NOUN AND VERB RETRIEVAL BY NORMAL SUBJECTS STUDIES WITH PET

PET activation studies identify significant local changes in regional cerebral blood flow (rCBF) in contrasts of behavioural tasks with cont rol states, and these local changes identify net changes in local syna ptic activity. A number of studies on word retrieval have all demonstr ated left frontal (dorsolateral and medial) involvement in the task. H owever, there have been differences in the responses observed in the l eft temporal lobe, with variously a deactivation (significant decrease in rCBF), no response and an activation (significant increase in rCBF ). In the four studies described here, we have examined word (verbs an d nouns) retrieval contrasted with a number of different control state s. The studies confirmed extensive activation of the left dorsolateral prefrontal cortex and, medially, the anterior cingulate cortex and th e supplementary motor area (SMA). Activations of the left posterior te mporal lobe and the inferior parietal lobe were consistently demonstra ted when word retrieval was contrasted with a rest state. Contrasts wi th other single word tasks controlled out the activation in the perisy lvian part of the left posterior temporal lobe, suggesting a role for this region in lexical processing. The left inferolateral temporal cor tex and the posterior part of the inferior parietal lobe were only act ivated by word retrieval, particularly verbs. It is proposed that thes e activated regions reflect access to semantic fields.