Stimulus-dependent form discrimination deficits in cats with superior colliculus lesions.

Previous studies have shown that cats with damaged superior colliculi are slower than normal in learning new visual discriminations based upon shape but show no losses in retention of discriminations learned prior to the tectal lesion. To elucidate the possible mechanisms producing this deficit, the rate at which cats with superior colliculus lesions learned to discriminate between targets differing either in (1) global features, or (2) a single, uniquely localized, feature was measured. Cats were first tested with problems shown in earlier studies to produce acquisition deficits and those with superior colliculus lesions showed the expected deficits. The rate of acquisition of form discriminations based on global stimulus features in these cats was, however, comparable to normals and sham-operated controls. When tested with targets differing only in a single, localized feature, acquisition of the discrimination was severely impaired compared to controls. The observed deficits did not depend upon damage to pretectum or total ablation of the superior colliculus. The results support the view that the superior colliculus is a major contributor to visuomotor integration (e.g. foveation of salient stimulus features), and that such visuomotor behavior is an important determinant in form discrimination learning.

Movement discrimination capacities in the cat.

Cats were trained to discriminate moving from nonmoving targets or one direction of movement from another. Various stimulus changes, e.g., size, direction, and rate, were then introduced as a test for generalization of the dimension of movement. Thresholds for detection for minimal movement were also determined. The results showed that (a) for cats, discrimination of movement is more difficult than discriminations based on brightness; (b) the dimension of movement is completely generalized across stimulus configuration but incompletely generalized for direction of movement; (c) the mean movement detection threshold was found to be 3.3 degrees/sec; (d) the thresholds for minimal movement and direction of movement were essentially identical; and (e) stimulus-viewing strategies were found to play an important role in the threshold determinations. The results of the generalization tests are consistent with the physiological properties of neurons found to be sensitive to movement in the cat visual system. The movement threshold values were found to lend support to the view that resolution and slow movement thresholds are correlated.

The role of superior colliculus in vision: visual form discrimination in cats with superior colliculus ablations.

The rate of acquisition of visual form discriminations in cats with superior colliculus damage was measured in a testing apparatus requiring a minimal amount of stimulus localization in space. In animals with bilateral superior colliculus ablations, acquisition was imparied relative to acquisition in normal animals. In these animals, the extent of the lesion was related to the severity of the deficit. In split-brain preparations with unilateral superior colliculus ablations, acquisition with the eye ipsilateral to the lesion was impaired relative to acquisition with eye contralateral to the lesion. The deficits observed did not depend upon damage to pretectum. The results support the conclusion that the superior colliculus participates in certain aspects of form vision.