DISINHIBITION OF THE SUPERIOR COLLICULUS RESTORES ORIENTING TO VISUAL-STIMULI IN THE HEMIANOPIC FIELD OF THE CAT

Following unilateral removal of all known visual cortical areas, a cat is rendered hemianopic in the contralateral visual field. Visual orie ntation can be restored to the blind hemifield by transection of the c ommissure of the superior colliculus or by destruction of the superior colliculus (SC) or the substantia nigra pars reticulata (SNpr) contra lateral to the cortical lesion. It is hypothesized that a mechanism me diating recovery is disinhibition of the SC ipsilateral to the cortica l lesion. The ipsilateral nigrotectal projection exerts a robust inhib itory tone onto cells in the SC. However, ibotenic acid destruction of SNpr neurons, which should decrease inhibition onto the SC, does not result in recovery. The failure of ipsilateral SNpr lesions to produce recovery puts into question the validity of SC disinhibition as a mec hanism of recovery. We directly tested the disinhibition hypothesis by reversibly disinhibiting the SC ipsilateral to a visual cortical lesi on with a gamma-aminobutyric acid (GABA)(A) antagonist, bicuculline me thiodide. In accordance with the hypothesis, transient disinhibition o f the SC restored visual orienting for several hours in three of eight animals. Recovery was not a volume or pH effect and was distinct from the release of irrepressible motor effects (i.e., approach and avoida nce behaviors) seen within the first hour after injection. Thus, in th e absence of all visual cortical areas unilaterally, disinhibition of the SC can transiently restore the ability of the cat to orient to vis ual stimuli in the previously ''blind'' hemifield. J. Comp. Neurol. 38 7:568-587, 1997. (C) 1997 Wiley-Liss, Inc.