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David W. Rhee, Jason Park, Donald C. Hood; The Cones Contribute More To The Sensation Of Brightness Than To The Pupil Response Under Mesopic Conditions. Invest. Ophthalmol. Vis. Sci. 2011;52(14):4907.
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To compare subjective brightness to pupillary light reflex (PLR) elicited by brief mesopic lights.
After dark adaptation, 4 naïve subjects with healthy vision were shown a series of blue (467 nm) and red (640 nm) full-field stimuli and were instructed to estimate the brightness according to the method of magnitude estimation [1-2]. The 1-second flashes ranged from -4 to 0.5 log cd/m2 in half-log steps. The inter-stimulus interval was long enough (10 to 20 seconds) for pupil baseline recovery. The stimuli were generated by a full-field LED Ganzfeld stimulator (Espion, Diagnosys, LLC). There were 4 identical runs; the data from the first run (practice) was discarded. The subjective values were normalized by the highest value in each trial. The PLRs to the same stimuli were recorded using an infrared camera system (Arrington Research) . Linear regression was used to fit lines to the data and determine the separation (relative effectiveness) of the blue and red stimuli.
The amplitudes of the PLRs to the blue stimuli were greater than to the photopically matched red. For the lower intensities (-4 to -1 log cd/m2), there was a 2.2 log unit separation between the blue and red data, while at the higher intensities (>0 log cd/m2), the PLRs to blue and red lights tended to converge towards an asymptotic pupil size. Because the rods are 2.3 log units more sensitive to the blue stimuli than to the photopically matched red, the PLR data suggest that the rods dominate the responses to both for the lower intensity flashes. Consistent with the PLR data, subjects reported the blue stimuli to be brighter than the photopically matched red stimuli. However, the separation between the data for the blue and red stimuli was 1.5 log units at the lower intensities, significantly smaller than for PLR, suggesting a larger cone contribution to brightness, at least for the red stimuli. The brightness curves at the higher intensities (-1 to 0 log cd/m2) remained separated (0.6 log units).
For brief dim lights, the cones make a larger contribution to the pathways controlling brightness perception than to those controlling the PLR. For brighter lights, the similarity of the PLRs to photopically matched blue and red lights is due to a compressive nonlinearity of the PLR response, rather than due to a saturation of the rods. 1. Stevens, Psychophysics, 1975; 2. Barlow & Verillo, Vision Res, 1976; 3. Kardon et al, Ophthalmology, 2009.
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