Interhemispheric transmission times in the presence and absence of the forebrain commissures: effects of luminance and equiluminance.

One subject (L.B.) with full forebrain commissurotomy, one (R.B.) with callosal agenesis and 20 normal controls were tested for simple reaction time (RT) with each hand, to visual stimuli in one or the other visual field. RTs for uncrossed conditions (hand ipsilateral to the visual field) were subtracted from RT to crossed conditions (hand contralateral to the visual field) to yield the crossed-uncrossed difference (CUD), taken to be a measure of interhemispheric transfer time. CUDs increased from an average of 4.9 ms among the control subjects, to 23.3 ms for R.B., to 53.1 ms for L.B. Although overall RTs in all subjects increased with decreasing luminance of the stimuli, the CUD was not systematically affected and remained largely unaffected even under equiluminance. The results support previous evidence that interhemispheric transfer, even in the split brain, depends on visually insensitive pathways.