CONDITIONED AND UNCONDITIONED REFLEXES 857 



association with salivary secretion, so that it becomes an exciter of se- 

 cretion if only it has been frequent! y presented at the same time with 

 the unconditioned reflex stimulus, food in the mouth." 



Work along lines similar to that devised by Pavlov has more recently 

 been undertaken by students of animal behavior, who have utilized the 

 acquired habits of an animal in searching for its food in order to study the 

 influence of conditioning circumstances on its procedure. The advantage 

 of this method depends mainly on the fact that it can be applied to all 

 groups of animals. In carrying out such an observation, the animal is 

 placed in one compartment of a cage, from which it is then released to 

 a second compartment, the end of which is divided into two passage- 

 ways, one leading to food, the other leading to some compartment in 

 which the animal is punished for its mistake as by receiving an electric 

 shock. Objects such as colored lights are placed in the different pas- 

 sageways, and the animal by repeated trial comes ultimately to learn 

 which particular colored light signifies the passage along which he 

 will receive food. A reflex has therefore become established conditioned 

 on the particular colored light. 



On account of the unavailability of his publications, it is impossible 

 at present -to give any complete account of Pavlov 's discoveries. A few 

 facts, however, are of such importance that it is necessary far us to 

 state them here as far as we know them. (See Bayliss, Physiology.) Two 

 mechanisms seem to be concerned in the conditioned reflexes: (1) that 

 of temporary association, and (2) that of analysis. Temporary associa- 

 tion is well illustrated in the above experiment in which the secretion of 

 saliva is induced by a sound. Temporary association of the sound with 

 the secretion of the saliva may readily be inhibited by all kinds of ex- 

 ternal phenomena; thus, if the dog's attention becomes diverted while 

 the conditioned reflex is being stimulated, the response does not occur. 

 In a dog that had been trained to secrete saliva to the sound of a par- 

 ticular metronome beat, inhibition occurred one day because, just as 

 the dog was being presented with the food, the laboratory servant made 

 a noise outside of the building which diverted the animal's attention. 

 The conditioned reflex may also be interfered with by internal inhibi- 

 tion, which is illustrated by experiments in which, after a dog has been 

 trained to respond to a given conditional reflex, several occasions follow 

 when food is not given to the animal after the particular sensation to which 

 it has been trained to respond. The condition for example, a sound loses 

 its effect. This is internal inhibition, but it is a temporary condition 

 since the reflex returns of itself after a period of rest. 



These experiments illustrate what is meant by the formation of tem- 

 porary associations occurring in conditioned reflexes, but in order that 



