THE NEURAL BASIS OF LEARNING 



■493 



havior on the basis that the first of these emerges in 

 the normal development of the organism while the 

 second appears only through appropriate learning 

 experiences. We will express this dichotomy in terms 

 of maturation vs. learning since many items of un- 

 learned behavior appear after birth and, strictly 

 speaking, are not "innate' even though they are 

 surely not learned. The important point is that basic 

 patterns of behavior are laid down in the nervous 

 system in the normal development of the system while 

 others are acquired only through learning. In general, 

 the behavior appearing early in the life of the organ- 

 ism is the result of maturation and what emerges later 

 is the result of learning, but the two stages overlap 

 and interact so that learning occurs in some sectors 

 before maturation is completed in others. 



'connections'. In both maturation and learning, 

 some change in the nervous system is necessary for 

 'new' patterns of behavior to occur. It is reasonable 

 to suppose that this change consists of new 'connec- 

 tions' formed between different organs, centers and 

 neurons in the system. The new connection or link, 

 whatever its exact nature, makes possible the How of 

 messages between points that were formerly 'uncon- 

 nected.' 



In maturation certain parts of the nervous system, 

 as well as the muscles, undoubtedly exert "attractive 

 influences' on the growth of neural fibers. Somehow 

 or other, motoneurons are guided to their respective 

 muscles and other neurons connect up appropriately 

 with the motoneurons. In addition, sensor) neurons 

 will connect up in exactly the 'correct' constellations 

 with nuclei in the central nervous system to perform 

 their functions. And we know that certain portions 

 of the nervous system, for example the medulla, 

 have a controlling and directing influence on the 

 growth of pathways formed in nearby structures. 



Some pathways laid down in maturation, when 

 once established, are so rigidly fixed that they can- 

 not be altered. The numerous studies of Stone and 

 of Sperry (225) clearly demonstrate this point. If, for 

 example, a motor nerve of the left leg of the rat is 

 surgically crossed to the right leg, and vice versa, a 

 noxious stimulus to the left leg reflexly evokes lifting 

 of the right leg and no amount of training of the 

 animal corrects this maladaptive response. The 

 animal continues to lift the 'wrong' leg. Similarly, in 

 amphibia, rotating the eyes or transplanting them to 

 the opposite sockets, thus twisting or reversing the 

 animal's visual field, causes the animal to respond to 

 visual objects in a direction opposed to the normal 



one. Prolonged learning experiences with such visual 

 fields, however, do nothing to alter these inappro- 

 priate responses. We are led therefore to conclude 

 that many connections or pathways, once established 

 through maturation, are not easily changed or altered. 

 Those established through learning, in contrast to 

 this, lend themselves easily to formation in the first 

 place and are more or less impermanent. 



Although the precise conditions for the formation 

 of the links are different in maturation and learning, 

 there is so far no reason to believe that the processes 

 are fundamentally different. That is to say, it is 

 probably our best assumption at present that the 

 neural changes taking place in maturation and learn- 

 ing are essentially the same and that only the condi- 

 tions or immediate causes are different. 



We might say that maturation rigidly fixes some 

 routes so that unlearned reflexes and responses can 

 be altered little or not at all, and it also prepares 

 other routes m> that they can become fixed through 

 the processes of learning. But there is no reason to 

 believe that the central processes that fix behavior 

 in maturation differ from those that fix them in 

 learning. 



the specific change in ii \K\iNo. What, then, is 

 the specific event thai fixes the connections in matura- 

 tion and learning? So far as maturation goes a num- 

 ber of such factors have been considered, among 

 these are a) mechanical, b) electrical and c) metabolic 

 or biochemical influences upon the direction and 

 State of growth (225). As for the changes in learning, 

 factors of the same general sorl have also been postu- 

 lated, but none of the evidence is critical as we have 

 already seen. We musl conclude, therefore, that both 

 the precise mechanism of guidance and control of 

 neurons in maturation, and the exact nature of the 

 specific event in learning are --till a mystery. 



MOTTA VTION WD ATTENTION. Psychologists and ph\ si- 



ologists have, from time to time, been led by their 

 observations to infer the existence of many events 

 going on within the mammal during learning. We 

 have considered one of these in detail, namely the 

 idea that some durable change— at synapses or else- 

 where — results from pairing CS with US to produce 

 the CR. There are two others, which we will call 

 here 'motivation' and "attention,' winch also appear 

 repeatedlv in formal treatments of the problem. An 

 animal "motivated' to learn (e.g. by reward for suc- 

 cess or by punishment lor failure) does better than 

 one not so motivated. Similarly, an animal (or a 



