Remembering episodes: a selective role for the hippocampus during retrieval.

Some memories are linked to a specific time and place, allowing one to re-experience the original event, whereas others are accompanied only by a feeling of familiarity. To uncover the distinct neural bases for these two types of memory, we measured brain activity during memory retrieval using event-related functional magnetic resonance imaging. We show that activity in the hippocampus increased only when retrieval was accompanied by conscious recollection of the learning episode. Hippocampal activity did not increase for items recognized based on familiarity or for unrecognized items. These results indicate that the hippocampus selectively supports the retrieval of episodic memories.

Material-specific lateralization of prefrontal activation during episodic encoding and retrieval.

Although numerous neuroimaging studies have examined the functional neuroanatomy supporting episodic memory for verbal material, there have been few investigations of non-verbal episodic encoding and retrieval. We used fMRI to directly compare prefrontal activation elicited by verbal and non-verbal material during encoding and during retrieval. Regardless of the mnemonic operation (encoding/retrieval), inferior prefrontal activation lateralized based on material type. Verbal encoding and retrieval resulted in greater left inferior prefrontal activation, whereas non-verbal encoding and retrieval resulted in greater right inferior prefrontal activation. The similarity between inferior prefrontal activity during encoding and during retrieval indicates that these mnemonic operations depend on shared processes mediated by inferior prefrontal regions.