FUNCTIONS OF THE CEREBRAL HEMISPHERES 459 



response takes place. Especially is this the case when the complex mesh- 

 work of neurons of the cerebral hemispheres is involved, or when the occur- 

 rence of the reaction is associated with the conscious processes of sensation 

 and volition. In the latter case the determination of the reaction time has 

 the added interest that it gives information as to the time-relations of the 

 psychical processes which are the representation in consciousness of the 

 physiological changes occurring in the neurons of the central nervous 

 system. 



Many methods are employed for the measuring of the reaction time of conscious 

 processes. In most methods the application of the stimulus is arranged so as to close 

 the circuit of a current which flows through an electro -magnet activating a lever which 

 writes on a rapidly moving blackened surface. The reaction of the individual who 

 is the subject of experiment is arranged so that the resulting movement activates a 

 key by which the same current is opened. We thus obtain a tracing on the blackened 

 surface showing the moment of application of the stimulus and the moment at which the 

 reaction takes place. Thus, if the reaction time for an auditory stimulus is to be 

 determined, the electric current is arranged so as to pass through : 



(1) A spring contact key which can be pressed so as to make a noise. 



(2) An electric signal writing on a rapidly moving surface. 



(3) A second key, which the subject will release as soon as he hears the noise of 

 the first key and so break the current. 



The recording surface may be a drum, a pendulum myograph, or a spring myograph, 

 such as the ' shooter ' of du Bois-Reymond. If the sensory impression is to be from 

 the skin the current may be made to pass through the primary coil of an inductorium 

 and wires be taken from the second coil to some part of the surface of the skin. In 

 this case the signal may be started by opening the circuit, and the subject of the experi- 

 ment will respond by closing the circuit by means of a spring key directly he feels 

 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 with 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 to the 

 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 



