THE ROLE OF REFLEX INHIBITION 609 



unwonted stimulus — e.g. a musical tone — is presented along 

 with the visual stimulus A. It is usually found that the reflex 

 salivation then fails to occur. The new stimulus inhibits the 

 conditional reflex to stimulus A. 



Again, suppose a conditional reflex to stimulus A to have 

 been established and that then some associate stimulus — e.g a 

 musical tone — is added to A and on a number of successive 

 occasions when this is done the feeding of the animal, which 

 regularly follows the presentation of the stimulus A alone, 

 is regularly withheld. Not only is there then no reflex saliva- 

 tion when the twofold stimulus is applied but on returning to 

 the original procedure and presenting the visual stimulus A 

 itself, the conditioned reflex acquired for that stimulus is found 

 also to have been lost. By training it can be reacquired. 

 But even when this reacquirement has taken place, the pre- 

 sentation once more of the note as an accompaniment of A 

 is found to inhibit the conditioned reflex which A otherwise 

 evokes. 



In assessing the significance of such results it is helpful to 

 remember that the conditioned reflex is acquired by training 

 and that in the training inhibition is a notable factor. The 

 first step is the directing of attention to the conditioned stimulus 

 — e.g. to the visual symbol, which denotes that food will be 

 offered. Unless that is successful the training cannot ensue. 

 In attention, besides the peculiar and characteristic free accen- 

 tuation of certain stimuli — i.e. those attended to — there is a 

 concomitant blocking of other stimuli seeking entrance to the 

 mind by other channels. This repression of reaction to other 

 stimuli is an essential element in the attentive process and it 

 amounts, as we know from our own experience, to a repression 

 so complete that we do not for the time being even perceive 

 other stimuli than the one attended to. And this concomitant 

 inhibition, even after the acquisition of the reaction, still remains 

 a sine qua non for the reaction ; it is a condition for the con- 

 ditioned reflex. Here, just as in motor reactions, excitation and 

 inhibition are complemental processes, combined to ensure a 

 unified response to the promptings of the outside world. 



8. Conclusions 



The power of the environment through the nervous system 

 to incite this and that activity of the organism has long been 



40 



