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Steven Buck, Corbin Cunningham; Rod influence on desaturated color mixtures. Journal of Vision 2009;9(14):55. doi: 10.1167/9.14.55.
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Studies using narrow-band spectral lights reveal three rod influences on hue perception: a red bias at short wavelengths, a blue bias at middle wavelengths, and a green bias at long wavelengths). Studies using less saturated lights have shown only some of these effects. Here, we assessed rod influence on mixtures of pairs of narrow-band lights that spanned desaturated regions of color space. The results reveal rod influence on R/G and B/Y opponent-hue dimensions at chromaticities extending from the spectrum locus toward white.
A staircase procedure measured observers' null points for R/G (unique blue and unique yellow) and B/Y (unique green). Stimuli were 5°-diameter discs centered 7° from fixation in temporal visual field. Stimuli were presented for 1 s with a minimum interval of 5.5 s between presentations. Rod influence was inferred from differences in null points measured under dark-adapted (maximizes rod influence) and cone-plateau (minimizes rod influence) conditions for stimuli presented at 0 to 1.4 log photopic trolands (CIE 10°). R/G and B/Y null points were determined for both spectral lights and mixtures of pairs of narrow-band lights that spanned the chromaticity diagram at constant photopic light levels.
Consistent with prior studies, along the spectrum locus, rods shifted all three unique hues to longer wavelengths: by up to 50 nm for unique green (blue bias), 10 nm for unique blue (red bias), and 9 nm for unique yellow (green bias). All three rod hue biases were also found for desaturated lights, extending toward white for distances that varied among observers. Some observers showed rod effects for stimuli of as little as ∼25% excitation purity, the least saturated we have tested so far. Rod influence was generally more pronounced for desaturated blues and greens than for desaturated yellows, even when light levels were adjusted to roughly equalize rod excitation for the three color ranges.
These results show that all three rod hue biases can alter the appearance of a wide range of desaturated hues like those found in the natural world, not just spectral hues of the laboratory.
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