CHAPTER XCVI] 



CONDITIONED AND UNCONDITIONED KEFLEXES 



In studying' the reflexes in the spinal animal, we have seen that the 

 effect of a given stimulus or of different stimuli is predictable with 

 absolute certainly. There is a fatality in the -responses. When the 

 higher centers are included in the reflex arc, the reflexes become modi- 

 fied or inhibited by events occurring in other parts of the central ner- 

 vous system and the results come to be more and more unpredictable. 

 The reflex comes to be a conditioned reflex (Pavlov). Studies of the 

 circumstances affecting these conditioned reflexes really constitute a 

 study of the function of the higher centers in the brain. Since such 

 experiments must be performed on the lower animals, Ave are limited in 

 the investigation to motor responses, for we have no way whatever of 

 studying the subjective sensations produced. The motor phenomena by 

 which the animal may express its sensations can be interpreted by us 

 only in terms of psychological ideas thai in large part are derived from 

 our own experiences. Obviously the conclusions that can be drawn 

 are subject to very great sources of error, and it must be considered as 

 one of the greatest advances of modern physiology that Pavlov and 

 others should have succeeded in evolving methods by which Ave may ar- 

 rive at conclusions regarding the nature of certain of the integrations 

 that occur in the higher centers of the neiwous system preceding the 

 motor manifestations by which the animal expresses its sensations. 



The methods employed for the study of these higher integrations of 

 the central nervous system all depend on the reactions of the animal 

 that are associated with the taking of food. When the food is actu- 

 ally placed in the mouth, it excites a secretion of saliva, Avhatever the 

 circumstances may be. This is an unconditioned reflex. Suppose, how- 

 ever, that every time food is given a particular sound is made; after 

 some time it will be found that the occurrence of the sound alone is- 

 sufficient to cause a secretion of saliva. In other Avords, a conditioned 

 reflex lias been formed. Similarly, sight or smell or any other type of 

 sensation m<ay be «made the excitant for the conditioned reflex. The 

 secretion now becomes psychic instead of merely physiological. To quote 

 Bayliss: "Any phenomenon of the outer world for which the animal in 

 question possesses appropriate receptors can be drawn into temporary 



856 



