Callosal inhibition: the key to the brain code.

A mechanism for interhemispheric communication is proposed based upon the assumption of homotopic callosal inhibition. The implied pattern of cortical activity in each cerebral hemisphere is shown to be the mirror-image and "photographic negative" of the contralateral pattern. Mirror-imagery is a direct consequence of (1) approximate cerebral hemispheric bilateral symmetry and (2) homotopic callosal connections. The photographic negative relationship is a direct consequence of (1) bilaterally symmetrical activation of the cortex via subcortical "attentional" mechanisms and (2) inhibition by a unilaterally established engram of an identical cortical pattern contralaterally. In language generation and understanding, excitation predominantly in the left hemisphere produces on the right inhibition of language-related neurons as well as excitation of all surrounding "contextual" neurons. The suggested dichotomy of verbal and contextual functions of the cerebral hemispheres is shown to be supported by previous findings concerned with cognition in brain-damaged patients. Unlike earlier suggestions of linguistic and paralinguistic specializations in the left and right, the mirror-image negative hypothesis is based upon an explicit physiological process rather than unspecified differences in "information processing."