A broader view of perirhinal function: from recognition memory to fluency-based decisions.

Although it is well established that the perirhinal cortex (PRC) makes an important contribution to recognition memory, the specific nature of this contribution remains uncertain. The finding that PRC activity is reduced for old compared with new items is typically attributed to the recovery of a long-term memory (LTM) signal. However, because old items are processed more easily or fluently than new items, reduced PRC activity could reflect increased fluency rather than LTM retrieval per se. We tested this hypothesis in humans using fMRI and a well-validated method to manipulate fluency: the masked priming paradigm. Some words during an old-new recognition test were preceded by conceptually related words (primes) that were subliminally presented (masked). The behavioral results replicated previous findings using this paradigm, whereby the fluency manipulation increased "oldness" responses to both old and new items. The fMRI analyses yielded two main sets of results. First, in the case of new items, which are independent from LTM retrieval, masked priming reduced PRC activity and predicted behavioral misattribution of fluency to oldness. Second, in the case of old items, the same PRC region showing fluency-related reductions for new items also contributed to "old" responding to old items. Individual differences in PRC attenuation also predicted oldness ratings to old items, and fluency modulated PRC connectivity with other brain regions associated with processing oldness signals, including visual cortex and right lateral prefrontal cortex. These results support a broader view in which the PRC serves a function more general than memory.