SENSORY CAPACITIES AND INTELLIGENCE OF DOGS 



from the situation, for the dog, more 

 perhaps than any other animal, is quick 

 to take advantage of the to us almost 

 imperceptible movements that the on- 

 looker often and quite unwittingly makes 

 in indication of the correct direction. 



This method has been used in the study 

 of brightness, size, and form discrimina- 

 tion in the dog but not, as yet, in the 

 study of color discrimination. Under 

 adequate experimental conditions positive 

 results as obtained by this method may 

 be accepted as final, but it is to be noted 

 that negative results cannot be considered 

 conclusive, since it is conceivable that an 

 animal might be capable of somewhat 

 more delicate discriminations in a less 

 arbitrary or complex situation. 



Szymanski (51) used this general 

 method in studying brightness discrimi- 

 nation. His dogs were a cross between a 

 Spitz and a fox terrier. His results were 

 entirely negative for slight differences in 

 brightness. Only when one alley was 

 brightly lighted and the other entirely 

 dark was he able to get a discrimination 

 habit. Very similar results were ob- 

 tained by Sutherland (46) using the same 

 method. His dogs, mongrels, discrimi- 

 nated between a light and no light, but 

 not between one light as opposed to two 

 lights, or even as opposed to four lights. 

 Form as well as brightness discrimination 

 would have been involved in these cases 

 of course, since the bulbs were actually in 

 view. Stone (45), however, using a male 

 and a female fox terrier found evidence for 

 a much finer degree of brightness discrimi- 

 nation. Having built up a discrimination 

 habit for two lights differing considerably 

 in brightness he gradually reduced the 

 intensity of the brighter until a point was 

 reached where the habit broke down. He 

 then increased the difference between the 

 two lights until the habit was reestab- 

 lished and again gradually approached 



the threshold value. Stone also tested 

 three human subjects by the same method 

 — except, of course, that these subjects 

 indicated by word of mouth rather than 

 by turning to the right or left which 

 light was in their opinion the brighter. 

 The men could, on the average, distin- 

 guish a light of 1 c.p. from 1.1 c.p. The 

 male dog discriminated between a 1 c.p. 

 and a 1.2. c.p. light, while the female dog 

 did slightly better. The dogs were thus 

 inferior to the men, but only slightly so. 



All the work on form and size discrimi- 

 nation except that of the Russian school 

 is in accord with the conclusion that the 

 dog is very deficient in these respects. 

 Johnson (2.3), using an English bull 

 terrier, was unable to establish evidence 

 for discrimination between vertical and 

 horizontal lines, these lines, alternately 

 black and white, being 3.9 mm. wide on a 

 circular field 6 cm. in diameter. Skia- 

 scope examination of the dog's eyes, 

 both with and without mydriatic, seemed 

 to indicate that objects at a distance of 

 twenty feet or more, i.e., at a distance such 

 that the light rays were approximately 

 parallel, were focussed on the retina. 

 Nothing is known of the ability of the 

 dog to accommodate, and thus it might 

 have been that failure to discriminate 

 in Johnson's case resulted from inability 

 to focus upon objects so close to the eyes 

 as the signals were placed. Johnson 

 considered overcoming this difficulty by 

 equipping the dog with a pair of spectacles. 

 Instead, however, he introduced a system 

 of lenses before the signals in such a way 

 as to throw the light reflected from them 

 onto the dog's eyes in parallel rays. But 

 even with all these advantages that science 

 offered the dog did no better. It could 

 not even distinguish between a plain field 

 and one of black and white stripes, 3.9 

 mm. in width. Each of these stripes 

 subtended a visual angle of 33' 3Z" or an 



