Focal and diffuse memory activation assessed by localized indicators of CNS metabolism: the semantic-episodic memory distinction.

: Methods for in-vivo measurement in humans of local brain activation are reviewed, to assess their potential contribution to the understanding of the brain organization of memory. Methodologically, it is argued that the instructive studies are those: (a) using fully normal subjects; (b) using sufficient sample sizes to account statistically for individual differences; and (c) using properly designed control tasks to isolate relevant independent variables that determine the brain response. The review of emerging data concentrates on the regional cerebral blood flow literature and proposes certain hypotheses for further test, based on extant findings. These include: (a) that hyperfrontality of cortical activation signals the type of processing that converts material from episodic to semantic memory; (b) that semantic remembering itself is likely to engage focal association areas relevant to the particular task (e.g. Wernicke's area in language tasks); but (c) that episodic remembering, even when task components such as language are controlled, will engender a less focal but more diffuse activation pattern than would a control semantic remembering task.