Activation of medial temporal structures during episodic memory retrieval.

Medial temporal lobe structures have been implicated in human episodic memory. Patients with medial temporal lesions show memory deficits, and functional neuroimaging studies have revealed activation in this region during episodic encoding and retrieval when data are averaged over a sample of subjects. The relevance of such observations for memory performance has remained unclear, however. Here we have used positron emission tomography (PET) to examine cerebral blood flow related to verbal episodic retrieval. We observed strong positive correlations between retrieval and blood flow in left medial temporal structures in individual normal human subjects. In addition, multivariate analysis showed that regions in the left medial temporal lobe were dominant components of a pattern of brain regions that distinguished a high-retrieval condition from conditions of lower retrieval. These results suggest that medial temporal activity is related to retrieval success rather than retrieval attempt, possibly by reflecting reactivation of stored patterns.