Neuronal allocation and sparse coding of episodic memories in the human hippocampus

Neurocomputational models hold that individual episodic memories are represented by a sparse, pattern-separated coding scheme in the hippocampus. Animal studies further suggest that the allocation of neurons to such codes is non-random and may be biased by their excitability at the time of encoding. Here, utilizing an independent dataset of single-unit recordings from epilepsy patients, we report that only remembered items that elicited a relative increase in firing at encoding were associated with a sparse, pattern-separated neural code at retrieval, and this effect was specific to the hippocampus. These findings provide evidence in humans that remembered episodic memories are represented by sparse codes in the hippocampus. Although excitability is not typically defined in terms of firing rates, the present results raise the possibility that excitability at encoding may influence the likelihood of recruitment into such codes.