COLLATERAL INHIBITION OF TRANSCALLOSAL ACTIVITY FACILITATES FUNCTIONAL BRAIN ASYMMETRY

The corpus callosum is the largest connection between the functionally asymmetric cerebral hemispheres. The objective of this study was to m easure functional activity of callosal fiber tracts during speech proc essing. We analyzed the regional glucose metabolism of the corpus call osum and of speech-relevant cortical areas in 10 normal individuals at rest and during word repetition. We used three-dimensionally register ed magnetic resonance imaging to visualize the individual brain morpho logy and high-resolution positron emission tomography for metabolic me asurements. The task-induced metabolic changes of the callosal midbody and isthmus had a significant negative correlation with key regions o f language processing in the left inferior frontal cortex (Brodmann's area 44) and in the right superior temporal cortex (Brodmann's area 22 ) (e.g., correlation of metabolic changes in the surface aspects of th e right Brodmann's area 22 and the callosal mid-body/isthmus: r = -0.9 1, P < 0.001). The study indicates that language processing in asymmet rically organized cortical areas inhibits the reciprocal transcallosal information exchange in favor of the lateralized mental operation. Ou r data agree with anatomic, electrophysiologic, and pharmacologic expe riments that point to the important role of collateral inhibition for the transcallosal information exchange.