372 SERGIUS MORGULIS 



This is a phenomenon of rapid but only temporary paralysis of 

 conditioned reflexes occurring whenever conditional stimuli do 

 not truly and precisely inform the central nervous system of 

 unconditioned stimuli, in other words when the conditioned 

 stimulus is not reinforced by the physiological irritation which 

 accompanied its formation. 



There are several kinds of internal inhibition. Waning con- 

 ditioned reflexes, due to a repeated application of the conditioned 

 salivary stimulus without the aid of an unconditioned stimulus, 

 is one kind. Another kind is the delayed reflex which appears 

 if the conditioned stimuli are regularly followed by feeding a 

 few seconds or even minutes after the conditioned stimulation 

 has ceased. Conditioned inhibition is likewise a form of internal 

 inhibition arising when an irrelevant factor is added to the 

 conditioned stimulus, the combination not being reinforced by 

 feeding. In such a combination the conditioned stimulus is 

 quite ineffective, but alone it exerts the usual influence. The 

 process of differentiation and concentration, already described 

 above, represents a still other type of internal inhibition — the 

 inhibition of differentiation. Furthermore, it is a very common 

 and very important occurrence that an inhibition checks another 

 inhibition, the result being a reactivation 3 of the inhibited reflex. 



It is quite evident from what has been said that inhibition 

 is an active process in the nervous system fully comparable to 

 the process of irritation. The results of all investigations point 

 strongly to the conclusion that the process of internal inhibition 

 is much less stable than the process of conditioned stimulation 

 and can be offset by the least outside disturbance. 



Recently a new type of conditioned reflex has been discovered, 

 the so-called secondary conditioned reflex. If a perfect con- 

 ditioned reflex to light has been worked out in a dog and a definite 

 sound is now added to it a few times in succession, while this 

 combination of the familiar light and unfamiliar sound remains 

 unsupported by the physiological stimulus, it has been observed 

 that the sound without the light causes a salivary reflex. This 

 secondary reflex is usually of very feeble action, causing the 

 secretion of one or two drops of saliva, and vanishes very quickly. 



3 The Russian term applied to this phenomenon has no appropriate English 

 equivalent. The German " Loshemmung bedingter Reflexe " is a literal translation. 



