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BRAIN MECHANISMS AND LEARNING 



The following fact is noteworthy. Although a strictly simultaneous 

 action of both stimuli or a certain minimal precedence of action of one of 

 them creates such conditions that the cerebral nervous structures cannot 

 elaborate new conditioned connections, or even abolishes all previously 

 conditioned reflexes, this does not preclude the possibility of a negative 

 anci even positive interaction between the cerebral structures which 

 correspond to these stimuli. Pakovich demonstrated that even in conditions 

 of a synchronous action of sound and electrical current there may take 



a 



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Fig. 7 

 Motor reflexes arising from different combinations in time of two stimuli — a tone and an 

 electric current. (17) — a motor reaction resulting from a strictly simultaneous action of the 

 stimuli; (fo) — absence of a conditioned reflex to the tone after 582 strictly simultaneous 

 applications of it in combination with an electric current; (c) — emergence of a conditioned 

 reflex to the same stimulus after fourteen applications of it in combination with an electrical 

 current delayed for 2 seconds. 



I. — record of movements of the leg; 2. — mark of conditioned stimulation; 3. — mark of 

 unconditioned stimulation; 4. — time in seconds. 



place both a reciprocal inhibition and a summation of excitation provoked 

 by each of these stimuli in corresponding central nervous structures. The 

 reciprocal inhibition is observed predominantly in the first period of the 

 combined action of the stimuli, when the orienting reaction to sound is 

 still strong. As to summation of excitation, it is observed predominantly 

 following the combined action of the stimuli. During these periods a 

 subthreshold excitation evoked by an electro-cutaneous stimulus applied 

 separately turns into an above-threshold excitation when an electric current 

 of similar intensity is applied in combination with a sound. In the same 



