Ablation study of the superior colliculus in the tree shrew ( Tupaia glis )

Abstract The main conclusions rest on the results obtained by ablating the superior colliculus in eight tree shrews. Additional animals either normal or with lesions of the superior collculus or striate cortex were studied to support or extend the basic findings. All eight tree shrews showed a profound loss of pattern discrimination which could not be attributed to some general factor such as inattention or an incapacity to learn. Thus, all succeeded in discriminating differences in the orientation of stripes but all failed to learn a habit based on discriminating the orientation of triangles. Two of the eight appeared normal in their capacity to orient toward, follow and track objects on the basis of vision. Six of the eight showed a remarkable unresponsiveness to visual stimuli manifested by a failure to follow, track, orient or even avoid what normal animals would regard as a threatening stimulus. This difference was clearly related to the extent of the lesion. In the two cases with normal visually guided behavior only the superficial three layers of the superior colliculus were removed. In the six cases with visual neglect the lesions included the deeper layers of the superior colliculus and invaded adjacent structures, notably the pretectum. It was concluded on the basis of differences between superficial and deep layers in their connections and in the behavioral effects of their ablation that the superior colliculus consists of at least two subdivisions.