VISUAL PERCEPTION OF THE CHICK 13 



A study of the relation of strength of stimulus to rate of 

 learning in the chick, though subsequent to Breed's work, is 

 reported by Cole (7) prior to Breed's account (5) of his dark 

 room work. Cole presented brightness stimuli to his chicks, 

 using the same apparatus as Breed. 



A further stage in the development of the combined "dark- 

 room-discrimination" method is indicated in a joint report by 

 Yerkesand Watson (35), on methods of studying vision. They 

 have greatly refined the apparatus used by Breed and Cole and 

 limited its use to studying light vision. In addition, they have 

 developed a special apparatus for studying color vision. 



The first use of the Yerkes light vision apparatus with chicks 

 is my study of size and form perception (2). The aim of this 

 study was to locate thresholds for standardized forms and sizes. 

 With a standard circle 6 cm. in diameter, the threshold was 

 found to be one-fourth to one-sixth. But instead of determining 

 a limit in form perception, the question became chiefly whether 

 the chick perceives forms. It was found that the chick could 

 discriminate between a circle and an upright triangle of equal 

 area, but the evidence of discrimination disappeared when the 

 triangle was inverted. Since Breed failed to make a similar 

 control, his statement that one chick "learned to discriminate 

 two optical stimuli on the basis of form" may be questioned. 

 A safer conclusion seems to be that the chick reacts to unequal 

 stimulation of different parts of the retina. 



With reference to this interpretation of the reactions to form 

 stimuli, Watson {?>2>) says: 



Bingham has raised the whole question as to what is meant by form.' 

 . . . • . While the chick can apparently respond to the difference in form be- 

 tween the circle and the square, and the circle and the triangle, when they are 

 equal in area, yet such responses, after all, are really nothing more than keen 

 perception of size differences. He draws this conclusion from the fact that 

 after the chick has learned the circle-triangle habit with the base of the triangle 

 down, the habit will disintegrate if the apex is placed down. (p. 366). 



Quite naturally, this interpretation of form perception has 

 called forth considerable comment. Hunter's contribution (13) 



3 For this suggestion I hasten to acknowledge my indebtedness to Professor 

 Yerkes who proposed this question as a part of my experimental task. Because 

 he conceived the possibility, however, it does not follow that he agrees with my 

 view. All responsibility for subsequent use of the conception as an explanation of 

 the facts, I alone assume. 



