Right prefrontal cortex responds to item familiarity during a memory encoding task

In a previous word-pair encoding study (Dolan & Fletcher, 1997), we examine d the effect of introducing novelty, either in studied words or in their mu tual associations. A left medial temporal lobe (MTL) sensitivity to novel w ords and left prefrontal cortex (PFC) to novel associations was observed. I n this further report on the data, we explored the extent to which the righ t PFC, more generally implicated in retrieval operations (Fletcher, Frith, & Rugg, 1997), was sensitive to these manipulations. Specifically, we chara cterised changes associated with increasing familiarity of study material. We demonstrate that the response in right ventrolateral PFC is preferential ly sensitive to a condition in which all material was familiar (that is, in which all material had been presented prior to scanning). A more dorsal re gion in right PFC was found to be relatively more active in association wit h a condition in which one item in the pair was familiar but was paired wit h a novel associate. Our results suggest that sensitivity to stimulus famil iarity is expressed in right PFC, even within the context of an encoding ta sk. The data also provide further evidence for functional heterogeneity wit hin right PFC, with a more ventral region responding to familiarity of comp lete word pairs and a more dorsal region responding to familiar single word s occurring in the context of new associative relationships.