Interhemispheric neural summation in the absence of the corpus callosum

One subject with full forebrain commissurotomy (L.B.), two with callosotomy (J.W. and M.E.), one with callosal agenesis (R.B.) and 10 normal subjects performed a simple reaction time task in which visual stimuli were either presented singly in one or other visual field, or in both visual fields simultaneously. Reaction times were faster to double stimuli than to single ones, but in the normal subjects this 'redundancy gain' did not exceed that predicted by probability summation (the horse-race model). In the four subjects lacking the corpus callosum, the gain did exceed that predicted by probability summation when the stimuli were brighter than the background, implying subcortical neural summation. In the three surgical cases (L.B., J.W. and M.E.) the gain was greatly diminished when the stimuli were equiluminant with the background, suggesting that neural summation occurred at the collicular level. In normal subjects, callosal transfer may ensure that at least some degree of interhemispheric neural summation occurs, even with unilateral input. The acallosal subject (R.B.) was anomalous in that neural summation was not diminished by equiluminance.