THE NEURAL BASIS OF LEARNING 



'479 



able to make progress in learning the discriminations. 

 Individuals using the hand on the side opposite to 

 their injuries were not, however, able to learn the 

 discrimination. This failure to learn was not related 

 to any sensory defect. Hence, we may conclude that 

 at least some kinds of learning depend on the hemi- 

 sphere receiving the main sensory projections of the 

 somesthetic system. 



This finding is related to a recent study of the 

 function of the corpus callosum in the contralateral 

 transfer of a somesthetic discrimination in the cat 

 (228). Cats were trained to use one paw to push the 

 correct one of two levers on the basis of tactual form, 

 softness or roughness (three different habits). After the 

 discrimination had been thoroughly learned, the cats 

 were required to make the same discrimination with 

 the forepaw not used in the original training. Normal 

 animals did this with relatively little additional train- 

 ing, but cats in which the corpus callosum had been 

 sectioned prior to the experiment took as many 

 trials to learn it as they had in learning the original 

 discrimination. Hence, the corpus callosum seems to 

 be essential to the transfer of somesthetic habits from 

 one side to the other just as it is in the experiments 

 cited above on intcrocular transfer. 



delayed reaction. As described earlier onl\ ;i slight 

 change in procedure is necessary to convert the 

 conventional discrimination problem into a test of 

 delayed reaction (or response). In this test, the 

 animal is shown the correct stimulus, then required 

 to wait for a few seconds or minutes before being 

 allowed to make its discrimination between two 

 stimuli. As first employed by Jacobsen (106), it was 

 regarded as a test of immediate memory. The factor, 

 however, that often determines success or failure in 

 the test is whether or not the animal pays attention 

 to the correct stimulus when it is first shown. 



Jacobsen first demonstrated that monkeys with 

 frontal lesions have difficulty in the delayed reaction 

 test (106). This fact has been confirmed many times 

 in subsequent studies (16, 65, 86, 155), but it is not 

 a universal finding for such monkeys sometimes can 

 succeed in the test (33, 167). Moreover, by making 

 such minor changes in the procedure that the animal's 

 chances of attending to the presentation of the correct 

 stimulus are increased, substantially all these animals 

 can succeed in passing the test (63, 151, 223). This 

 fact indicates that the impairment is more one of 

 attention than of immediate memory (85, 87). There 

 is some evidence that an area lying in the dorsolateral 

 frontal region anterior to the precentral motor cortex 



is especially important in delayed response per- 

 formance (194, 197). There is also evidence that 

 areas outside the frontal lobe are not important 

 ( I0 7> ! 54)» although a deficit in delayed reaction is 

 reported occasionally in animals having posterior 

 lesions (130). 



Closely related in principle to the delayed reaction 

 are tests of double alternation or delayed alternation 

 (144, 164). In double alternation tests, the animals 

 must respond in a pattern of RRLLRRLL. In delayed 

 alternation, it must respond RLRL after a delay 

 between each response. In all such tests, the important 

 element is that the animal must remember what it 

 has previously done or experienced in order to know 

 what to do next. In all such tests, too, frontal abla- 

 tions are usually followed l>\ substantial impairment 

 (1 14, 144, 164, 196). 



conditioned inhibition. Konorski has recently re- 

 ported (28, 122) a series of experiments on the role 

 of the frontal lobes in conditioned inhibition. Dogs 

 and cats were taught a conditioned discrimination 

 b\ setting up an excitatory reflex to one stimulus 

 and an inhibitory reflex to another. Following this 

 training, ablations were made in frontal areas and 

 in parietal areas, usually in different animals but 

 sometimes in the same animal. The general result of 

 frontal ablation was to impair inhibitory reflexes with- 

 out affecting excitatory reflexes. That is to say.frontal 

 animals made correct responses to positive stimuli 

 but also made these responses, which they had been 

 trained not to make, to negative stimuli. Parietal 

 lesions, however, had no significant effect on the 

 retention of conditioned inhibitory reflexes. These 

 conclusions apply about equally well to salivary 

 responses (classical or Type 1 conditioning) and to 

 motor foreleg flexion (Type II conditioning). 



anticipation and perseveration. Learning tasks can 

 be so designed that they involve a series of responses 

 leading up to a goal. A maze constitutes such a task, 

 but so also do some problem boxes in which the 

 animal is required to do two or three things in a 

 particular order. If the animal is unable to perform 

 the task correctly, two kinds of mistakes are fairly 

 common: one is to anticipate a correct response, 

 making it too soon in the series of the response; the 

 other is to perseverate, making the same response two 

 or three times in succession. 



Disorders of serial learning appear with almost any 

 large lesion of the cerebral cortex, but they are 

 especially prominent after frontal ablations. Thus 



