VISUAL PERCEPTION OF THE CHICK 103 



pages, it is quite probable that the reactions observed were 

 made, not to the visual details which the experimenters sought 

 to present to the chick, but to unsuspected visual complexes , 

 similar to the "complex remainder" which is here designated 

 as "brightness and general illumination." It is desirable that 

 subsequent studies further analyze our visual complex and 

 determine the visual acuity of the chick with each element as 

 has been done with size and form. 



When presented with a standard circle 6 cm. in diameter, 

 it has been found that a variable circle 4.5 cm. in diameter 

 can be discriminated by the chick. If one were to increase 

 the size of the variable by increments no greater than one 

 millimeter, it is probable that the chick would be able to 

 discriminate a larger variable than the 4.5 cm. circle. The 

 threshold of difference with a standard 6 cm. circle, therefore, 

 is certainly one-fourth and perhaps less. In the perception 

 of size the ability of the chick is evidently inferior to that of 

 the crow. 



Earlier experiments on the chick's perception of forms have 

 failed to eliminate all discriminable possibilities other than the 

 factor of form. The chick can discriminate between circles 

 and triangles and circles and squares which are equal in area, 

 but, with the conditions as described in this study, none of 

 the subjects were able to discriminate between visual stimuli 

 on the basis of form alone. Reactions to visual stimuli which 

 have been interpreted by observers as indicating form discrimi- 

 nation have probably been made on the basis of unequal 

 stimulation of different parts of the retina. If local inequality 

 of excitations be the basis of these reactions, then the apparent 

 discrimination of form by the chick is, in reality, a keen percep- 

 tion of size differences. In the perception of form, also, the 

 ability of the crow seems to surpass the ability of the chick. 



The chick evidently has a keener perception of flicker than 

 of form stimuli. The present experiment indicates that it 

 perceives at least a two to one flicker relation. How much 

 lower the threshold may be, this study has not indicated. 



The discrimination method employed in the present investi- 

 gation furnishes a means of recording the learning process of 

 the chick. An essential part of the discrimination habit is a 

 maze problem. The formation of this maze habit, it was 

 found, can be quantitatively measured in terms of approaches 



