no BRAIN MECHANISMS AND LEARNING 



corresponding to (S) become more excited after the emergence of condi- 

 tioned connections than the central nervous structures corresponding to 

 (P). This is due to the fact that the structures corresponding to (S) are in 

 this case excited by two sources — by (P) in a conditioned reflex way and 

 by (S) in an unconditioned reflex way. There does not exist the initial 

 relative equality in intensity of excitation of the central nervous structures 

 which corresponds to the combined stimuli. The excitation of the nervous 

 structures of (S) prevails to a great extent over the excitation of the nervous 

 structures corresponding to (P). As a result the direct conditioned connec- 

 tion begins to prevail over the reverse connection in all characteristics. 

 This explanation was corroborated by Varga in a series of experiments 

 (1958) in which she demonstrated that the transition from a variable 

 sequence of combined stimuli to a stereotyped sequence entails a con- 

 siderable increase in the excitability of the central nervous structures 

 which correspond to (S) and prevalence in this respect over the one 

 corresponding to (P). 



We are inclined to offer a similar explanation for Struchkov's findings 

 concerning the conversion of food to a signalling stimulus for somato- 

 motor and vasomotor reflexes. In the first phase of combining food 

 (which is the preceding stimulus (P)) with a passive lifting of the paw or a 

 local cooling of the skin (which is the .subsequent stimulus (S) ) the central 

 nervous structures corresponding to the latter stimuli become excited to 

 a relatively high degree on account of the novelty of the stimuli for the 

 experimental animals. Owing to this, the level of excitation of the central 

 nervous structures corresponding to these stimuli approximates the level 

 of excitation of the central nervous structures of the alimentary reflex 

 which is the preceding stimulus. This creates favourable conditions for 

 the elaboration of the double conditioned connection between these 

 structures. Subsequently, after elaboration of this double connection, the 

 above-mentioned factor comes into effect, namely, a more intense 

 excitation of the central nervous structures of (S) coming from two 

 sources by way of summation — at first, from the food in a conditioned 

 reflex way, and then, against this background, from adequate stimuli of 

 the nervous structures (a lifting of the paw or cooling of the skin) in an 

 unconditioned reflex way. This maintains the previously created relative 

 equality in levels of excitation of the nervous structures corresponding to 

 the combined stimuli, i.e. the precondition necessary for maintaining the 

 double conditioned connection is preserved. 



Facts concerning relatively different duration of efficiency for the condi- 

 tioned comiections arising after combination of various stimuli of different 



