1122 DURATION OF PSYCHICAL PROCESSES. [BOOK in. 



between their respective reaction periods, and that the difference, 

 forming the basis of the so-called ' personal equation/ varied from 

 time to time according to the personal conditions of the observers. 



692. The events taking place in the central stage are of 

 course complex, and this stage may be subdivided into several 

 stages. Without attempting to enter into psychological questions, 

 we may at least recognize certain elementary distinctions. The 

 afferent impulses started by the stimulus, whatever be their 

 nature, when they reach the central nervous system undergo 

 changes, and as we have seen, probably complex changes before 

 they become sensations ; and further changes, now of a more 

 distinctly psychical character, are necessary before the mind can 

 duly appreciate the characters of these sensations and act accord- 

 ingly. Then come the psychical processes through which these 

 appreciated sensations, or perceptions, or apperceptions as they are 

 sometimes called, determine an act of volition. Lastly, there are 

 the executive processes of volition, the processes which, psychical 

 to begin with, end in the issue of coordinate motor impulses, or, 

 in other words, start the distinctly physiological processes of the 

 efferent stage. We may thus speak of the time required for the 

 perception of the stimulation, of the time required for the action 

 of the will, and of the time required for the complex psychical 

 processes which link these two together. Accepting this elemen- 

 tary analysis, it is obvious that the total length of the central 

 stage may be varied by differences in the length of each of these 

 parts ; and a more complete analysis would of course open the 

 way for further distinctions. Hence, by studying the variations 

 of the whole reaction time under varying forms of psychical 

 activity, we may form an estimate of time taken up by various 

 psychical processes. 



We may take as an instance the case in which the subject 

 of the experiment has to exercise discrimination. The mode of 

 making the signal being the same, and the stimulus being of 

 the same order in each trial, that is to say, visual, or cutaneous, 

 or auditory, &c., and general circumstances remaining the same, 

 two different stimuli are employed, and the subject is required 

 to make a signal in response to the one stimulus, but not to the 

 other; the subject has to discriminate between the psychical 

 effects of the two stimuli. Suppose, for example, the stimulus 

 is the sound of a spoken or sung vowel, and the subject is 

 required to make a signal when a is spoken or sung, but not 

 when o is spoken or sung. If the subject's whole reaction period 

 be determined (i) in the usual way, with either a or o spoken (and 

 the result will be found not to differ materially whether a or o -be 

 used), the subject knowing that only a or 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, 



