The nature of the visual discrimination impairment after neonatal or adult ablation of superior colliculi in rats.

The superior colliculi were removed in rats at either one or five days of age or in adulthood. Seven months later they were tested on four successively presented two-choice intensity discriminations. The intensity difference between the discriminanda was reduced across the four problems to encourage choice by comparison. The purpose was to establish whether impoverished scanning is a feature of rats with collicular lesions and whether the age at which the lesion is incurred is important. The number of door-push and approach errors made in reaching criterion were used as measures of performance and the number of head-scans during acquisition was counted. The results provide no evidence that either one- or five-day operated rats exhibit sparing or recovery of the ability to scan discriminanda since all operated animals were impaired. Furthermore, novel retinal projections, present in one-day operated animals, fail to mediate such sparing. Finally, the results did not demonstrate a selective increase in approach errors following collicular lesions and were therefore inconsistent with the view that the impairment is one of visually-guided locomotion. It is concluded that visual discrimination learning is impaired following collicular lesions in circumstances where scanning of discriminanda is required for efficient performance.