Wavelength discrimination in the leopard frog: a reexamination. 
A reexamination of wavelength discrimination using a 2-choice paradigm and both chromatic-chromatic and chromatic-achromatic stimulus pairs has reaffirmed the positive tendency of Rana pipiens to approach short wavelengths.  In addition, a strong avoidance of long wavelengths was observed and an interaction of these two opposite response tendencies was demonstrated.  Thus, the percent choice frequency for a short-wavelength stimulus depends upon the stimulus with which it is paired.  A positive phototaxis appears to influence response to some short-wavelength stimuli when they differ in brightness from an achromatic stimulus, but not with wavelengths above 471 nm.