Simultaneous and successive visual discrimination by monkeys with inferotemporal lesions
Journal of Comparative and Physiological Psychology
Impairment in the performance of visual discriminations following inferotemporal cortical resections appears to be related to the parameter of "difficulty" (3, 5, 7), difficulty being defined in terms of the performance scores achieved by a control group on a given series of tasks. In a recent experiment (4) the difficulty of a discrimination was gradually increased by varying the physical dimensions (size) of the discriminanda: Although the animals with inferotemporal resections achieved
... ions achieved perfect scores on the initial discrimination, they showed a decrement in performance (compared with controls) when the difference between the stimuli was reduced. The present experiments were designed to investigate whether such decrement occurs only when difficulty is a function of the physical dimensions of the discriminanda, or whether the impairment may be revealed, as well, when difficulty is a function of certain "situational" variables which determine the differential response. In the two experiments reported below all animals were first trained to criterion on a particular discrimination and then transferred to one or more variations of that discrimination using the identical discriminanda. lIIETHOD Subjects In the first experiment nine immature rhesus monkeys were used; in the second experiment six of these monkeys plus four others served as 5s. All animals were trained in apparatus described previously (5). Procedure Experiment 1. The nine 5s were divided into three groups of three animals each. All groups were trained, with the rerun correction technique, to a criterion of 90 correct in 100 consecutive trials, in the discrimination of a plus vs. a square each painted yellow on a gray 1 We wish to express our appreciation to Mrs. Marilyn Tucker and Miss Lila Rupp for their technical assistance and to the Department of the Army which, through grant No. DA-49-007-MD-401, made this study possible. 198 background. To two of the groups (5s 4, 15, 26 and 2, 3, 5) the discriminanda were presented simultaneously: correct choice, rewarded with food, depended on opening that one of two containers covered by the plus.sign. To the third group (5s 51, 52, 54), the discriminanda were presented successively: correct choice depended on opening the single centered cup (baited) when it was covered by the plus sign, and not opening the single cup (un baited) when it was covered by the square ("gono-go"). Animals were permitted 5 sec. in which to respond. The correct "no-go" responses were not rewarded (except by termination of the correction trials). Following the training in the initial discrimination each group was given the other discrimination, i.e., the animals trained first in the simultaneous procedure were then trained in the successive, and conversely. After the preoperative training had been completed all animals were subjected to bilateral one-stage cortical resections. The "successive-simultaneous" group and one of the "simultaneous-successive" groups received inferotemporal lesions; the other "simultaneoussuccessive" group received anterolateral-frontal lesions. The surgical and anatomical methods have been described elsewhere (3). Reconstructions of the temporal removals are shown as the first six diagrams in Figure 1 . The figure also shows representative cross sections through the lesions and enlarged sections through the thalami. Reconstructions of the frontal lesions are reported separately (6). Postoperatively, all groups were retested on both discriminations, presented in the preoperative order. Experiment 2. The ten 5s were divided into three groups. Four monkeys (5s 4, 15, 26, 37) had received bilateral inferotemporal resections; four (5s 2, 3, 5, 11) had received bilateral a,nterolateral-frontal resections; and two animals (5s 31, 36) served as nonoperated controls. (Three of the temporal and three of the frontal operates had comprised the two groups given identical training in Experiment 1.) Reconstructions of the lesions of the inferotemporal operates are shown as the last four diagrams in Figure 1 . Those of the frontal controls are reported separately (6). In the present experiment all groups were trained in the same manner. The discriminanda, a tobacco tin and an ash tray, were presented in three different situations: (a) simultaneously, choice consisting of opening one of two containers covered by the stimuli; (b) successively, choice consisting of opening or not opening a single centered cup covered by the single stimulus ("go-no-go"); and (c) successively, as heretofore, with a single stimulus presented in the center, but with opportunity for response the same as in the simultaneous procedure, i.e., opening the right or left of two simultaneously presented containers ("conditional"). Animals were trained with the rerun correction technique, and were permitted 5 sec. in which to respond.