Segregating the functions of human hippocampus

It is now accepted that hippocampal lesions impair episodic memory. However , the precise functional role of the hippocampus in episodic memory remains elusive, Recent functional imaging data implicate the hippocampus in proce ssing novelty, a finding supported by human in vivo recordings and event-re lated potential studies. Here we measure hippocampal responses to novelty, using functional MRI (fMRI), during an item-learning paradigm generated fro m an artificial grammar system. During learning, two distinct types of nove lty were periodically introduced: perceptual novelty, pertaining to the phy sical characteristics of stimuli tin this case visual characteristics), and exemplar novelty, reflecting semantic characteristics of stimuli tin this case grammatical status within a rule system). We demonstrate a left anteri or hippocampal response to both types of novelty and adaptation of these re sponses with stimulus familiarity. By contrast to these novelty effects, we also show bilateral posterior hippocampal responses with increasing exempl ar familiarity. These results suggest a functional dissociation within the hippocampus with respect to the relative familiarity of study items. Neural responses in anterior hippocampus index generic novelty, whereas posterior hippocampal responses index familiarity to stimuli that have behavioral re levance (i.e., only exemplar familiarity). These findings add to recent evi dence for functional segregation within the human hippocampus during learni ng.