234 BRAIN MECHANISMS AND LEARNING 



Consideration of the time course of acquisition will illustrate this concept. 

 A monkey learning to avoid shocks by pressing a lever at a signal appears 

 to pass through a series of behavioural stages in the process. At first the 

 signal arouses much 'emotional' activity such as pilo-erection and 

 vocalization. Later, when learning has progressed to 50 per cent correct 

 responses, this 'emotional' behaviour can be partly replaced by an 'alert 

 or attentive' attitude. The fully trained animal, in fnial complete com- 

 mand of the situation, seems undisturbed 'emotionally' by the signal and 

 sits relatively 'inattentive' as he delays making the correct response until 

 the very last moment. These successive behavioural stages presumably 

 reflect a progressive reorganization of brain structures or processes during 

 acquisition. If this is true, brain events measured at a particular place in the 

 early stages of learning might be absent there later; while at another brain 

 locus characteristic brain events might appear only when learning has 

 become complete. Evidence accumulates that learning is just such a 

 dynamic orderly march of events that involves much or all of the neural 

 apparatus the organism possesses. 



6. There is reason to believe that there are different kinds of learning, 

 that the brain changes accounting for these are not uniform or identical, 

 and that what happens in the brain during solution of one problem need 

 not happen in the solution of the next one. As has been adequately shown 

 (Brady anci Hunt) a rat trained both'(r7) to press a lever to get a drop of 

 water (an instrumental or Type II response) and (/)) to 'expect' a shock at 

 the termination of a signal (a classical or Type I response), loses only the 

 second of these habits after experiencing a number of convulsive seizures. 

 Furthermore, the lost habit returns after a time without retraining. That 

 convulsive seizures temporarily abolish one CR while leaving another 

 entirely intact defines a large and presumably highly significant difference 

 in the neural basis of the two CRs. 



7. All problems are not equally easy for animals to learn. Teachers 

 know that what one child learns easily another may never acquire, and 

 everyone will concede that speaking, writing and reading are much more 

 readily learned by man than by any other species. Besides the obvious 

 species and individual differences in learning capacities which are apparent 

 upon a moment's reflection, there is the fact that whereas many or very 

 many combinations of CS and US are ordinarily required to establish a 

 CR, a single exposure to CS alone proc^uces lifetime retention in the case 

 of imprinting (Hess). What are the neural events in this exceptional 

 instance, where for only a few hours in the life history of an organism its 

 brain is prepared to make a particular set of functional connections ? The 



