Extrageniculostriate vision in the monkey III. Circle vs triangle and "red vs green" discrimination

Four monkeys were trained on 3 visual discriminations using a two-choice pulling-in technique with movable transilluminated stimuli, before and after histologically verified total bilateral removal of area 17, most of 18, and part of 19. Test 1 consisted of one small bright circle vs a large, dim circle closely matched for total luminous flux. Test 2 was a circle vs a triangle of equal brightness and approximately equal area and luminous flux. Test 3 contained a "red" circle vs a "green" circle of equal area. Preoperatively, all monkeys mastered the 3 tests. Postoperatively, test 1 was relearned by all animals with 6.7 fold increase in the mean error score. Test 2 was solved by all subjects to a criterion of 18 correct out of 20 responses, and by only 3 to a level of 90% correct in 200 trials after considerable training. Test 3 was mastered by all monkeys. Controls were carried out following each of the tests. The principal type involved the introduction of a fixed additional density to the right side stimulus box. After test 3, this manipulation was made in increasing steps to the point of complete occlusion of the stimulus on the right side. For test 2, variations in orientation, relative size and brightness were also part of the controls. The results of the original tests and controls confirmed and extended previous findings showing that monkeys without striate cortex can discriminate luminous flux-equated figures, that prior training on luminous flux differences is not a prerequisite, and that they can master a circle vs triangle, and "red vs green" type of visual discriminations.