CHAP, ii.] THE BRAIN. 823 



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 of so to speak the 

 volitional side of psychical processes. 



In a somewhat similar way we may obtain a measure of the 

 time required for perception. A strong sensation following 

 too closely upon a weak one will prevent the psychical recog- 

 nition 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 afterwards 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 increases, until at an interval of 

 about -05 sec. 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 discrimination period may be conve- 

 niently used for measurements of the varying effects of circum- 

 stances. Practice shortens the discrimination period as it does 

 the simple reaction period. One of the most powerful - influ- 

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

 of the period is greater \vhen 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 

 processes to the psychical action which we call attention. On the 

 other hand, the period is distinctly prolonged if the observer be 

 distracted by concomitant sensations. For example, the period 

 for discriminating between two visual sensations is prolonged 

 if powerful auditory sensations be excited at the same time. 



The same method of measurement may be used in other 

 ways and under other circumstances with reference to psychi- 

 cal processes. It must be remembered, however, that all such 

 observations are open to many fallacies and need particular 

 caution. It not unfrequently happens that false results are 

 obtained ; for instance, the subject, expecting the stimulus to 

 be brought to bear upon him and straining his attention, makes 

 the signal before the stimulus actually comes off. And the 

 interpretation of the results obtained are in many cases very 

 difficult; but it would be out of place to dwell upon these 

 matters any further here. 



