222 BRAIN MECHANISMS AND LEARNING 



In the existing literature afferent impulses of this type are ascribed 

 mainly to proprioceptive formations. At this point I want to stress the 

 fact that this conception has nothing in common with that of return 

 afterentation. Proprioceptive signalling is also of a return nature, but it 

 regulates the trend of the very action in the sense of its biomcchanical 

 characteristics. Proprioceptive signalling will never be able to inform the 

 brain of the results of the adaptive effect obtained by means of the given 

 movement. Asa matter of fact, we cannot assay by means ofa proprioceptor 

 whether we have grasped a knife or fork, although the impulses of a 

 tactile nature, suggesting that the hand has touched the object and the 

 visual impulses fixing the moment of grasping the object, by means of 

 return afferent impulses together ensure exhaustive information of the 

 fact that the movement has ended in a certain positive result. 



Here we approach the most critical point of the physiological architec- 

 ture of the conditional reflex from which all the new adaptive acts, 

 yielding more perfect results of action, begin. We shall ask the question in 

 the following manner: where is the return afterentation ot the results of the 

 action directed to, what apparatus of the central nervous system perceives 

 it and, so to speak, 'assays' the correspondence of the result achieved with 

 the result of the afferent synthesis performed by it earlier ? 



The experiments and cc^nsiderations discussed above, as regards the 

 formation of the afferent apparatus of the acceptor of action, have led us to 

 the conclusion that the meeting of the excitations of the acceptor of action 

 and of the return aff'erentation from the results of the performed action is 

 the moment at which the organism is always informed of the satisfactory 

 precision of the act of behaviour that has taken place. It is precisely at 

 the pcnnt where these two excitations — the excitation of the acceptor 

 of action and the stream of return afferentations from the results of the 

 action — meet that, in our opinion, the necessary condition for any 

 co-ordinated and regulated relation oi the animal and man to the external 

 environment lies. Only when these two streams of excitation exactly 

 coincide do the effector excitations cease to reach the functioning apparatus 

 and is the given behaviour stage in the chain of individual reflex actions 

 completed (Fig. i6). 



Conversely, if the streams of return afferentations entering the brain 

 along different analysers fail to coincide with the system of excitations, 

 which has formed at the end of the afferent synthesis and is the apparatus 

 of the acceptor of action, this lack of coincidence immediately involves 

 other reactions, primarily, the orienting-investigatory reaction. By its 

 very essence the orienting-investigatory reaction leads to an accumulation 



