92 ROBERT M. YERKES. 



to difference in strength of stimulus, but it is of interest to en- 

 quire whether the quality of the stimulus is not of importance. 

 We may ask, for example, whether the reaction-time to the thres- 

 hold stimulus of all modes of stimulation is the same. If it is 

 not, quality of stimulus is evidently significant. Wundt l presents 

 the following figures in support of his statement that the reaction- 

 time to the threshold intensity of all modes of stimulation is the 

 same. 



The results which I have gotten with frogs in working with 

 electric and tactual stimuli cause me to question the appli- 

 cability of this statement to the reactions of all organisms. It 

 seems highly probable that the just perceptible stimulus reac- 

 tion-time is by no means the same for different qualities of 

 stimulus. Those modifications of the vital processes which make 

 survival possible appear even in the responses to minimal stimuli. 

 In one case the just perceptible stimulus may cause nothing 

 more than a slight local change in circulation, excretion, muscu- 

 lar action, in another it may produce, just because of the particu- 

 lar significance of the stimulus for the life of the organism, a 

 violent and sudden motor reaction. 2 



ABSOLUTE AND RELATION VARIABILITY. 



As already pointed out 3 it is generally useless to compare re- 

 action-times with respect to variability unless the reaction-time 

 value as well as the absolute variability is considered. If, for 

 example, to an electric stimulus Gonionemus reacts in 2.0 seconds, 

 with a variability of 0.5 sec. ; and to a photic stimulus in 6.0 

 seconds, with a variability of 1.5 sec., it is not correct to say that 

 the reaction-time to light is three times as variable as that to 

 electricity. As a matter of fact the two variabilities as such are 



1 Wundt, Wm. : " Grundziige der physiologischen Psychologic," Fiinfte Auflage, 

 Leipzig, 1903, Dritte Band, S. 428-429. 



2 Harvard Psychological Studies, Vol. I., 1903, p. 625. 



3 Amer. Jour. Phvsiol., Vol. 9, 1903, p. 291. 



