THE QUARTERLY REVIEW OF BIOLOGY 



angle larger than that subtended by the 

 sun's disc at the earth. Johnson states 

 that this coarsely striped field is "closely 

 comparable with that of a plank fence, the 

 units of which are 6 inches wide and 6 

 inches apart, viewed at a distance of 50 

 feet." Most dog-lovers would refuse to 

 believe that their pets were incapable of 

 such a discrimination. 



Data on form vision have been reported 

 by Williams (56) and by Szymanski (51). 

 In neither case were the experimental 

 conditions so admirably controlled as in 

 Johnson's study. Williams was unable 

 to establish discrimination between a 

 square and a circle of equal area, although 

 he did find discrimination between a con- 

 stant and an intermittent light, the latter 

 flashing at z second intervals. Szymanski 

 was unable to establish response to a 

 white pyramid standing in bold relief 

 against a black background. 



Johnson alone of American workers has 

 studied size discrimination (zz). His 

 dog apparently discriminated between a 

 circle 6 cm. in diameter and circles with 

 diameters of 1, z, and 3 cm. respectively. 

 The 4 and 5 cm. circles were not certainly 

 discriminated from the 6 cm. circle. The 

 intensity of the light transmitted through 

 the pairs of circles used was always con- 

 stant. Thus it was impossible to say 

 whether the animal's success was due to 

 size discrimination or to luminosity. To 

 test this matter Johnson equated the 6 and 

 the 3 cm. circles for luminosity by reducing 

 the larger circle to Z5 per cent of its 

 original brightness. As a result the dog 

 no longer discriminated, choosing now 

 one and now the other circle. His dis- 

 crimination had therefore been based on 

 luminosity differences. 



The students of Pavlov, working in 

 Leningrad, have reported data bearing on 

 the sensory capacities of dogs. Unfortu- 

 nately we are at present largely dependent 



on secondary source material, and thus a 

 critical evaluation of the Russian work 

 cannot readily be made. The method 

 employed is quite different from that used 

 by these other investigators, and the 

 work still awaits verification in other 

 laboratories. It involves the establish- 

 ment of a conditioned salivary response 

 to a given stimulus, as for example a black 

 patch, by presenting this stimulus and 

 the unconditioned stimulus, food, simul- 

 taneously a large number of times. After 

 this conditioning is firmly established a 

 second stimulus to be discriminated from 

 the conditioned stimulus (a white patch, 

 for example) is presented. Food is never 

 presented with the white patch, whereas 

 the black patch is often reenforced by the 

 simultaneous presentation of food. Such 

 training finally results in a differential 

 response to the white and to the black 

 patch, each presented alone, the one 

 causing secretion of saliva, the other 

 inhibiting such secretion, provided, of 

 course, the sensory capacity of the dog is 

 such as to permit such discrimination. 

 Not only the visual capacity of the dog, 

 but its auditory, olfactory, thermal, tact- 

 ual, and gustatory capacities have been 

 studied to some extent by this method. 

 In general it may be stated that where the 

 results of the Russian workers and those 

 of other investigators are not in agree- 

 ment, the findings of the former group 

 seem to indicate greater sensory capacity. 

 There are at least two possible explana- 

 tions for these discrepancies. The sali- 

 vary response method may be a more deli- 

 cate measure of discriminatory ability 

 involving, as it does, a more simple 

 learning problem. On the other hand 

 it may very well be that, due to technical 

 errors in the application of the method, 

 the dogs studied by the Russian students 

 were responding to secondary criteria. 

 That the latter is true of much of the 



