REFLEX ACTION : REACTION TIME : EXCITATION OF CORTEX 99 



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. 



Volitional time. Similar arrangements are made, but with the 

 understanding 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 with the employment of 

 different sounds and the exhibition of different colours, but the methods for re- 

 cording the reaction times are essentially the same. For rapid and accurate 

 work it is usual to employ a specially constructed 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 

 du Bois key in the secondary circuit of an induction-coil (use the Helmholtz 

 modification) and applied to various spots in the excitable region of the frontal 

 lobe, the first temporal gyms, and the occipital lobe, and the results are 

 noted. 



The rnonopolar 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 the other [stimulating] electrode is a small spiral 

 platinum wire with blunt point, which is applied to the excitable areas of 

 the cortex. 



