Pupil increment thresholds are influenced by color opponent mechanisms.

Pupillomotor and sensory spectral sensitivity curves of light-adapted eyes were compared. Experimental conditions were designed to favor (despite the threshold criterion) opponent processing of signals: long (500-ms) stimuli of large diameter (60 degrees) were presented centrally on an intense (16 cd/m2) white background. Narrow spacing of monochromatic stimuli permitted the recording of peaks and troughs in the increment spectral sensitivity curve which neither coincided with the characteristics of receptor pigment absorption spectra nor with the V-lambda curve. They are considered as showing influences of opponency mechanisms. Pupillomotor spectral sensitivity paralleled the sensory one, exhibiting the influence of opponency mechanisms on the pupil. Using more physiological conditions, human photopic spectral sensitivity was revealed as a three-peaked function, in sensory as well as in pupillomotor terms.