REFLEX ACTION: REACTION TIME H5 



of a time tracing, indicates the time between stimulus and response 

 i.e., the reaction time in the case of each of the three senses. To 

 determine this with any accuracy several observations must be made 

 with each method of stimulation and the shortest time taken. 



Discrimination time. For the measurement of this the observed 

 person places one finger over each lever. It is agreed beforehand 

 that he is only to react to a stimulus received on the one side, not 

 on the other. The experimenter may stimulate either. It will be 

 found that the reaction time is lengthened by a certain interval, 

 and this increase of reaction time is termed the discrimination time. 

 Determine and record this as before. 



Volition time. Similar arrangements are made, but with the un- 

 derstanding that it is only the hand on the side which receives the 

 stimulus which is to be used for the response. The reaction time is 

 now found to be still more lengthened because the observed person 

 has to make a double decision ; viz., to determine not only which of 

 the two hands has been stimulated, but also which one he has to use 

 in response to the signal. 



Variations of the above experiments can be made by the employ- 

 ment of different sounds and exhibition of different colours, but the 

 methods for recording the reaction times are essentially the same. 

 For rapid and accurate work it is usual to employ a specially con- 

 structed clock which can register the time of a reaction to a fraction 

 of a second. 



Excitation of the cortex cerebri. A monkey, anaesthetised with 

 ether, is used for this demonstration. A considerable portion of the 

 skull cap is removed on one side by trephining the skull and enlarging 

 the aperture by bone-forceps. The dura mater is then cut through 

 below and reflected towards the middle line, thus exposing the cerebral 

 surface. A pair of blunt-pointed platinum electrodes, with their 

 points 1 mm. apart, is connected with a key in the secondary 

 circuit of an induction coil (use the Helmholtz modification) and is 

 applied to various spots in the excitable region of the frontal lobe, the 

 first temporal gyrus and the occipital lobe, and the results are noted. 



The monopolar method of stimulation may also be employed for 

 these observations. In this case one electrode is a flat pad of wash- 

 leather wetted with strong salt solution and laid on any part of the 

 body (the skin should first be shaved) ; the other [stimulating] 

 electrode is a small spiral platinum wire with blunt point, which is 

 applied to the excit-able areas of the cortex. 



