Simple reaction times to lateralized visual stimuli in a case of callosal agenesis

Simple reaction times (RTs) of a young acallosal woman were measured using three different intensity levels of the visual stimulus. It was found that "'crossed" stimulus/hand combinations yielded finger RTs about 20 msec longer, on average, than "uncrossed" combinations. In addition, an inverse relation was found between stimulus intensity and the magnitude of this crossed/uncrossed difference in RT. It was also found that vocal RTs displayed a large advantage (18 msec) for right visual-field stimulation. It is suggested that the stimulus information in crossed finger reactions must cross the midline through visually-coded neurones in the acallosal brain.