THE NEURAL BASIS OF LEARNING 



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environmental events. How activity of the neurons of 

 the limbic midbrain circuit would select only some 

 afferent impulses for transmission, which is the proc- 

 ess required to explain 'attention,' is by no means 

 clear (but see io8, 142). Neither, unfortunately, can 

 any critical suggestion be advanced at the present 

 time as to how an influence upon, say, the alerting 

 function of the reticular formation would create con- 

 ditions in the cortex favorable for the specific changes 

 of learning which is our central problem. 



The simple scheme outlined here is, however, sup- 

 ported by a sufficient amount of anatomical, physio- 

 logical and behavioral evidence to make it worth con- 

 sideration. It does not, of course, account for all the 

 known facts about learning gleaned from any of the 

 experimental sciences. Its main virtue, if it has any 

 at all, is that many experimenters are concentrating 

 their attacks upon it at the present time (see 108, 

 142, 145, 174). Figure 1 summarizes the anatomical 

 plan that forms the basis of the ideas under discus- 

 sion here. 



Summary 



At least two classes of events appear to provide the 

 neural basis for learning. One of these includes tin- 

 durable neural change that constitutes the new link 

 between previously unconnected parts of the brain. 

 Whether this durable change is to be explained l>\ .1 

 synaptic change or a new neural circuit, or in some 

 other terms, is still a matter for speculation. Its locus, 

 too, is unsettled. Certainly the cerebral cortex is not 

 exclusively the place where such changes occur. 



The second class of neural events in learning con- 

 sists of those that prime or prepare the brain for the 

 durable change it will undergo. Among these arc the 

 so-called 'motivational' and 'attentive' states that 

 commonly precede and accompany the learning 

 process and without which learning is unlikely or im- 

 possible. Study of the motivational and attentive 

 mechanisms can be expected to supply at least some 

 of the answers to our questions, for the brain changes 

 they produce underlie the brain change we wish to 

 understand. 



Learning is consequently best conceived not as a 

 particular event in a particular place but rather as a 

 sequence of events that involves various organ sys- 

 tems of the brain in a certain order. The end result, 

 to be sure, is the production of a more or less per- 

 manent change somewhere, but antecedent events 

 determine where, and even whether, it will occur. 



fig. 1. Simplified anatomical plan of the neural connections 

 involved in learning. The classical afferent systems disseminate 

 information about current environmental events to the cortex 

 and to the reticular formation. Efferent sensory tracts originating 

 from these latter structures terminate in the afferent nuclei 

 and even in the sense organs themselves. The limbic-midbrain 

 hi, ml, which starts in, and distributes to, the reticular sub- 

 stance brings the phylogenetically oldest parts of the cortical 

 mantle, as well as the hypothalamus, into functional contact 

 with the rest of the brain structures at a highly strategic point. 

 1 Ixperimental information on the vertebrate, while fragmentary 

 as we have seen, is consistent with the idea that each neural 

 structure and circuit outlined here plays its special part as the 

 durable brain change in learning is produced. [U. S. Army 

 photograph. 



CONCLUDING REMARKS 



People in the past have ruefully commented upon 

 the primitive state of our knowledge about the neural 

 basis of learning and, as this review makes it clear, 

 such comments are fully justified. Our ignorance, 

 however, is not wholly due to lack of industry, as this 

 summary also clearly demonstrates. What, then, have 

 been the main obstacles to progress and what is being 

 done to overcome them? 



The major obstacle has undoubtedly been the 

 inability to break through the barrier of the cal- 

 varium in order to expose the brain of normal tin- 

 anesthetized animals to direct experimental investiga- 



