38 K. S. LASHLEY 



with retention of others or a general weakening of the whole 

 mechanism. In some instances habits are possibly aggre- 

 gates of relatively independent activities linked together by 

 simple associative bonds, as in the case of reproduction of 

 lists of words, where the forgetting of one link may block 

 reproduction of the whole series, or in the maze habit which 

 has been held to consist of a simple somaesthetic-motor chain. 

 The habit of visual discrimination seems, on the contrary, to 

 be a unitary act. Although it requires the coordination of 

 many acts in response to at least two stimuli, the actual dis- 

 crimination cannot be analyzed into independent parts, but 

 seems to function either in its entirety or not at all. A similar 

 type of learning is presented by the conditioned reflex, in 

 which a single reaction is associated with the conditioned 

 stimulus. During training the reaction sometimes follows the 

 conditioned stimulus, sometimes fails to do so, but whenever 

 the reaction does appear, it is as perfect as any later amount 

 of practice can make it. The improvement can be stated only 

 in terms of the proportion of trials in which the conditioned 

 reflex is elicited. So with the discrimination reaction, the 

 loss can be stated only in terms of the percentage of trials 

 in which discrimination does not occur. 



The behavior of the subjects, both in conditioned reflex and 

 discrimination experiments, suggests that the incompletely 

 formed habit is subject to inhibition by distracting stimuli 

 (Bechterew, '13) which gradually lose their effectiveness as 

 the habit is more firmly established. With the conditioned 

 reflex the distracting stimuli are easily observed. With the 

 discrimination habit we cannot always detect the source of 

 the interference. The condition rather resembles the fluctua- 

 tions of function described by Franz ( '24) for aphasia, where 

 the subject may at times inexplicably have command of a 

 sizable vocabulary, at other times be speechless. Yet, 

 although we cannot detect the interfering agent, we may not 

 assume that the engram is now present, now absent. We must 

 rather take it for granted either that irrelevant neural proc- 

 esses sometimes inhibit the reaction or that, in a condition 



