THE NEURAL BASIS OF LEARNING 



1477 



No particular locus of lesion proved to be more 

 important than any other in impairing either learning 

 or retention of the maze habit. Instead, the degree of 

 impairment correlated well with the size of the lesion, 

 irrespective of its locus. This general finding has been 

 given the name 'mass action.' Moreover, the impair- 

 ment depended on difficulty of the maze. With easy 

 mazes, relatively large lesions were required to 

 produce a given deficit; with difficult mazes, a rela- 

 tively small lesion. Since maze tasks involve several 

 different sensory and psychological capacities, and 

 since a lesion made almost anywhere in the cortex is 

 likely to impair one or more of these, it is to be ex- 

 pected that the larger the lesion, the greater the 

 resulting deficit. Some consider this an adequate 

 explanation of mass action (64). But Lashley has 

 held (129) that in addition to specific functions of 

 areas there are general, nonspecific functions of 

 cortical tissue. Experimental difficulties, however, 

 have so far blocked the resolution of this issue. 



Problem-Box Learning 



Several studies have utilized the so-called problem 

 box —a compartment so arranged that the animal 

 must perform some simple act cither to escape or to 

 secure a food reward. In the simplest case, a rat must 

 learn to push two platforms in succession to obtain 

 food (127). Learning such a simple habit, Lashley 

 found, is unaffected by ablations of 50 per cent or 

 more of the cortex; larger lesions than this may 

 impair learning, I mi ii matters little where they are 

 placed. If the problem, however, requires much 

 manipulative skill, such as pulling a string or turning 

 a latch, it is more susceptible to impairment by 

 cortical ablation, and there is a high correlation 

 between the size of the lesion and the degree of 

 impairment. This result is probably explained by the 

 fact that rats subjected to operation are less resourceful 

 and variable in their behavior and therefore have less 

 chance of hitting on the correct solution to the 

 problem. 



Somewhat comparable experiments have been done 

 with monkeys (104, 105). As one might expect, their 

 ability to learn problems involving manipulative skill 

 is impaired by lesions of the motor, premotor or both 

 areas. However, it seems unlikely that the resulting 

 impairment concerns learning ability or memory per 

 se; rather it is probably the result of motor disability. 

 In fact it is reasonable to believe that difficulties in 

 problem-box learning by rats and monkeys are due to 

 sensory-motor impairments and not to interference 

 with learning or memory processes. 



Discriminative Learning 



Both Type I and Type II techniques have been 

 widely employed in attempts to determine the effects 

 of removal of some specified area of the cerebral 

 cortex upon the learning or retention of a particular 

 discrimination. It is most convenient to consider 

 these studies by grouping them according to the 

 sensory modality invoked : vision, hearing and so- 

 mesthesis. (For studies concerning only sensory ca- 

 pacity rather than learning and retention per se, see 

 Chapter LX by Neff in this Handbook.) 



visual discrimination. In general, simple visual learn- 

 ing and retention are unaffected by removal of the 

 striate cortex. This is true of the conditioned evew ink 

 in the dog (152), of ley flexion either to avoid shock 

 (245) or to obtain food reward (246), and even of 

 discrimination of a change in visual intensity (247). 



When, however, an animal is required to make a 

 choice between two stimuli, the results arc somewhat 

 different. While striate ablation usually has no effect 

 on the learning of such a discrimination, when made 

 after learning is complete it causes partial or complete 

 loss of memory lor the discrimination (117, 125, 128, 

 222). In the typical ease of a rat, for example, the 

 Striate lesion causes complete amnesia for a discrimina- 

 tion of lights of different intensity, but the animal 

 can relcarn the habit in about the same number of 

 trials as was required originally (125). This difference 

 in the effects ill striate lesions on learning and on 

 retention is an important phenomenon that has not 

 been satisfactorily explained (163, p. 470). 



Animals need the striate cortex to perceive detail; 

 without it they are completely unable, regardless 

 of the amount of training, to discriminate forms 

 like triangles, circles, squares, etc. (u(>). The discus- 

 sion of this problem, therefore, belongs under the 

 heading of visual capacities rather than learning. 

 The possible role of the so-called association areas 

 of the cerebral cortex in the learning and retention 

 of visual form discriminations has been investigated. 

 The prestriate areas lying adjacent to the striate 

 cortex have often been regarded as the 'visual associa- 

 tion areas.' Animal experiments, however, cast serious 

 doubt on this supposition. Although results are not 

 entirely consistent (1,2 ), ablation experiments indicate 

 little participation of these areas on the learning or 

 retention of form (130, 203) or color discrimination 

 (58) — at least when only these areas are removed. 

 On the other hand, when relatively large lesions are 

 made in the 'p oster ' or association areas,' involving 

 some combination of the prestriate, parietal and 



