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D. Cao, J. Pokorny, A. J. Zele; Rod Contributions to Color Perception: Linear With Rod Contrast. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3176.
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© ARVO (1962-2015); The Authors (2016-present)
At mesopic light levels an increment in rod activation causes changes in color appearance that can be characterized by a higher M-cone than L-cone weighting, and a higher S-cone contribution at low light levels (Cao, Pokorny & Smith, 2005, Vision Research, 45, 2119-2128). Here we investigated how rod percepts varied with the contrast of the rod increments.
Stimuli originated from a colorimeter that provided 4 primaries in both a 2 deg circular test and 13 deg annular surround fields to allow independent modulation of the rods and each of the S-, M- and L-cone types. The test field was viewed at 7 deg in the temporal retina. The center rod activation was incremented (0.3-0.8 Weber contrast) in a 1 Hz square-wave temporal profile. Rod percepts were measured with a temporal matching technique. The observer toggled between stimulus and matching epochs and adjusted the cone chromaticity and luminance in the matching epochs to match the rod percepts seen during the stimulus epochs. Data were collected with a chromaticity of L/(L+M) = 0.7, S/(L+M)= 0.2 at 3 retinal illuminances: 2, 10 and 100 photopic Td.
We can characterize two features of the matches: the chromaticity (L/(L+M), S/(L+M)), and the luminance contrast. At all light levels, the rod contribution to the M-cone system was linearly related to rod contrast, with the highest cone contrast settings at the lowest light level (2 Td). At 2 Td rod matches required additional S-cone weightings, which were also linearly related to rod contrast. At all light levels, the matching luminance (L+M) contrast was linearly related to rod contrast.
The linear rod/cone relations for M/L, S and luminance suggest that rod and cone signals combine at an early site in the visual system, in accord with the theory that at mesopic light levels the rod signals are conveyed via rod/cone gap junctions (Sharpe & Stockman, 1999, Trends in Neuroscience, 22, 497-504).
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