THE TIME CONSUMED BY PSYCHOPHYSICAL PROCESSES 673 



feel that back of the world of phenomena powers are at work for which human 

 knowledge can scarcely find an adequate metaphor. 



4. THE TIME CONSUMED BY PSYCHOPHYSICAL PROCESSES 



There remains for us to summarize briefly what has been discovered with 

 reference to the time occupied by psychophysical processes. 



The point of departure in all investigations of this subject is the time 

 required for a person to react with a definite voluntary movement to some 

 external stimulus (simple reaction time). It will be apparent at once that 

 some measurable time is required for such reactions, if we but reflect that 

 the propagation alone of a stimulus along the afferent and efferent nerves con- 

 cerned consumes a certain amount of time (cf. page 417). But in the process 

 which we are considering now, something takes place in the sensorium and 

 something in the motor region, and between these something, probably, else- 

 where in the cortex, all of which constitute the psychophysical factors. The 

 special interest attaching to these determinations is that they will give us 

 some information as to how much time is required for these purely central 

 processes. 



Experiments in this field are usually carried out by having the subject of 

 the experiment open the current of an electric signal the moment he receives 

 a given stimulus. If, for example, the reaction time is being taken for an 

 auditory stimulus, it is necessary to have in the same electrical connection: 

 (1) the signal, (2) a key for the subject, (3) a key by which the current is 

 closed by the director of the experiment and which at the same instant causes 

 a sound to be made by (4) an electric bell or other device. The sound which is 

 made when the current is closed constitutes the stimulus to which the subject 

 reacts by opening the current. If the electric signal be arranged so as to record 

 on a moving surface the instants of opening and closing, the time which inter- 

 venes will be the reaction time. This time can be determined directly if a 

 clockwork whose hands mark thousandths of a second be set in motion by the 

 closing and stopped by the opening of the current. Such an instrument is 

 known as a chronoscope. 



The simple reaction time varies all the way from 0.11 to 0.55 second 

 according to the nervous organization of the subject, and the kind of sensory 

 stimulus employed. Likewise if a series of experiments be carried out on 

 the same person and with the same kind of stimulus, only varying in strength, 

 a considerable variation in the reaction time is noted, which cannot be due 

 to any variation in the rate of propagation in the nerves exercised, conse- 

 quently must be accounted for by differences in the time consumed in the 

 psychophysical processes. Such variations moreover can be perceived sub- 

 jectively, so that one can tell within 0.05-0.06 of a second whether a given 

 reaction time was longer or shorter than a previous one. Remembering that 

 the propagation of the sensory stimulus before it rouses the conscious sensa- 

 tion, as well as the motor impulse after it has once been discharged, is an 

 entirely unconscious process, we see that the time subjectively estimated covers 

 the span between perception of the sensory impulse and release of the volun- 

 tary impulse. 



