112 BRAIN MECHANISMS AND LEARNING 



GROUP DISCUSSION 



KoNORSKi. The problem investigated by Professor Asratyan and his group is 

 very important, and has not been solved as yet, although it has been studied for 

 many years. This is the old problem whether the so-called backward conditioning 

 exists, or not. Both in Russia and in America there were authors strongly support- 

 ing the idea of backward conditioning, as well as strong opponents of this view. 

 While the first group of authors claimed that any association between stimuli 

 (both forward and backward) leads to the formation of connections between them, 

 the second group argued that the phenomena of backward conditioning were only 

 pseudo-conditioning due to sensitization. In fact, while the biological role of 

 forward conditioning is obvious, the biological significance of backward condi- 

 tioning could hardly be understood. 



Professor Asratyan has brought out more extensive material concerning this 

 problem than any other author. According to his data backward conditioning does 

 exist, although the connections formed in the direction from the subsequent 

 stimulus to the preceding one are much weaker than those established in the 

 opposite direction. This is also in agreement with the results obtained by Czech 

 authors on chimpanzees, and recently by Mrs Budochovska in the Psychological 

 Department of the Nencki Institute on man. 



Chow. I would like to ask Professor Asratyan how these very interesting 

 experimental results are to be incorporated, if they are to be incorporated, in the 

 concept of reinforcement during the formation of the conditioned reflex. 



Asratyan. These conditioned reflexes also need reinforcement. If one of the 

 stimuli is applied without any other, this, leads to the extinction of the reflex in 

 every case. But in cases where we have a weak stimulus, the so-called 'indifferent 

 stimulus' — I say so-called because I think that these indifferent stimuli are only 

 relatively indifferent — indeed if they evoke a reaction at all in the organism they 

 cannot be indifferent. They are indifferent in relation to another reaction evoked by 

 another stimulus but by themselves they also originate reflexes like unconditioned 

 stimuli, althoush their histological sia;n and their strength are different. When we 

 combine two indifferent stimuli, or weak stimuli, in these cases absence of reinforce- 

 ment leads to a more rapid extinction than in the case of strong stimuli. Reinforce- 

 ment is necessary. 



Naquet. How do you explain the limit oi loo msec? 



Asratyan. I am afraid we cannot answer this question in a satisfactory manner. 

 It may be a question of mutual influences, facilitation and inhibition, not elaborated 

 connections. I really don't know. But it should be pointed out that tacilitation alone 

 is not sufficient for the elaboration of conditioned connections. 



Fessard. I believe that new connections within the central nervous system 

 cannot be formed unless the associated afferent messages converge towards common 

 neurones so that the limit of lOO msec, might be explained by the recovery period 

 of these neurones, at least of those having discharged aiter the first of the afferent 

 signals reaching them. 



Naquet. I think loo msec, represents the recovery time of the reticular excitability 

 and you may have some facilitation which permits the establishment of condition- 

 ing. 



Hebb. I have been very interested for a long time in the problem of the sequential 



