PATTERN-DISCRIMINATION IN VERTEBRATES 199 



AN ALTERNATIVE METHOD OF TESTING VISUAL ACUITY 



A very convenient measure of an animal's visual acuity is 

 the angle subtended at the eye by a single dark or bright stripe 

 in a system which the animal can just distinguish as striate at 

 a given distance. In the earlier experiments mentioned above, 

 I obtained this value by training the animal to distinguish a 

 striate field from a plain field at a distance of 60 cm. from the 

 eye, and then reduced the width of the striae on the positive 

 field until discrimination ceased. An alternative method con- 

 sists in training the animal, as I did in the present work, to 

 discriminate between a horizontal and a vertical system of equal 

 width, and then gradually reducing the width of the striae in 

 both systems simultaneously until the animal ceases to dis- 

 criminate at the given distance. The work of Casteel 4 suggests 

 this possibility, although Casteel did not attempt to control 

 the factor of distance or to ascertain the limits of the animal's 

 sensitivity. A priori, we should not expect the results yielded 

 by the two methods to differ greatly, provided the animal's eyes 

 were free from astigmatism in the vertical or horizontal direc- 

 tion, and provided otherwise that in both cases the animal is 

 responding by choosing the positive field, rather than by reject- 

 ing the negative field. If experimental data confirm this ex- 

 pectation, and if the second problem should prove as easy for 

 an untrained animal to learn as the first, the time required 

 for learning the first problem might as well be saved. I decided 

 to test the practicability of this method on Chick 2. The daily 

 results appear in table 2-A, between the dates of August 18 

 and August 26, 1914. From August 27 to August 31 inclusive 

 I repeated the test by the first method used — that of plainstriate 

 discrimination, which is designated as method No. 1, and the 

 other as method No. 2. In these tests, the minimal distance 

 between test-field and eye at which comparison could be made 

 without a choice being registered was 60 cm. The results are 

 summarized in table 3. The values given under the heading 

 " Width of striae (mm.)" are the widths of striae on the positive 

 field for method No. 1, in which the striae on the negative field 

 were invisibly small, and the widths of the striae on both fields 

 in method No. 2, in which the difference in direction was 90°. 



4 Casteel, D. B. Discriminative ability of the painted turtle. This journal, 

 vol. 1, 1911, pp. 1 ff. 



