Encoding and retrieval in declarative learning: a positron emission tomography study.

We present neuroanatomical correlates of encoding and retrieval in an episodic memory task using visually presented highly imaginable word-pair associates. A total of 13 right-handed normal male volunteers took part in the study. Each subject underwent six (15)O-butanol PET scans. On each of the six trials the memory task began 30 s before the injection of a bolus of (15)O-butanol. The subjects had to learn and retrieve 12 word pairs (highly imaginable words, not semantically related, hard associations). The presentation of nonsense words served as a reference condition. Recall accuracy after 2-4 presentations was 66.1%+/-21.1 correct during the PET measurement so that scanning during the retrieval of word pair associates was appropriate to capture the brain activity associated with retrieval. The results obtained support the hypothesis of the presence of an asymmetric network consisting of distributed brain structures subserving associative memory. We show left dorsolateral prefrontal activation during the encoding of visually presented word pair associates, whereas retrieval led to bilateral frontal activation. Furthermore, the importance of the precuneus in the retrieval of highly imaginable word-pair associates using visual imagery as a mnemonic strategy is demonstrated.