Simple reaction times to lateralized light flashes. Varieties of interhemispheric communication routes.

Simple unimanual reaction times to lateralized light flashes were measured in 40 normal subjects, 4 commissurotomized patients, and a boy with callosal agenesis. In all subjects, reaction times tended to be shorter when the stimuli were presented on the same side as the response hand (uncrossed condition) than on the opposite side (crossed condition). In 2 experiments, the magnitude of the crossed-uncrossed difference (CUD) was greatest in the commissurotimized patients (ranging from 35 to 96 ms), the acallosal boy showed an intermediate CUD (20 and 12 ms for the first and second experiment, respectively), and normal subjects exhibited the smallest CUD (an insignificant 1 ms difference and a significant 3 ms difference in the first and second experiment, respectively). For all subjects, reaction times increased when the stimulus light intensity was decreased (Experiment 1) or when stimulus eccentricity was increased (Experiment 2). However, the effects of these visual parameters on the CUD revealed group and individual differences. Neither stimulus light intensity nor eccentricity affected the CUD in the normals, in 1 of the commissurotomized patients, and in the acallosal boy. The CUDs in these subjects are attributed to the transfer of nonsensory information via the corpus callosum, via ipsilateral corticospinal tracts, and via the anterior commissure or ipsilateral motor pathways, respectively. The CUDs of 2 commissurotomized patients varied with stimulus eccentricity but not intensity. Subcallosal interhemispheric visual transfer is thus apparently more sensitive to stimulus eccentricity than to intensity. The final commissurotomized patient, R.Y., had slower and less accurate responses to stimuli presented in the right visual hemifield during left, but not right, hand responses. An asymmetry in subcortical visual interhemispheric communication is suspected in this patient. The disparate results amongst these subjects are discussed and contrasted with previous findings. Finally, an information processing model of cortical and subcortical interhemispheric communication is presented.