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Christopher Shooner, Kathy T. Mullen; Different effects of a chromatic pedestal on the detection of luminance and contrast. Journal of Vision 2019;19(8):101. doi: https://doi.org/10.1167/19.8.101.
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Psychophysical interactions between responses to chromatic and achromatic stimuli provide insight into how luminance and color are processed jointly or independently. It has been shown that thresholds for detecting a luminance increment of a small spot stimulus are reduced by an added chromatic pedestal, suggesting a shared pathway for luminance and color. Related experiments with grating stimuli, however, have found no facilitation for detecting achromatic gratings on chromatic grating pedestals. It is unclear whether this difference stems from the use of different types of chromatic pedestal, or different tests of achromatic sensitivity. We addressed this question by measuring the effect of a single pedestal type (a chromatic patch) on 2 types of achromatic test. In a forced-choice design, subjects discriminated the interval containing pedestal+test from that with pedestal only. The pedestal was a foveal, 4-degree, raised-cosine patch, with isoluminant red or green (L/M-cone) chromaticity of varying strength. The test was either an increment in the luminance of the patch, or a superimposed 1 c/deg achromatic modulation. We found that luminance-increment thresholds were reduced by a factor of 2 or more by the chromatic pedestal, over a wide range of pedestal intensities. Grating detection, however, was unaffected by pedestals at any level. This suggests that these two forms of luminance signal engage different neural pathways, which overlap with chromatic mechanisms in different ways. In ongoing work, we measure interactions among a wider range of chromo-luminance stimuli to specify this effect more fully.
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