VISUAL PERCEPTION OF THE CHICK 57 



unnatural. Under natural conditions, it probably relies upon 

 combinations of many visual factors. The perception of an 

 isolated visual quality is a technical human problem, not a 

 natural animal problem. 



Johnson^ criticises the use of the visual complex in prelimi- 

 nary training on the ground that one is risking a large waste of 

 time. He says: 



It has happened that an experimenter discovers after several weeks of training 

 that the animal had learned to react to difference of e.g. brightness, and had not 

 been affected by the other stimulus-differences. It would seem that a difference 

 in luminous intensity is especially objectionable since in dark surroundings the 

 animal can choose the brighter or darker alley without attending specifically to the 

 stimulus-forms. 



Now the value of the visual complex lies precisely in the fact 

 that the natural inclination of an animal is to attend to some 

 other factor than the one under investigation. A discrimina- 

 tion habit is hastened because the animal may resort to the 

 most conspicuous cue. Whatever this cue may be, it is used 

 as a valuable ally in directing the animal's attention to the 

 proper detail. A patient and alert experimenter can eliminate 

 brightness, luminous intensity, et cetera so gradually that the 

 animal's basis of discrimination is quite simply switched to the 

 detail in question, provided that the detail is perceivable for the 

 animal. The change should not be made abruptly, nor is it 

 necessary to wait for a perfect habit. Slight changes should 

 be commenced as soon as there is evidence of an incipient habit. 

 My results with chick 3, in contrast with later results from other 

 chicks, compel me to favor the use of the visual complex in pre- 

 liminary training. The success of one method or another 

 evidently depends to a great extent upon the temperament of 

 the experimenter. 



Coburn's study (6, Chapter 2) of the vision of the crow fur- 

 nishes the only available results on size perception in birds that 

 are comparable with my results. His study of the crow's per- 

 ception of size was not primarily a threshold study, but his pre- 

 liminary results roughly indicate in centimeters a 3-2, 5-3, 6-4, 

 and 9-6 discrimination of circles. A hasty series of tests to- 

 wards the close of his observations indicate a discrimination 

 between a 5 cm. circle and a 4.5 cm. circle. He thinks a finished 



^Journal Animal Behavior, vol. 4, p. 324. 



