CHAP, ii.] THE BRAIN. 



tin- \vhole reaction period will be found to 1> <ii-t in<-tl\ 

 ih M-cond case. The experiment may be varied by making use 

 of all the vowel sounds taken irregularly 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 wh-n 

 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 required 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 suppose 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, auditory, &c., and 

 according to the kind of difference in the sensation 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, tne 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 following 

 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 instance, 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 will is formed upon that recognition ; these are 

 the same in the two cases; it takes place in the processes by 

 which the will is brought to bear on the nervous motor apparatus 

 for making the signal, on the cortical origin, for example of the 

 pyramidal tract ; these processes take a longer time in the case of 



