Retention of conditional responses to light following lesions to the striate cortex
Phoenix, Charles H
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It is generally agreed that an animal conditioned to a visual intensive stimulus retains that response, at least to some degree, after ablation of the striata area. The conclusion is based on studies that have been restricted in several respects. First, the responses conditioned have been limited to the eyeblink and leg flexion. Second, the experiments hage been carried out only on monkeys and dogs. Furthermore, the research in this area, with the exception of an experiment by Wing in which he measured the post-operative retention of a response to direction of light change, has been restricted to use of a single light stimulus. From the available data it is impossible to determine whether a differential response to two stimuli would survive removal of the striate area when the responses are acquired in a conditioning situation. The present study was undertaken (1) to determine the effect of partial and total extirpation of the striate area of the cortex on the retention of a conditioned differentiation of two light stimuli; (2) to investigate the effect of partial lesions to the striate cortex on previously acquired conditioned response; (3) to determine whether or not findings based on research with dogs and monkeys apply to animals lower on the phylogenetic scale; and to learn whether or not lesions to the striate area of the cortex produce the same effect on the retention of previously learned gross bodily responses as are produced on segmental responses acquired in a conditioning situation. Two studies were carried out in order to investigate these problems. The first study employed cats as subjects and the second study used rats. In Study I, four adult male cats were trained in a conditioned avoidance situation. A light served as the conditioned stimulus and electric shock as the unconditioned stimulus. The CS-US interval was 2.8 seconds. Responses occurring within this interval resulted in shock avoidance and were termed conditioned avoidance responses. The response latency was recorded by the experimenter. Each day, the animals were given thirty trials in the apparatus, and when the percentage of conditioned responses to the light stimulus had reached a comparatively high and stable level a second conditioned stimulus, a buzzer, was introduced. Training was continued to the two stimuli, presented in random order, until the animals had received a minimum of three hundred trials (15 trials to each stimulus per day) with a variation of less than four conditioned responses in the daily score to either stimulus for six consecutive days. After achieving criterion the animals were allowed a "forgetting" period which varied from 53 to 61 days. At the end of this period the cats were given a retention test. The test consisted of fifteen trials with the shock circuit disconnected, and with the stimuli again presented randomly. Following the test, the animals were given several days additional training to restabilize the response. When retraining was complete the cats were brought to surgery. Lesions were made in the marginal gyrus and parts of the suprasylvian gyrus. The cortical area removed or destroyed corresponded approximately to the area described by anatomists as containing the projections of the lateral geniculate bodies. Following a fourteen day recovery period, the animals were tested for retention using the procedure employed preoperatively. At necropsy the brains were examined and drawings of the lesions made with the aid of a camera lucida. The second experiment (Study II) was performed specifically to determine whether or not an animal conditioned to respond differentially to two light stimuli would retain that response when tested following lesions to the striate cortex. The training apparatus consisted of a circular grid runway 88 inches in circumference enclosed in acetate walls and divided electrically into six sectors. Switches were so connected as to allow the experimenter to energize any of the six grid sectors of the apparatus. The subjects consisted of twenty-four, naive, male albino rats, appaoximately ninety days old at the beginning of the experiment. The animals were given ten trials per day to each stimulus. When presented with the positive stimulus (Sp) the animal could avoid shock by going to a different sector within 2.5 seconds in which event the animal was credited with a conditioned response. If the animal failed to leave the sector on which he was located at the onset of the positive stimulus within 2.5 seconds, he received approximately .4 milliamperes of current delivered through his feet as he stood on alternate grids of the floor. He could escape the shock by going to a different sector. Movement from one sector at any time during training was termed a response. If the animal failed to respond within five seconds, the stimulus went off automatically and the shock circuit was opened and reset automatically for the next trial. The light stimulus was turned off the moment the animal responded. The latency of the response was recorded. Training to the negative stimulus (Sn) followed essentially the same procedure used in training to the positive stimulus except that in no case was the animal shocked following the onset of the negative stimulus. Response within 2.5 seconds of the onset of the negative stimulus was considered an error and termed a generalized conditioned response. Animals received training to the two stimuli in blocks of ten trials. The blocks or sessions were separated by a five hour period in a given day. The order of presentation was reversed from day to day. The learning criterion consisted of 80 per cent conditioned responses or more to the Sp and no more than 20 per cent generalized conditioned responses to the Sn. Upon achieving criterion the rats underwent extirpation of the striate area of the cortex. Following a five day recovery period, they were given ten trials to the Sp and ten trials to the Sn with the shock supply also disconnected. On the following day, retraining trials were started and continued until the animals again achieved criterion. After reattaining criterion, four of the animals were given a series of extinction trials. A number of control animals were used to insure that (1) the task set for the animals could not be solved by use of non-visual cues; (2) loss, should it occur, could not be attributed to the trauma associated with the operation; (3) the task would be retained by normal animals over a period of time comparable to the recovery period following operation. Seventeen days after operation, the animals were anesthetized with ether and bled to death. The brains were removed from the skulls and fixed. They were stained by Ranson's pyridine-silver modification of the Cajal method. The following results were obtained: Two cats with partial lesions to the striate cortex gave approximately 100 per cent conditioned responses to both the visual and auditory stimulus in the first half of the test session following operation. The responses showed no significant increase in latency. (p > .05), as determined by the Mann-Whitney U Test. A third cat gave similar results with respect to the auditory stimulus but failed to respond to the visual stimulus on one half of the trials. A fourth cat died before he could be tested. Three of the rats underwent complete extirpation of the striate area. These animals gave between 0 and 40 per cent conditioned responses to the Sp in a series of ten extinction trials following operation. The percentage of responses to the Sn remained low. The remaining animals with partial lesions gave from 0 to 80 per cent conditioned responses in the extinction test to the Sp. The percentage of generalized conditioned responses to the Sn remained low in all cases. The response latencies, for all but two animals, in the extinction test to the Sp were significantly greater (p < .05) than in the last preoperative session as determined by the Wilcoxon T. Savings scores were calculated by dividing the difference between the number of initial and postoperative trials to criterion by the number of trials initially required. The amount of savings varied from 73.3 to 100 per cent. The results of the two studies lead to the following conclusions: (1) Animals undergoing complete extirpation of the striate area of the cortex following acquisition of a differential conditioned response to two light stimuli differing intensity retain that differentiation postoperatively when retention is measured by the savings method. Animals with partial lesions give similar results; (2) Both cats and rats undergoing partial lesions to the striate area following acquisition of a conditioned response to light retain that response postoperatively; (3) Survival of a conditioned response to a visual intensive stimulus following complete ablation of the striate area is not restricted to dogs and monkeys but applies also to rats; (4) postoperative retention of a conditioned response to light is not restricted to behavior involving segmental responses. Rats with complete extirpation of the striate area, and both cats and rats with incomplete extirpation of the striate cortex retain a conditioned response postoperatively when the response involves gross bodily movement; (5) The latency of conditioned responses to light as measured in extinction trials was found to increase in all but two rats following lesions to the striate area of the cortex. The response latencies returned to their preoperative level after few reinforcements; (6) Cortical lesions involving the projection area of the lateral anterior nucleus has no effect on the retention of a conditioned differentiation of two light stimuli; (7) Conditioned avoidance responses to light stimuli are not particularly resistant to extinction following partial lesions to the striate area.
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