822 DURATION OF PSYCHICAL PROCESSES. [Book hi. 



only o will be spoken, and then be determined again (ii) when 

 he has to discriminate in order that he may make the signal when 

 a is spoken but not when o is spoken, he not knowing which 

 is about to be spoken, the whole reaction period will be found to 

 be distinctly longer in the second case. The experiment may 

 be varied by making use of all the vowel sounds taken irregu- 

 larly as the stimulus, the subject responding by a signal to one 

 only, as arranged beforehand. And of course other orders of 

 stimulus may be used, either visual, the signal being made 

 when a red light is shewn but not when other colours are 

 shewn, or tactile, the signal being made when one part of the 

 body is touched but not when other parts are touched, and 

 the like. 



In such experiments where the subject has to distinguish, 

 to discriminate between two or more events, the prolongation 

 of the reaction period is obviously due to the longer time re- 

 quired for the psychical processes taking place during what we 

 have called the central stage. In the two cases, one without and 

 the other with discrimination, not only are the afferent and 

 efferent stages the same in both, but we have no reason to sup- 

 pose that in the central stage is there any difference between 

 the two cases as to the time taken up by the transformation of 

 simple sensory impulses into perceptions, or as to that taken up 

 by the will in gaining access to the motor apparatus and so 

 starting the processes of the efferent stage ; the delay takes 

 place in the psychical processes intervening between these two 

 parts, and the amount of delay is the measure of the time 

 needed for the processes involved in the discrimination. This 

 " discrimination period " has been found to differ in the same 

 individual according to the sensation employed, visual, audi- 

 tory, &c, and according to the kind of difference in the sensa- 

 tion which has to be discriminated, for instance in visual 

 sensations between colours or between objects in different parts 

 of the field of vision. In a series of observations made in this 

 way, the discrimination period, i.e. the prolongation of the 

 simple reaction period due to having to discriminate, was found 

 to range from 0-011 sec. to 0-062 sec. 



Another series of observations may be made in the follow- 

 ing way. The signal being one made with the hand, the 

 simple reaction period for a stimulus is determined with the 

 signal given by the right hand. Two kinds of stimuli are then 

 employed, both of the same order, two vowel sounds for in- 

 stance, and the subject is directed to respond to one vowel 

 with the right hand and to the other with the left hand. It 

 is found, the subject being right-handed, that the reaction 

 period is greater when the signal is made with the left hand. 

 In this case the delay takes place not in the recognition of the 

 effects of the stimulus, nor in the processes through which the 



