The effect of the sequence of contralateral cortical and collicular lesions on the rate of relearning a visual discrimination by cats.

Visual field deficits which follow ablation of the posterior two-thirds of one cerebral hemisphere in the cat have been shown to be ameliorated by ablation of the contralateral superior colliculus. The present study was designed to determine whether the sequence in which contralateral cortical and collicular lesions are made is a significant factor affecting the rate of monocular relearning through the eye ipsilateral to the cortical lesion. Nine adult cats had the optic chiasm sectioned, were trained to criterion monocularly on a dark-light discrimination, and were divided into two groups. Four cats (Group S) had one superior colliculus ablated, followed one month later by ablation of the contralateral two-thirds of the neocortex. Five cats (Group C) had the same lesions in the reverse order. One month after the second brain lesion, each cat was retrained monocularly through each eye on the dark-light discrimination for food reward in a Y-maze. Through the eye ipsilateral to the cortical lesion, all cats in Group S relearned to criterion more rapidly than cats previously reported with only unilateral neocortical lesions. Four of the five cats in Group C were severely retarded in relearning and two of these cats failed to relearn. The results show that when chiasm-sectioned cats are trained monocularly in a Y-maze on a dark-light discrimination, ablation of the superior colliculus prior to ablation of the contralateral neocortex facilitates relearning through the eye ipsilateral to the cortical ablation. When ablation of the superior colliculus follows the neocortical lesion, learning occurs but is not facilitated by the additional lesion.