Extrageniculostriate vision in the monkey. VIII. Critical structures for spatial localization.

Eight monkeys (Macaca mulatta) were trained on a spatial localization task before and after either total ablation of the striate cortices with partial damage to circumstriate cortices, complete colliculus removals, or combined ablations. The lesions were histologically verified. The task, given under normal room illumination, required the animals to reach for a target randomly placed in one of eight equal segments of a while disk located at arm's length. An apple cube, affixed to the center of the target, served as the reward for accurate reaching. A correct response consisted of an initial contact with either the target or the apple cube directly. The animal was trained sequentially to a criterion level of performance set at 90% correct responses over four 56-trial consecutive sessions using black disks of decreasing diameter (i.e., 90 mm, 55 mm, 35 mm, and 15 mm). Finally, only the apple cube, about 10 mm on a side, was presented. Four animals with complete or almost complete bilateral superior colliculus removals were unimpaired in their performances. The other four subjects with total striate cortex removals could be trained to reach accurately for all targets, but with marked deficits on the first and last tests, i.e., the 90 mm disk and the apple cube. When the striate cortex was ablated in the colliculectomized animals, they failed to attain a criterion level of performance in almost 6,000 trials. These results indicate that the superior colliculus is at least one structure that is critical for spatial localization in the absence of striate cortex, but it is not crucial for this capacity in the otherwise intact animal. In addition, the findings confirmed previous studies which showed that monkeys with total bilateral striatectomies could require the ability to execute accurate visually guided reaches. The present and earlier studies emphasize the primary importance of the geniculostriate system for the function explored and do not support a sharp dichotomy between two independent visual systems in the monkey.