INTEGRATIVE ACTION OF THE CEREBRUM 957 



lation of the inactive spot must have caused an inhibition to be set 

 up in the nerve centers concerned in the reflex. This inhibition only 

 gradually passes away, disappearing first in the spot farthest removed 

 from that made inactive, but it may take several minutes before all the 

 active spots have reacquired their original sensitivity. 



The persistence of the inhibition produced by stimulating the inac- 

 tive spot in the above experiment indicates an important factor in con- 

 nection with the production of conditioned reflexes. For example, an 

 animal can be trained to know that in a certain number of minutes after 

 the sound of a given bell food will be presented to him; the condi- 

 tioned reflex will become established so that he salivates at exactly 

 the same time after the bell is sounded. Something must be going on 

 in the centers during this time something inhibiting the reflex. If 

 during this interval of inhibition some other sensory stimulus is applied, 

 it will be likely to cut short the inhibition; in other words, it produces 

 an inhibition of inhibition, so that the secretion of saliva occurs. 



Another most curious combination of conditioned stimuli is illustrated 

 in the following experiment. Suppose, for example, that a given light 

 and sound are each separately made a stimulus for a conditioned reflex, 

 but that when they occur together there is no reflex. Suppose now that 

 while one of these active stimuli is being presented, the other stimulus 

 is also presented; the result will be that the secretion produced by the 

 one stimulus will stop. Evidently, although each is in itself a stimulus, 

 acting together they cause inhibition. 



By studying the conditioned reflexes after a certain part of the cere- 

 bral cortex has been removed, it has been found that the power of estab- 

 lishing certain kinds of conditioned reflexes becomes abolished, while 

 that for others is retained. 



The writing of Sherrington, 37 Loeb, 38 Watson, 39 and Margulis 41 should 

 be consulted for further details concerning the material in this chapter. 



