Loss of stereopsis following lesions of cortical areas 17–18 in the cat
Seventy-five cats learned a shape discrimination with zero, one, or two irrelevant cues. They were then subjected to sham operations (n = 34), ablation of the marginal and splenial gyri (n = 9), or lesions in the extramarginal (EM) cortex. The 32 EM cats comprised four groups, three with small (EM1, n = 9), intermediate (EM2, n = 10), and large (EM3, n = 9) decortications, and a fourth group with both EM lesions and heavy degeneration in the lateral geniculate nucleus (LGN, n = 4). The cats with marginal or extensive extramarginal lesions were severely impaired in shape and size discrimination. Two lines of evidence indicate that their behavioral defects resulted from different neural dysfunctions. (a) The errors made by marginal gyrus cases increased sharply as a function of the number of irrelevant cues present in shape discrimination training; no other group, including Group EM3, was affected by this variable. (b) Cats with extramarginal ablations and strong LGN degeneration were no more severely impaired than were subjects with comparable extramarginal damage and little or no LGN degeneration. While the nature of the two kinds of deficits remains unclear, they seem parallel to those following posterior cortical lesions in monkeys.