102 BRAIN MECHANISMS AND LEARNING 



same picture of relative excitability or excitation of the central nervous 

 structures of these two stimuli. 



III. New and interesting data obtained by our collaborator Struchkov 

 (1958) also relate to the problem under discussion. While elaborating 

 conditioned reflexes in dogs, he combined 'indifferent' stimuli with an 

 unconditioned stimulus. This special method is known in the literature as 

 the method of 'covering'. In this method the 'indifferent' stimuli are 

 applied against a background of action of the unconditioned stimulus. 

 Struchkov used food as an unconditioned stimulus and chose only those 

 'indifferent' stimuli which by themselves evoke in the organism specific 

 reactions that can be objectively observed and recorded. For example, in 

 one series of experiments he applied a passive lifting of one of the dog's 

 hind legs and in another a momentary cooling of a limited area of the skin 

 on one side of the animal's body, which evokes a local vasomotor reaction. 

 This sequence of stimuli was applied systematically in all experiments of 

 each scries. The following very interesting facts were brought to light. 

 After repeated combination of an alimentary reflex with one of the 

 'indifferent' stimuli, presentation of food alone began to evoke reactions 

 peculiar to these stimuli. In one series of experiments the reaction was an 

 active lifting of the leg, and in another a local vasomotor reflex. This is 

 shown in Fig. 5. In other words, in Struckhov's experiments the classical 

 unconditioned stimulus — food — played the role of a conditioned or 

 signalling stimulus, while the so-called 'indifferent' stimuli — passive 

 movement of the leg or local cooling of the skin — appeared as reinforcing 

 stimuli. These remarkable experiments show that it is precisely the 

 position in time of the preceding stimulus (P) which determines the 

 acquisition by the food of a signalling significance in relation to other 

 reflexes, that is, the formation in one case of a somato-motor conditioned 

 reflex to the presentation of food, and in the other case a vasomotor 

 conditioned reflex. It may be assumed therefore that this is the consequence 

 of the formation of a direct conditioned reflex connection between central 

 nervous structures of the alimentary reflex and corresponding structures of 

 the somato-motor or vasomotor reflexes. It was subsequently established 

 that in these experiments there also arises a double (or two-way) con- 

 ditioned connection between the unconditioned and indifferent stimuli 

 which are combined in a definite, stereotyped sequence. This means that 

 with the direct conditioned reflex connection there exists a reverse connec- 

 tion which conducts excitation from the nervous structures corresponding 

 to (S) (i.e. to the movement of the leg or local coohiig of the skin) to the 



