SEMANTIC ENCODING AND RETRIEVAL IN THE LEFT INFERIOR PREFRONTAL CORTEX - A FUNCTIONAL MRI STUDY OF TASK-DIFFICULTY AND PROCESS SPECIFICITY

Prefrontal cortical function was examined during semantic encoding and repetition priming using functional magnetic resonance imaging (fMRI) , a noninvasive technique for localizing regional changes in blood oxy genation, a correlate of neural activity. Words studied in a semantic (deep) encoding condition were better remembered than words studied in both easier and more difficult nonsemantic (shallow) encoding conditi ons, with difficulty indexed by response time. The left inferior prefr ontal cortex (LIPC) (Brodmann's areas 45, 46, 47) showed increased act ivation during semantic encoding relative to nonsemantic encoding rega rdless of the relative difficulty of the nonsemantic encoding task. Th erefore, LIPC activation appears to be related to semantic encoding an d not task difficulty. Semantic encoding decisions are performed faste r the second time words are presented. This represents semantic repeti tion priming, a facilitation in semantic processing for previously enc oded words that is not dependent on intentional recollection. The same LIPC area activated during semantic encoding showed decreased activat ion during repeated semantic encoding relative to initial semantic enc oding of the same words. This decrease in activation during repeated e ncoding was process specific; it occurred when words were semantically reprocessed but not when words were nonsemantically reprocessed. The results were apparent in both individual and averaged functional maps. These findings suggest that the LIPC is part of a semantic executive system that contributes to the on-line retrieval of semantic informati on.