REACTIONS OF CHICKS TO OPTICAL STIMULI 287 



training to this end was begun. The habit proved a compara- 

 tively easy one for the chicks to form, as evidenced by the first 

 training series, 1-8. Each of the four animals finished the 

 training, including the final twenty perfect reactions, within a 

 total of 80 trials. When all the chicks had formed the red- 

 blue habit perfectly, a slightly different intensity of blue was 

 opposed to red at 10 and ico alternately. The chicks con- 

 tinued the specific reaction to red, two of them without error. 

 They had evidently not formed a habit of specific reaction to 

 red or blue of a particular brightness value. As soon as the 

 original habit was ascertained to be in perfect condition in all 

 the chicks, they were tested further with red and blue, each color 

 this time being used equally often at the value of 10 against the 

 other color at 100 in a series of ten trials. Surprisingly, not a 

 single error was made in this series by any chick. It is well 

 to bear in mind that blue 1 o was without question much brighter 

 than red 100; and red 10 than blue 100. 



Elsewhere 3 the writer has said something about Yerkes' dis- 

 crimination method and the peculiar emphasis that sometimes 

 attaches to the stimulus in connection with which the electric 

 shock is administered. This emphasis on the blue was very 

 noticeable in the results from all four chicks. When white light 

 was opposed to blue in the control test, the chicks uniformly 

 rejected blue, regardless of which stimulus had the greater 

 brightness value. When white was substituted for blue and 

 the factor of brightness eliminated by variation, the score was 

 red 29, white 27, in a total of 56 trials. No. 61, however, with a 

 little training formed a perfect red-white habit, the element of 

 brightness being again eliminated in the manner just mentioned. 



What shall be our interpretation of the above facts? When 

 the stimuli were red 70 versus blue 10 in the preference tests, 

 no marked preference was shown for either stimulus. Prefer- 

 ence shifted decidedly to blue in the combination red 100 

 versus blue 10, and just as decidedly to red in the combination 

 red 40 versus blue 10. That is, from the indifference point prefer- 

 ence varied as the relative brightness of the stimuli varied. It 

 seems reasonable to suppose, therefore, that at the above in- 

 difference values, the two stimuli were approximately equal in 



3 hoc. cit.j p. 69. 



