T20 BRAIN MECHANISMS AND LEARNING 



4. ON THE PHYSIOLOGICAL MECHANISM OF RECENT MEMORY 



It is now almost generally accepted that recent memory is based on the 

 activity of reverberating circuits of neurones which are connected with a 

 group of neurones excited by the actual operation of a given stimulus. 

 Such activity causes this group to continue to be excited for some period 

 after the cessation of the stimulus itself (Hcbb, 1949), and either dies out 

 spontaneously within a lapse of time or is knocked out by some inhibitory 

 influence arising from other foci of antagonistic excitation (the so-called 

 external inhibition). 



In order to develop this hypothesis a little further and make it more 

 precise, the following well-known facts from the field of CR studies 

 should be taken into account. 



1. If a CS is suddenly discontinued before the moment of its usual 

 reinforcement, the conditioned response proceeds uninterruptedly almost 

 with the same intensity as if the CS were still acting (Kupalov and Lukov, 

 1932). On the contrary, if the reinforcement is usually presented several 

 seconds after the CS is discontinued, the conditioned response appears 

 already during its operation. This shows that a high degree of generaliza- 

 tion exists between the actual action of a stimulus and its traces. 



2. If an actual stimulus is not reinforced, but the 'trace stimulus' is, then 

 dift'erentiation of the two stimuli is 'gradually established. As a result the 

 animal always displays the conditioned response only after the cessation 

 of the stimulus. 



3. The animal is also able to differentiate early traces from late traces of 

 the stinuilus (cf section i). This differentiation is, however, much more 

 difficult than that between the stimulus itself and its traces. 



If we hold the view developed in detail elsewhere (Konorski, 1948) 

 that generalization, or similarity, of stimuli is due to the overlapping of 

 their central representations, while differentiation of them is possible when 

 in at least one of these representations there are elements which do not 

 belong to the other one, then the above facts can be understood as 

 follows : 



According to the vast evidence of facts concerning the responses of 

 various nerve-cells to the incoming impulses on all levels of the nervous 

 axis, we may admit that the central representation of a given stimulus 

 consists of the following types of neurones: Neurones which are activated 

 only at the beginning of the operation of the stimulus (pure on-clements, 

 group la); neurones which are activated during the whole action of the 

 stimulus but not after its cessation (group ib); neurones which are 



