EFFECT OF SAWTOOTH POLARITY ON CHROMATIC AND LUMINANCE DETECTION

Psychophysical studies have documented that many observers show lower thresholds for rapid-off than for rapid-on sawtooth luminance modulati on. This finding, together with physiological findings from chromatica lly opponent ganglion cells of the macaque monkey, prompted a search f or a similar bias in psychophysical detection of chromatic increments and decrements of light. Using a luminance pedestal in conjunction wit h a luminance background to favor detection by chromatic mechanisms, w e measured spectral sensitivity for rapid-on and rapid-off sawtooth st imuli presented spatially coextensive with the pedestal. There were tw o different pedestal chromaticities: one broadband, and the second com posed only of long-wavelength light to enhance short-wavelength-sensit ive, cone-mediated detection. Spectral-sensitivity measurements for di fferent wavelength stimuli revealed no systematic differences across t he visible spectrum as a function of sawtooth waveform polarity or ped estal chromaticity. Similarly, temporal contrast-sensitivity functions for hetero-chromatically modulated red-green sawtooth stimuli did not reveal an asymmetry in sensitivity for rapid-red and rapid-green chro matic change. Some of the observers showed a higher sensitivity for lu minance modulated rapid-off sawtooth stimuli, as also noted in previou s studies. This asymmetry was not found when a white luminance pedesta l and background was used. These results suggest that the cone inputs to chromatically opponent ON- and OFF-center cells are sufficiently ba lanced to provide equivalent psychophysical thresholds for chromatic i ncrements and decrements of light.