Regional cerebral blood flow was examined by PET recordings during multiple-trial learning in healthy volunteers tested at Guy’s and St Thomas’s Hospitals and the MRC Cyclotron Unit, Hammersmith Hospital, London. Eight adult subjects were presented visually with a 15-word list, either novel or repeated. Incremental learning occurred with repeated word lists and was associated with PET activation in the left medial temporal area, but not the left frontal. The left medial temporal region is activated by novel word visual stimuli and also in incremental and consolidating learning of repeated stimuli. The right frontal and precuneal regions, participating in retrieval of already learned memories, are also involved in this activation. Memory function requiring effort and elaboration of stimuli involves the left frontal region. 
COMMENT. The left medial temporal region is involved in memory and learning of novel stimuli and also during incremental learning and the consolidation or “binding’ of new memories. These findings based on PET studies are supportive of the etiological importance of left temporal lobe arachnoid cysts associated with attention deficit and learning disorders, the TLAC/ADHD syndrome.