The cerebral hemispheres may be mutually inhibitory. An injured hemisphere may continue to be inhibited while the uninjured hemisphere is released from inhibition, inducing attentional bias. Patients with neglect might therefore benefit from transection of the corpus callosum. To test this hypothesis, we ablated the frontal arcuate gyrus alone or several months after corpus callosum transection in monkeys. Neglect was significantly worse in animals with a corpus callosum transection. Therefore, the hemispheres may be mutually excitatory or compensatory, and a callosal transection would not be therapeutic. Because the rate of recovery in the two groups was not different, recovery seemed to be an intrahemispheric phenomenon. © 1984 American Academy of Neurology.