GENERAL CONDITIONS OF BRAIN-AGTIVITT. 95 



Fatigue lengthens it. 



Concentration of attention shortens it. Details will be 

 given in the chapter on Attention. 



The nature of the signal makes it vary.* Wundt writes : 



"I found that the reaction-time for impressions on the skin with 

 electric stimulus is less than for true touch-sensations, as the following 

 averages show: 



Average. /jf^ 



Sound 0.167 sec. 0.0221 sec. 



Light 0.223 " 0.0319 " 



Electric skin-sensation 0.301 " 0.0115 " 



Touch-sensations 0.213 " 0.0134 " 



"I here bring together the averages which have been obtained by 

 some other observers : 



Hirsch. Hankel. Exner. 



Sound 0.149 0.1505 0.1360 



Light 0.200 0.2246 0.1506 



Skin-sensation 0.182 0. 1546 0. 1337 " t 



Thermic reactions have been lately, measured by A. 

 Goldscheider and by Viutschgau (1887), who find them 

 slower than reactions from touch. That from heat espe- 

 cially is very slow, more so than from cold, the differences 

 (according to Goldscheider) depending on the nerve-ter- 

 minations in the skin. 



Gustatory reactions were measured by Vintschgau. They 

 differed according to the substances used, running up to 

 half a second as a maximum when identification took place. 

 The mere perception of the presence of the substance on 

 the tongue varied from 0".159 to 0".219 (Pfliiger's Archiv, 

 XIV. 529). 



Olfactory reactions have been studied by Vintschgau, 



* The nature of the movement also seems to make it vary. Mr. B. I. 

 Gilmau and I reacted to the same signal by simply raising our hand, and 

 again by carrying our hand towards our back. The moment registered was 

 always that at which the hand broke an electric contact in starting to 

 move. But it started one or two hundredths of a second later when the 

 more extensive movement was the one to be made. Orchansky, on the 

 other hand, experimenting on contractions of the masseter muscle, found 

 (Archiv f. (Anat. u.) Physiol., 1889, p. 187) that the greater the amplitude 

 of contraction intended, the shorter grew the time of reaction. He 

 explains this by the fact that a more ample contraction makes a greater 

 appeal to the attention, and that this shortens the times. 



+ Physiol. Psych., ii. 223. 



