IBOTENIC ACID LESIONS OF THE SUBSTANTIA-NIGRA PARS RETICULATA IPSILATERAL TO A VISUAL CORTICAL LESION FAIL TO RESTORE VISUAL ORIENTING RESPONSES IN THE CAT

Unilateral removal of all known visual cortical areas in tile cat rend ers the animal hemianopic in the contralateral visual field as measure d by visual perimetry and other behavioral tests. We have shown that v isual orientation behavior can be restored to the previously blind hem ifield by destruction of a critical zone in the substantia nigra pars reticulata contralateral to a cortical lesion (Wallace et al., J. Comp . Neurol. 296:222-252, 1990). The model proposed to explain this recov ery postulates that damage to the crossed nigrotectal projection disin hibits the superior colliculus ipsilateral to the cortical lesion and this leads to recovery. If disinhibition can account for recovery, the n destruction of the uncrossed nigrotectal projection, which is known to exert a tonic inhibition on the superior colliculus, should also re sult in recovery. We made unilateral visual cortical ablations and ips ilateral ibotenic acid lesions of the substantia nigra pars reticulata . Visual orienting behavior was assessed in animals for a period of 4 to 31 weeks. Contrary to the prediction of the model, we failed to obs erve a recovery of visual orienting behavior in the blind hemifield in any of 23 animals. (C) 1997 Wiley-Liss, Inc.