Simple reaction time: evidence for focal impairment from lesions of the right hemisphere.

Simple reaction time is significantly increased in patients with unilateral lesions of either cerebral hemisphere responding with the hand ipsilateral to the lesion, but the effect is much greater when the lesion is in the non-dominant hemisphere. This difference cannot be attributed to asymmetries in the size or type of lesion. It applies over the complete distribution of reaction times as well as to the means, and is not diminished by practice. Neither the classical conduction model of reaction time nor the mass-action hypothesis proposed by De Renzi and Faglioni (1965) can account for the findings. Although the increase of about 100 ms observed with lesions of the dominant hemisphere probably represents a non-specific effect of brain disease, the much larger increases resulting from lesions of the non-dominant hemisphere appear to be of focal origin. The critical areas cannot yet be specified with certainty, but maps of the brain scans for patients with the highest reaction times suggest that structures in or near the basal ganglia and the posterior parietal region of the non-dominant hemisphere are involved, and other studies show that the focal effect is not found when patients with contralateral motor involvement are excluded.