ON THE TIME TAKEN UP BY CEREBRAL OPERATIONS. 7'27 



nate, 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 there is 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, 

 etc., and according to the kind of difference in the sensation which has to 

 be discriminated, for instance in visual sensations between colors 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 second to 0.062 second. 



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 the 

 unaccustomed left hand than in the case of the usual right hand. In this 

 way we obtain a measure, so to speak, of the volitional side of psychical 

 processes. 



In a somewhat similar way we may obtain a measure of the time re- 

 quired for perception. A strong sensation following too closely upon a weak 

 one will prevent the psychical recognition of the weaker one. If, for 

 instance, two or three letters in white on a black background be presented 

 to the eye, and a large white surface be presented afterward at an interval 

 which is made successively shorter and shorter, it is found that when the 

 interval is made very brief indeed the letters cannot be perceived at all. 

 In proportion as the interval is prolonged, the recognition of the letters in- 

 creases, until at an interval of about 0.05 second they are fully and clearly 

 recognized. That is to say, the time required for perception is in such a 

 case of about that length. * 



The duration of all these psychical processes, as of the simple reaction 

 period itself, varies of course under different circumstances, and the dis- 

 crimination period may be conveniently used for measurements of the vary- 

 ing effects of circumstances. Practice shortens the discrimination period as 

 it does the simple reaction period. One of the most powerful influences is 

 that of attention. And it is stated that the shortening of the period is 

 greater when the attention is concentrated on the making of the signal than 

 when it is more especially directed to recognition of the stimulus; in other 

 words, the volitional processes are more amenable than are the perceptive 



