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S.L. Buck, L.P. Thomas; The Generality of Rod Hue Biases: Size, Light Level & Photopic-Scotopic Spectral Sensitivity Differences . Invest. Ophthalmol. Vis. Sci. 2003;44(13):1910.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: For large, dimly-mesopic, extrafoveal stimuli, we have previously shown 3 types of rod hue biases, all of which shift the spectral unique hues (blue, green, yellow) to longer wavelengths. We are currently exploring the generalizability of these rod hue biases to a wider range of conditions, to clarify their overall impact on human color vision. Methods: The present studies address this generalizability question in two ways. First, we measured rod hue biases with three different sized targets (7.4°, 2°, 0.6°), across five mesopic light levels, with stimuli presented for 1-s each at 7° eccentricity in nasal retina. Second, we measured rod hue biases with spectral stimuli that were photopically equated, rather than scotopically equated, as we have in the past. We inferred rod hue biases from measurements of rod influence on the wavelengths of the spectral unique hues by means of a discrete-trials staircase procedure. Rod influence was assessed by comparing unique hue wavelengths for the same stimuli under cone-plateau (minimized rod contributions) and dark-adapted (maximized rod contributions) conditions. Results: We found rod hue biases for 2° targets to be very similar to those observed for 7.4° targets (the size we have most often studied in the past). Consistent with past results, at the lowest light level (1 log scot td), rods shifted all three spectral unique hues to longer wavelengths. Rods produced the greatest wavelength shift (~35nm) for unique green (blue bias) and 5-15nm shifts for unique blue (red bias) and unique yellow (green bias). As we increased light level over a 2-log-unit range the rod green bias tended to be the most constant while the rod blue and red biases tended to disappear. At lowest light levels, which are optimal for rod hue biases, we have not yet been able to reliably measure or interpret spectral unique hue settings for the 0.6° target. We are continuing to test at higher light levels. For photopically-equated stimuli, the rod blue bias was the largest and the most consistent with what we find with scotopically-equated stimuli. The rod red and green biases were more variable in magnitude and incidence across observers. Conclusions: The rod hue biases we have previously described appear to generalize to moderate-sized stimuli and to conditions in which stimuli are photopically equated. Generalizability to very small stimuli remains uncertain. At least some of the variability between scotopically- and photopically-equated stimuli may be related to differences in relative light levels of dark-adapted and cone-plateau conditions occasioned by photopic-scotopic spectral sensitivity differences.
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