134 MENTAL EVOLUTION IN ANIMALS. 



ception.* The last-named author found that an exposure of 

 ^o second is required for the perception of a row of six or 

 seven letters. 



Other experiments prove that the more complex an act of 

 perception, the more time is required for its performance. 

 Thus Donders has shown that when an experiment in re- 

 action-time is made to consist, not merely in signalling a 

 perception, but in signalling one of two or more perceptions, 

 the reaction-time is lengthened, owing to the greater time 

 required for performing the more complex psychical process 

 of distinguishing which of the expected stimuli is perceived, 

 and in determining to make or to withhold the response 

 accordingly. The state of matters thus presented to the mind 

 is called by Donders a " Dilemma," and the following is his 

 table of results : — 



Dilemma between two spots of the skin, right or left foot 

 stimulated by an electric shock ; signal to be made in one 

 case only 0-066" 



Dilemma of visual perceptions between two colours, sud- 

 denly exhibited,; signal to be made on seeing the one but 

 not on seeing the other .. .. .. .. .. 0184 



Dilemma between two letters ; signal to be made on seeing 



one only 0166 



Dilemma between five letters ; signal as before . . . . 0"170 



Dilemma of hearing ; two vowels suddenly called ; signal to 



be made on hearing one only . . . . . . , . 0'056 



Dilemma between five vowels ; signal as before . . . . 0088 



The above table gives, in each case, not the whole period 

 between the occurrence of the stimulus and the occurrence 

 of the response, but the difference between the time required 

 for this whole period when a single stimulus has to be 

 answered, and when only one of two or more possible stimuli 

 has to be answered. It will thus be seen that the time 

 required for the act of meeting a dilemma is from £ to ^ of 

 a second longer than that which is required to signal a 

 simpler perception.! 



This " Dilemma- time " has also been estimated where the 

 other senses are concerned by Kries and Auerbach, with the 

 following results : — | 



* Archiv. f. d. ges. Physiol., Bd. IV, p. 329 ; Monatsber. d. Per. Acad., 

 June, 1871. 



f For Donders' investigations, see Archiv. f. Anat. und Physiol., 1868, 

 pp. 657-81. 



X Archiv. f. d. ges. Physiol., 1877, pp. 293-380. 



