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Nine positron emission tomography (PET) studies of human visual information processing were reanalyzed to determine the consistency across experiments of blood flow increases during active tasks relative to passive viewing of the same stimulus array. No consistent blood flow increases were found in cerebral cortex outside of the visual system, but increases were seen in the thalamus and cerebellum. Although most tasks involve increases in arousal, establishing an intention or behavioral goal, setting up control structures for sequencing task operations, detecting targets, etc., these operations do not produce blood flow increases, detectable with the present methods, in localized cortical regions that are common across tasks. Common subcortical regions, however, may be involved.

A left cerebellar and a medial cerebellar focus reflected motor-related processes. Blood flow increases in these regions only …