Tactile Discrimination Performance in the Monkey: The Effect of Bilateral Posterior Parietal and Lateral Frontal Ablations, and of Callosal Section

Abstract

Four groups of monkeys were trained to discriminate by touch in the dark between five pairs of test objects. One group consisted of four animals having bilateral ablations of posterior parietal cortex; another group consisted of four animals with bilateral removals of lateral frontal cortex; four monkeys having callosal section served as operated controls; and three animals formed an unoperated control group. A further group of two animals with posterior bi-parietal removals received the same training as did the remaining 15 animals, but these two animals made the discriminations in the light, by vision. Two of the five tactile discrimination tasks were given both before and after surgery, the other three only after surgery. The four parietal animals that were tested in the dark were significantly impaired on four of the five tactile discrimination tasks; but there was no evidence of impairment in the two parietal animals tested in the light. The frontal animals were significantly impaired only on two of the five tactile discrimination tasks, but unlike the parietal group were severely impaired at learning a test of delayed alternation. The callosal animals were not impaired at any stage of their training. The nature of the tactile defect in the parietal animals received detailed consideration. It was concluded that the more recent findings accord with an earlier suggestion, namely that posterior parietal ablations give rise in the monkey to a selective motor retardation which, in the dark, is the origin of the defective performance on tactile tasks. Further discussion concerned tactile performance in the frontal animals. It was concluded that no single factor (such as the requirement that a preference be overcome, that the test be difficult or unfamiliar, or that the animal be severely impaired at delayed alternation) was of especial importance in giving rise to tactile defect in the lateral frontal animals; and that more consistent and severe tactile defect might follow from ablations of the adjacent orbital frontal cortex.