SCIENCE.-SUPPLEMENT. 



FRIDAY, SEPTEMBER 10, 1886. 



AN EASY METHOD OF MEASURING THE 

 TIME OF MENTAL PROCESSES. 



It is justly considered one of the triumphs of 

 phj'siological psychology to have made the ele- 

 mentary processes involved in perceiving and think- 

 ing more real and better known, bycomparmgthe 

 times necessary for their performance. It has made 

 the connection between mental action and the 

 function of the brain closer, by showing that all 

 processes take time, and that this time is varied 

 by abnormal conditions of the brain. These psy- 

 chometrical observations, though of but recent 

 date, form one of the favorite fields of present 

 psychological research. 



The usual method of measuring one's reaction 

 time is somewhat as follows : The subject is seated 

 with his liand in contact with an electric key: his 

 attention is to be directed, we wiU say, to a flash 

 of light electrically produced before him. The 

 operator controls the appearance of the spark by 

 simply breaking an electric connection : at the same 

 instant he sets in motion (by the same current) a 

 Hipp cln-onoscope, ' which in turn is stopped im- 

 mediately on the closure of the key by the subject. 

 The interval during which the clock was recording 

 will then be the time necessary for the subject to 

 perceive the light. But in this time several ele- 

 ments are involved. Tliese can be separately in- 

 vestigated by other means. We have, 1^, a series 

 of afferent processes, such as the time necessary 

 for the sense-organ (in this case the retina ^) to be 

 affected, the time necessary for the impulse to 

 travel along the sensory nerves to the brain ; 2°, 

 the reception of the sensation in the brain (plus, 

 perhaps, the generation of the wUl) ; and, 3°, a 

 series of afferent phenomena, including the trans- 

 mission of the impulse from tlie brain to the spinal 

 cord, down the cord to the anterior nerve-roots, 

 thence along the afferent nerves to the muscles, 

 the latent time of the muscles, and, finally, the 

 contraction of the muscles closing the key. The 

 phenomenon in which the psychologist is interested 



1 This Is an instrument which, by a clock-work arrange- 

 ment, records to the thousandth of a second. It is set in 

 motion electrically by the release of a magnet, and stopped 

 by the closure of the same. A tuning fork recording on a 

 revolving drum, or similar arrangement, is often used in its 

 place. 



2 It the stimulus excited the touch, we should also have 

 the time for transmission along the nerve to the spinal 

 cord, and the slow travelling up the cord. 



is included under 2°. But to determine that, he 

 must eliminate 1° and 3°. And here we see how 

 essentially physiological a real psychology is : it has 

 need of facts which none but a physiologist would 

 undertake to discover. We want to know the rate 

 at which the nervous impulse travels. This Helm- 

 lioltz measured in 1850, only a few years after Jo- 

 hannes Muller despaired of our ever ascertaining it, 

 and found to be about 38 metres (108 feet) per sec- 

 ond for both motor and sensory nerves. The trav- 

 elling along the cord is much slower, — about 10 

 metres (83 feet) per second. The very minute 

 times involved in the delay in the sense-organ, 

 ganglion of the spinal nerves, and muscle, have 

 also been accurately determined. The whole 

 operation, i.e., the complete reaction time, takes 

 about Y of a second, of wliich the process included 

 under 2° consumes a share subject to great varia- 

 tion according to the conditions of the experi- 

 ment, but always small. 



Let the operation be somewhat more complex. 

 Say that the light shall not always be of the same 

 kind, but that at times it shall be red, and at times 

 blue. The subject is not to react until he has per- 

 ceived the blueness or redness of the light. If we 

 subtract the simple reaction time from the total 

 time intervening between the appearance of the 

 colored light and the closing of the key after the 

 subject lias seen whether it is a red or a blue light, 

 we shall have the time required to distinguish red 

 from blue. Tliis we will caU the ' distinction ' 

 time. We can evidently make the distinction 

 more difficult by having three, four, or more 

 colors. The average distinction time between two 

 sensations, though largely variable, is about from 

 tV to -^3^ of a second, or less. 



In the above experiment it has been assumed 

 that the nature of the reaction has remained un- 

 altered ; that is, in each case the subject closed the 

 one key before him. This, too, is capable of com- 

 plication. We can agree that the subject is to 

 react by a key on his right hand if a red light 

 appears, and by one on his left if a blue light ap- 

 pears. If we subtract the time necessary for all 

 the f)rocesses up to the color distinction from the 

 time requu-ed to close the appropriate key, we shall 

 obtain the time necessary for making a choice be- 

 tween two reactions. While before we were test- 

 ing the readiness of the subject's sensibility and of 

 his judgment, we are now testing the alertness of 

 his will. That time necessary for this new yito- 

 cess we wiU call the ' choice ' time. According to 



