CONTRIBUTIONS FROM PAWLOW'S LABORATORY 369 



as distinct from that of 1,000 vibrations, or 94 strokes of the 

 metronome as different from 100 strokes. 



The gradual concentration of the nervous process with the 

 resulting differentiation of the stimulus is due to the develop- 

 ment in the central nervous system of inhibitory processes. 

 This can be demonstrated by numerous facts. If a conditioned 

 reflex has been worked out to a sound of 1,000 vibrations with 

 such precision that 1,012 vibrations are differentiated into a 

 distinct stimulus the following interesting things can be observed. 

 Whenever the sound of 1,000 vibrations is produced, a definite 

 quantity of saliva is secreted; whenever the 1,012 vibrations are 

 tried no salivary reflex is obtained. If, however, the effective 

 stimulus is applied immediately after the ineffective, it is found 

 to have either no influence or only a very feeble influence on the 

 salivary secretion. Its original effect as a conditioned stimulus 

 of the salivary gland is regained only after considerable time. 

 It is obvious, therefore, that the differentiated stimulus caused 

 an inhibition, and in virtue of this inhibitory action of unusual 

 stimuli the sphere of influence of the usual stimulus becomes 

 gradually restricted until at last it travels over a completely 

 isolated path in the cortical portion of the analyser. 



This hypothesis of differentiation as the result of inhibition 

 is supported by several considerations. In the first place it is 

 possible to offset an accomplished differentiation by reactivating 

 those portions of the cortex whose susceptibility for the differ- 

 entiated stimulus has been checked. This is, indeed, a common 

 experience and can be demonstrated by a simple experiment. 

 Here is a dog with an auditory conditioned reflex differentiated 

 to one-eighth of a tone. The differentiation is so sharp that in 

 no instance will a sound one-eighth of a tone different from the 

 familiar one stimulate the secretion of a single drop of saliva. 

 If, however, some unusual noise should be made during the experi- 

 ment which excites the animal the differentiation of the auditory 

 conditioned reflex will be temporarily lost and the salivary reflex 

 will occur in response to any tone. This state of affairs may last 

 fifteen to thirty minutes, when the differentiation becomes once 

 more very rigid. 



The hypothesis receives further proof from the fact that the 

 inhibition can be increased by repeating the differentiated 



