128 BRAIN MECHANISMS AND LEARNING 



This difficulty can now be overcome if we take into account the fact 

 that each of the stimuh taking part in conditioning leaves transient traces 

 in the nervous system such that the respective central representations of 

 the stimuli are excited for a much longer time than the duration of the 

 stimuli themselves. 



If the proposed mechanism of the transformation of recent memory 

 traces into stable memory traces is correct, then one should predict that 

 cutting the first ones short in some way or other, for instance by electro- 

 convulsive shock, would lead to slowing down or even preventing the 

 process of conditioning. 



Such experiments were in fact performed by a number ot authors and 

 gave clear positive results. Duncan (1949) was the first to apply ECS after 

 each conditioning trial in rats and he showed that the closer the application 

 of the shock is to the trial, the stronger is its disrupting effect upon learn- 

 ing. Later the co-workers of Gerard (1955) and Thompson and Dean 

 (1955) applied ECS at various times after a single session of discrimination 

 learning and again proved that the shorter the interval between the 

 learning session and the shock, the poorer is the animal's retention after 

 24 hours. 



All these data show that after the termination of the learning trial, or a 

 massed series of trials, the consolidation of the CR continues and that this 

 phenomenon depends on some on-going process in the nervous system. 

 It is quite reasonable to believe that this is exactly the same process which is 

 involved in the recent memory phenomena described in earlier sections. 



The important role played in the formation of CRs by recent memory 

 may explain some other facts connected with learning which would 

 otherwise be difficult to understand. It has been shown by Chow (195 1), 

 Mishkin and Pribram (1954) and others that the ablation of posterior 

 ventral parts of the temporal lobes in monkeys abolishes or greatly 

 impairs visual discriminations established before operation. However, 

 Orbach and Faiitz (1958) found recently that if before operation the 

 animals were given prolonged, post-criterional overtraining, then the 

 discrimination habit suffered little or no decrement after inferotemporal 

 lesions. If we assume that the 'learning to criterion' of a visual discrimina- 

 tion is largely based on recent memory, i.e. that the animal remembers 

 from trial to trial and from day to day which figure is positive and which 

 negative, then this would explain why after a partial destruction of the 

 visual association area the habit is greatly impaired. On the other hand if 

 with long training the habit becomes based on stable memory, then no 

 post-operative deficit would ensue. It is worth while to stress that the 



