LXX.] BEACTION-TIMK. 323 



LESSON LXX. 



RBAOTION-TIMB— CEREBRAL HEMISPHERES. 



Reaction-Time is the interval that elapses between the applica- 

 tion of a stimulus to a sense-organ and the moment the stimulus 

 is responded to by the individual. For simple reaction-time, or 

 sensori-motor reaction-time, all discrimination and choice are elimi- 

 nated by repeating the same sensation and using the same response. 

 Rutherford's results {Proc. Roy. Sor. Edin., July 10, 1894) give 

 rather longer periods than some German observers. He finds the 

 pendulum-myograph very advantageous in experiments on hearing 

 and touch, as successive curves can be superimposed. The mean 

 reaction-time he found to be, for sight, o.2o"-o.2 2"; hearing, 

 0.1 5' -0.1 6"; touch, o.i4"-o.i5" ^cheek), o.i^"-o.i8" (skin of 

 finger). 



Reaction-Time for Touch in Man. 



1. Pendulum-Myograph Method {Rutherford). 



Two persons are required, and the observed person should not 

 see what the observer is doing. 



(a.) Arrange the apparatus as in fig. 243. The stimulation is 

 done always at the same moment when the pendulum in its swing 

 breaks the primary circuit. It is convenient, as shown in the 

 figure, to use an electro-magnet for releasing the pendulum. 



{h.) The electrodes from the secondary coil are applied to any 

 part of the skin, and the observer, when he feels the shock, closes 

 the " response key," whereby a mark is made on the gla«s plate. 

 Time should be recorded on the plate beforehand (60 or 100 D.Y. 

 per second). 



(c.) If it is desired for sound, a telephone is placed in the 

 secondary circuit and the observed person responds when he hears 

 the click at the moment of breaking the primary circuit. 



Fig. 244 shows the result obtained for the reaction-times for 

 touch by the pendulum-myograph method (fig. 243), chronograph 60 

 D.V. per second. The vertical line indicates the moment at which 

 an induction shock was given to (i) skin of left cheek ; (2) 

 left side of neck ; (3) left upper arm at insertion of deltoid ; (4) 

 left little finger ; (5) dorsum of left foot at root of toes. Response 

 signal was always given by right forefinger. The vibrations 

 following each signal of response result from the momentum of the 

 lever {Rutherford), 



