192 BRAIN MECHANISMS AND LEARNING 



conception ascribes the decisive role to an isolated stimulus, something 

 that has found expression in the evaluation ot the conditioned stimulus as 

 a decisive factor in determining the quality and strength of the conditioned 

 reflex effect. 



However, one physiological phenomenon which has considerablv 

 shaken this widespread idea was described in detail in Pavlov's laboratory. 

 I refer to the phenomenon of the dyitaiiiic paneni (Pavlov, 1932). 



As is well known, a definite and precise sequence of the selfsame con- 

 ditioned stimuli, trained without any alterations over a long period of 

 time, becomes the principal factor determining the quality and strength of 

 the conditioned reactions. Conversely, the role of the itidiviihial condi- 

 tioned stimulus is eliminated and the latter serves only as a noii-spccific 

 iiiipetiis for the appearance of the conditioned reflex, whereas the quality 

 and nature of the conditioned reaction does not depend on any condi- 

 tioned stimulus in particular. This stimulus does not even have to be a 

 conditioned stimulus, but may be an entirely new, i.e. indifferent (in 

 Pavlovian terminology) stimulus. Nevertheless, it will evoke the condi- 

 tioned reaction characteristic of the absent conditioned stimulus which 

 was always used in the former experiments at tiic given point of the (dynamic 

 pattern (Pavlov, 1932). 



Thus these experiments already revealed that the conditioned reflex, as 

 regards its quality, strength and time'of appearance, is a synthetic result of 

 the action of the conditioned stimulus and the preceding action of a 

 greater sum of af]:erent stimuli representing the conditions of the experiment 

 as a whole. 



A direct EEG analysis of the processes of the cerebral cortex conducted 

 in our laboratory has shown that a sound stimulus, applied at the point of 

 the dynamic pattern where light was always used, causes a desynchroniza- 

 tion of cortical electrical activity, although applied at its usual place 

 15 seconds previously it did not produce such desynchronization (Fig. 2) 

 (Anokhin, 1956). Thus a real sound cannot alter the electrical activity of the 

 cerebral cortex, this activity alternating according to the preceding 

 training of the dynamic pattern. 



This dependence of each conditioned reflex on the synthetic nature of 

 the external afferent influences was particularly clearly revealed in a 

 special experiment conducted in our laboratory. The selfsame comhtioneil 

 stinnihis (bell) was reinforceti by food in the morning and by an electro- 

 cutaneous pain stimulus in the evening (Laptev, 1937). In the end the dog 

 elaborates a perfectly clear differentiation: experimented upon in the 

 morning it responds to the bell with pronounced food reflexes, whereas 



