1 66 COMPARATIVE PHYSIOLOGY 



reflexes. The possibility of isolating a conditioned reflex 

 for study implies that some inhibitory agency preserves the 

 normal surroundings of the laboratory from exerting very 

 much influence. Inhibition in conditioned phenomena 

 presents four distinct aspects depending upon inherent pro- 

 perties of the central nervous S3^stem ; these are : — 



1. Inhibition by extinction. When an indifferent stimulus 

 has become a conditioned stimulus for salivary secretion, and 

 is allowed to act alone on several occasions without the con- 

 ditioned stimulus, it gradually loses its potency, but recovers 

 it after a period. 



2. Conditional- inhibition. If the conditioned stimulus in 

 a conditioned reflex is accompanied by another indifferent 

 stimulus the extinction referred to under (i) takes place more 

 rapidly than it would if allowed to operate alone. 



3. Differential inhibition. Stimuli which resemble a 

 conditioned stimulus fairly closely may at first evoke response 

 when applied alone, but lose this efiicacy more readily than the 

 original conditioned stimulus. 



4. Retardation. If in the formation of a conditioned 

 reflex, the new stimulus precedes by a definite interval (from 

 a half to three minutes) the unconditioned stimulus, the re- 

 sponse to the conditioned stimulus when the new reflex is 

 established is delayed by a corresponding interval of time. 



In addition to the above may be mentioned the generalised 

 form of inhibition of the activity of the cortex known as sleep. 

 This can be regularly evoked in dogs by application of warmth 

 or cold to an area of the skin. A further complication is 

 introduced by the fact that external agencies not only give 

 rise to inhibition but to release from inhibition. The phenome- 

 non of " inhihition of inhibition " may be illustrated thus. 

 By repeated synchronous action of the sound and presentation 

 of food, an organ note of 1000 vibrations per second becomes 

 a conditioned stimulus evoking salivary secretion in absence 

 of the food itself. If repeated too often alone it suffers 

 inhibition by extinction, but recovers its efficacy with a sufficient 

 period of rest. If during the indifferent period, there is 

 superimposed on the now ineffective sound stimulus a second 



