theta oscillations, using spike-field coherence metrics.