E. A. ASRATYAN III 



biological significance (two 'indifferent' stimuli, two unconditioned 

 stimuli, or one 'indifferent' and one unconditioned stimulus) can be best 

 understood when considering the important role of the concrete physio- 

 logical strength of the corresponding stimuli. There are grounds for 

 assuming that this factor plays an important role in the further fate of 

 the conditioned connections, whereas the relative strength of excitation of 

 the central nervous structures corresponding to these stimuli and the 

 sequence of application play an outstanding role in elaborating these 

 connections and forming some of their properties. If we assume that the 

 level of excitation of the central nervous structures originating conditioned 

 reflex comiections expresses approximately the number ot functional 

 units which come into activity, and that the strength of the conditioned 

 reflex connection is its derivative, the following conclusion may be 

 drawn : for maintenance and functioning of conditioned connections it is 

 necessary that they should possess a certain threshold strength, i.e. consist 

 of a definite 'threshold' number of functional units. Weak conditioned 

 connections, that is, connections consisting of a small number of functional 

 units, become rapidly fatigued and exhausted even after moderate activity; 

 a protective inhibition occurs which blocks them. At present we arc unable 

 to explain satisfactorily why such inhibition develops in structures corres- 

 ponding to weak conditioned connection. We can point out, however, 

 with some justification, that there is a similarity between this phenomenon 

 and the rapid development of inhibition during a protracted action of weak 

 conditioned and even indifferent stimuli. This phenomenon was known a 

 long time ago, trom numerous investigations carried out by Pavlov in his 

 laboratories. Moreover, the development of inhibition in coimection with 

 a protracted action of weak stimuli seems to belong to the category of 

 phenomena which are common to all nervous structures. 



For the time being we cannot give a satisfactory explanation for the 

 facts obtained by Pakovich, which show that a conditioned reflex cannot 

 be formed when two combined stimuli act synchronously. Nor can we 

 explain the disappearance of an existing conditioned reflex when a 

 conditioned stimulus and an unconditioned one are combined in a similar 

 way, although in this case the possibility of a positive and negative 

 interaction of these stimuli remains. It is possible that a certain role in this 

 respect is played by mutual inductive inhibitory influences of the simul- 

 taneously excited cerebral structures upon each other. These influences, 

 however, do not preclude the summation of excitations that have 

 originated in them. 



