Visual recognition: evidence for two distinctive mechanisms from a PET study.

In this study, we examined the hypothesis that two distinct sets of cortical areas subserve two dissociable neurophysiological mechanisms of visual recognition. We posited that one such mechanism uses category specific cues extractable from the viewed pattern for the purpose of recognition. The other mechanism matches the pattern to be recognized with a pre-encoded memory representation of the pattern. In order to distinguish the cortical areas active in these two strategies, we measured changes in regional cerebral blood flow (rCBF) with positron emission tomography (PET) and (15)O Butanol as the radiotracer. Ten subjects performed pattern recognition tasks based on three different short-term memory conditions and a condition based on visual categories of the patterns. When subjects used representations of the patterns held in short-term memory for the purpose of recognition, the precunei were bilaterally activated. Recognition based on visual categories of the patterns activated the right (R) angular gyrus, left (L) inferior temporal gyrus, and L superior parieto-occipital cortex. These findings demonstrate that the R angular gyrus, the L inferior temporal gyrus, and the L superior parieto-occipital cortex are associated with recognition of patterns based on visual categories, whereas recognition of patterns using memory representations is associated with the activity of the precunei. This study is the first to show functional dual dissociation of active cortical fields for different mechanisms of visual pattern recognition.