XI 



CONDITIONING 



A most important property of neural circuits is that their ac- 

 tivity may continue indefinitely after cessation of the stimulus. The 

 possible application of this property to memory is evident, but to 

 conditioning it is much less so. We now suggest a mechanism for ex- 

 plaining conditioning and learning. 



Consider first a few properties of the structure of Figure 1 



i x 



Figure 1 



(Rashevsky, 1938) . For the present we shall ignore the presence of the 

 dotted neurons. This structure consists of two neuron-chains, leading 

 through a final common path to a response R , together with a uni- 

 lateral interconnection and a simple circuit C . The chief character- 

 istic of conditioning is that a particular response R , normally pro- 

 duced by the "unconditioned" stimulus S u but not by the stimulus S c , 

 may after the repeated concurrence of the stimuli S c and S u become 

 capable of being evoked by S c alone. This may require one or more 

 concurrences of S c and S u , and while S c and S u need not be presented 

 at exactly the same time, the time between them cannot be too long. 

 Suppose, only for the sake of simplicity, that all the neurons are of 

 the simple excitatory type, and let <£o represent the maximum value 

 of <f> for any S c . Then for the net of Figure 1, let 



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