jg6 BRAIN MECHANISMS AND LEARNING 



after the removal of the frontal parts of the cerebral cortex this animal 

 behaves perfectly quietly during the intervals between the applications of 

 the conditioned stimuli: it sits on oiiv side of the stand and performs no pendn- 

 lar nioi'cnieiits. 



However, if we feed this quiet animal trom the opposite feeding 

 trough but once, it immediately begms to perform the well-known 

 continuous pendulous movements to both sides of the stand. 



These experiments suggest that the 'pendular' movements are a direct 

 result of the situation stimulation which includes conditions of alternate 

 choice and behaviour. 



Why then does the intact animal sit quietly in the middle ot the stand 

 during the intervals between the applications ot the conditioned stimuli? 



The interest of this phenomenon consists precisely in the fact that the 

 numerous stimulations of the experimental conditions (situational afteren- 

 tation) affect the nervous system all the time, whereas the conditioned 

 reaction manifests itself only at the moment the conditioned stimulus 

 begins to act (trigger ali'erentation). At the same time the extreme change 

 in the experimental situation convinces us that the situational stimuli form 

 an organic component part of the afferent integral. Thus it becomes clear 

 that all forms of afferent excitations affecting the animal's nervous system 

 at the given moment undergo a synthetic processing and that only after 

 this stage does the efferent complex of working excitations begin to form. 



In recent years, neurophysiological studies have convinced us that all 

 the afferent influences on the organism come to the cerebral cortex along 

 two channels: (i) the specific or lemniscus pathway and (2) the non- 

 specific, i.e. the brain stem reticular formation. The latter excitation is an 

 indispensable condition for any interaction and interconnection of 

 specific excitations on the level of the cerebral cortex. 



Specially for the physiology of the conditioned refiex this means that 

 any form of association of stimulations acting on the organism simul- 

 taneously or successively becomes possible only if the activating effect of 

 the reticular formation has spread to the substrate of the cerebral cortex. 

 The constant mediator between the new external conditions and the 

 associative activity of the cerebral cortex and subcortical structures is the 

 orienting-investigatory reaction of the animal taking place under an 

 uncommonly high activation of the apparatus of the brain stem reticular 

 formation (Heniandez-Peon, Shumilina, Havlicck). 



The physiological and biological purport of the orienting-investigatory 

 reaction was very well revealed by Pavlov himself (Pavlov, 1925). It has 

 now been shown that it occurs under a constant excitation not only of 



