The effects of lesions of the superior colliculus on visually guided behaviour and general activity in the rat.

From an initial review of the literature on the superior colliculus it was apparent that there were a number of discrepancies concerning the effects of lesions upon general activity and discrimination performance in the rat. Subsequent experiments further investigated these effects. It was demonstrated that colliculectomised rats were transiently impaired in retention of multi-choice discriminations and that the deficit transferred to a two-choice task. In this task, it appeared that operated animals were unable to inhibit on-going responses. Initial learning of simple discriminations was normal. Later tests of the retention deficit demonstrated that operated rats performed faster than normals in the initial stages of a trial, but that simple running speeds were the same. This latency difference lasted throughout and after relearning. It was suggested that post-colliculectomy deficits in such tasks resulted from a locomotor disinhibition effect which caused animals to initiate responses before completing processes necessary for a successful outcome of the trial. In open-field tests, colliculectomised rats were hyperactive, but less responsive to novel visual stimuli than normals. Operated animals responded to novel auditory stimuli with startle whilst normal animals exhibited orienting responses. In the home cage, there was no difference in activity between the normal and operated rats, and there was no running speed difference in a simple runway. Operated and normal animals in the home cage responded differently to changes in the light cycle. A hypothesis was proposed to account for deficits in certain discrimination tasks in terms of locomotor disinhibition, which was also used to account for locomotor hyperactivity in the open-field. The hypothesis proposed that the rat tectum mediates orienting responses to novel stimuli, and that integral with this response is an inhibition of on-going locomotor activity. This hypothesis was reviewed in the light of previous literature regarding rats and other species.