96 BRAIN MECHANISMS AND LEARNING 



was the attempt to select and apply only those 'indifferent' stimuli which 

 provoke in experimental animals reflex reactions which can be objectively 

 observed and graphically recorded. 



I. In 1958, our collaborators Varga and Pressman pertormed a scries 

 of experiments in dogs in which they combined two so-called 'indifferent' 

 stimuli, namely, a sound and a passive lifting of the animal's leg. In some 

 of these experiments stimuli were always applied in the same stereotyped 

 sequence, m others in a variable sequence. The advantage of a passive 

 movement of the leg as one of the 'indifferent' stimuli is that it gives 

 direct and immediate indication of a connection between the combined 

 stimuli when the movement of the leg or the electromyogram of its 

 muscles is observed. In addition, with these indicators the investigators 

 were able to study directly the whole process of formation of conditioned 

 connections when these stimuli are combined from the very beginning of 

 the connections to any subsequent stage of their evolution. 



((?) In experiments by previous investigators of this problem (Podko- 

 payev and Narbutovich (1936), ZeHony (1928), Oreshuk (1950), Roko- 

 tova (1952), Karmanova (1955), Bregadzc (1956), Sergeyev (1957) and 

 others, this was not possible. The reason was that they used only the 

 method of intermediated and, besides, sporadic indication ot the process 

 of elaboration of connections between 'indifferent' stimuli. Data obtained 

 by Varga and Pressman clearly show that between the brain points of 

 these stimuli a double (or two-way) 'associative connection', i.e. actually 

 a conditioned reflex connection is established. This occurs not only when 

 sound and passive lifting of the animal's leg are combined in a variable 

 sequence, but also when they are combined in a stereotyped one. (Fig. i) 

 Further, in the course of their experiments, Varga and Pressman also 

 obtained data which on the one hand corroborate facts previously estab- 

 lished by other workers, and on the other are new. For example, they have 

 shown, in conformity with the findings of other authors, that the connec- 

 tions between 'indifferent' stimuli are characterized by extreme fragility 

 and instability. After being established, they rapidly begin to weaken and 

 soon disappear after a period of more or less regular function. They 

 recover again for a very short time but only after a certain interval, 

 during the experiment, or when an alimentary conditioned reflex has been 

 elaborated in response to one of the combined stimuli. 



(/;) At the same time the experiments of Varga and Pressman established 

 other facts which illustrate the great signiticancc of a detnntc sequence in 

 combining stinndi. Their experimental data show that although the 



