Sparse Coding of Episodic Memory in the Human Hippocampus is Related to the Excitability of Neurons at Encoding and Predicts Subsequent Memory

Neurocomputational models hold that individual episodic memories are represented by a sparse, pattern-separated coding scheme in the hippocampus. In addition, recent theories of neuronal allocation suggest that the assignment of individual neurons to a sparse code is non-random and is associated with intrinsic neural excitability. Here, utilizing an independent dataset of single-unit recordings from epilepsy patients, we demonstrate that a relatively small proportion of high-firing hippocampal neurons represent a single item within a recognition memory test. Critically, only items that were both remembered and showed heightened excitability during encoding were preferentially allocated to a sparse, pattern-separated code, one that was selectively present in the hippocampus. Our findings suggest that individual episodic memories are represented by a sparse distributed coding scheme and that neuronal excitability guides the preferential allocation of hippocampal neurons into sparse codes, which in turn supports subsequent retrieval.