4fiO PHYSIOLOGY 



the shock caused by the break of the primary circuit. If the reaction period is to be 

 determined for sight, a white piece of paper may be placed on an electro-magnet in the 

 primary circuit and the person will respond directly he sees this move. Many other 

 instruments have been devised for the same purpose. 



The average reaction times obtained tfith the different senses are as 

 follows : 



Electrical 

 Sight. Hearing. Stimulation of Skin. 



0-186 to 0-222 sec. 0-115 to 0-182 sec. 0-117 to 0-201 sec. 



The two figures given for each case are the extremes obtained in different 

 series of observations. 



The times vary according to the condition of the person that is the subject 

 of the experiment. They are lengthened by fatigue ; they are shortened 

 up to a certain point by continued practice. Within limits also they are 

 shortened by increase of the strength of the stimulus. 



DILEMMA. When the subject has to make a deliberate choice between 

 the parts of the body stimulated, the reaction time is considerably longer. 

 To show this, the wires from the secondary coil are connected by a switch 

 to two pairs of electrodes which are applied, one to the right and one tot lie 

 left half of the body. It is agreed beforehand that the subject shall react 

 only to stimulation, say, of the right side. The switch is removed from 

 the observation of the subject and the stimulus is applied irregularly to one 

 side or to the other-. It is found that the additional neural processes 

 involved in determining whether the stimulus is on the right side, and there- 

 fore should be followed up as agreed, adds considerably to the length of the 

 reaction time (on an average -006 sec.). It is possible to complicate the 

 dilemma to almost any extent. Thus the experiment may be so arranged 

 that either a red or a white disc appears and the subject has to react with 

 the right hand to the red disc and with the left hand to the white disc. In 

 such an experiment the reaction time was found to be be 0-154 sec. longer than 

 the^ simple reaction time. A still more complex process would be involved 

 in the experiment in which a word was spoken, and the subject had to speak 

 some other word which had some association with the word which formed tin- 

 stimulus, e.g. horse mammal; paper- pen, &c. In such an experiment 

 the reaction time was found to be as long as 0-7 to 0-8 sec. 



We see that the recording of the time of occurrence of any physical event 

 can occur only after a certain lost time, which represents the observer "> 

 reaction time for the stimulus in question. This applies however only 

 to movements carried out in response to single stimuli or to stimuli repeated 

 at irregular intervals. When the stimuli are rhythmic the lost time applies 

 only to the first one or two of the stimuli. The observer or subject is 

 conscious of the interval elapsing between the physical event and his reaction, 

 and anticipates the later stimuli so that his reaction becomes synchronous 

 with the stimulus. This synchronism of stimulus and reaction characterises 

 all rhythmic movements, such as dancing or the playing of an orchestra in 

 time with the beat of the conductor's baton. 



