Two kinds of memory signals in neurons of the human hippocampus

Some studies of the neural representation of memory in the human hippocampus have identified memory signals reflecting the categorical status of test items (novel vs. repeated). Others have identified pattern-separated, episodic memory signals reflecting recognition of particular test items. Here, we report that both kinds of memory signals can be found in the hippocampus, and we consider their possible functions. We recorded single-unit activity from four brain regions (hippocampus, amygdala, anterior cingulate, and prefrontal cortex) of epilepsy patients as they performed a continuous recognition task. The generic signal was found in all four regions, whereas the sparse, pattern-separated signal was limited to the hippocampus, as predicted by longstanding computational models.