Light wavelength and energy preferences of the bullfrog: Evidence for color vision.

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Abstract
Bullfrogs'' preferences for lights of various wavelengths and energies were studied in 4 experiments with a forced-choice, paired-comparisons procedure. Short wavelengths were preferred with lights of equal energy. The preference was heightened when lights of "equal brightness" were used, i. e., lights equated on the basis of bullfrogs'' electroretinogram spectral sensitivity. With 460 mu and 540 m[mu], but not with 620 m[mu], higher energies were preferred among stimuli of different energies of the same wavelength. The preference for 460 m[mu] over 620 m[mu] could not be reversed by varying the energy of each over a 10, 000-fold range. Frogs discriminate among wavelengths as well as among energies of light stimuli.