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 modulation. This finding, together with physiological findings from chromatically opponent ganglion cells of the macaque monkey, prompted a search for a similar bias in psychophysical detection of chromatic increments and decrements of light. Using a luminance pedestal in conjunction with a luminance background to favor detection by chromatic mechanisms, we measured spectral sensitivity for rapid-on and rapid-off sawtooth stimuli presented spatially coextensive with the pedestal. There were two different pedestal chromaticities: one broadband, and the second composed only of long-wavelength light to enhance short-wavelength-sensitive, cone-mediated detection. Spectral-sensitivity measurements for different wavelength stimuli revealed no systematic differences across the visible spectrum as a function of sawtooth waveform polarity or pedestal 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 chromatic change. Some of the observers showed a higher sensitivity for luminance modulated rapid-off sawtooth stimuli, as also noted in previous studies. This asymmetry was not found when a white luminance pedestal and background was used. These results suggest that the cone inputs to chromatically opponent ON- and OFF-center cells are sufficiently balanced to provide equivalent psychophysical thresholds for chromatic increments and decrements of light.
Visual Sciences Center, University of Chicago.
P J
PJ
DeMarco
V C
VC
Smith
J
J
Pokorny
D000328:Adult; D003118:Color Perception / Q000502:physiology*; D015350:Contrast Sensitivity / Q000502:physiology; D006801:Humans; D008027:Light; D008875:Middle Aged; D011601:Psychophysics; D017949:Retinal Cone Photoreceptor Cells / Q000502:physiology; D012684:Sensory Thresholds / Q000502:physiology; D014795:Visual Pathways / Q000502:physiology