PATTERN-DISCRIMINATION IN VERTEBRATES 209 



It now became necessary to determine whether the negative 

 results which the dogs gave were due to the inappropriateness 

 of the conditions under which the latter had worked. The 

 first matter to be settled was that of errors of refraction in 

 the dog's eyes. This examination should have been made be- 

 fore the experiments were begun, as is made evident by the 

 results. My colleague, Dr. P. W. Cobb, was kind enough to 

 make the examination and to assume responsibility for the 

 results. He administered atropin to ■ the animals for about a 

 week, until the pupils ceased to respond to light. He then 

 made a careful skiascopic examination. The results obtained 

 during mydriasis were very consistent. Dog 1 showed about 

 0.25 to 0.50 D. hyperopia during mydriasis. (This condition is 

 characteristic of the emmetropic human eye.) No astigmatism 

 was discoverable. The examination without mydriasis yielded 

 variable results, but none of them indicated the presence of a 

 refractive error which the animal did not overcome by accom- 

 modation or by an analogous process. We assumed, therefore, 

 that this dog's eyes were for practical purposes emmetropic. 

 The correction for Dog 2 was +1.75 D. sphere with +0.75 

 cylinder, the axis lying in the horizontal meridian. The neces- 

 sary correction was the same for both eyes. A week later 

 Dr. Cobb attempted to test the animals for extent and range 

 of accommodation, but got no consistently positive results. It 

 seemed useless, therefore, to continue experimentation of Dog 2, 

 since even the best accommodation could hardly have overcome 

 so large a refractive error, and since it was inconvenient to equip 

 her with spectacles. 



At this time I was working some other animals on the prob- 

 lem of discriminating between a vertical system of striae and 

 a horizontal system differing from the former only in direction. 

 The problem had proved itself quite easy for some of the animals. 

 I had also found it feasible to test an animal's visual acuity by 

 making the stripes on both fields rather coarse, and training the 

 animal to choose the food-box indicated by the horizontal system. 

 The next step was to reduce the width of the stripes in both 

 systems simultaneously, until the members became so numerous 

 and their width and separation so small that the animal could 

 not distinguish the horizontal system from the vertical system 

 at the given distance. I decided to test Dog 1 by this method. 



