726 THE BRAIN. 



individual, whether fresh or fatigued, fasting or replete, having taken more 

 or less alcohol, and the like. 



The reaction period of vision has long been known to astronomers. It 

 was early found that when two observers were watching the appearance of 

 the same star a considerable discrepancy existed 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 

 condition of the observers. 



604. The events taking place in the central stage are, of course, com- 

 plex, and this stage may be subdivided into several stages. Without at- 

 tempting to enter into psychological questions, we may at least recognize 

 certain elementary distinctions. The afferent impulses started by the stimu- 

 lus, 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 accordingly. 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 r 

 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 elementary 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 reac- 

 tion time under varying forms of psychical activity, we may form an esti- 

 mate of time taken up by various psychical processes. 



We may take as an instance the case in which the subject of the experi- 

 ment 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, etc. 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 (1) 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 (2) 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 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 shown but not when other colors are shown, 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 discrimi- 



