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H.J. Hofer, B. Singer, D.R. Williams; Microstimulation and Imaging of the Trichromatic Cone Mosaic in the Living Human Eye . Invest. Ophthalmol. Vis. Sci. 2003;44(13):3189.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To characterize the chromatic sensations produced by single cones in the human retina. It has previously been reported that tiny, brief, dim, monochromatic flashes fluctuate in color from flash to flash, an effect often attributed to the granularity of the trichromatic mosaic. However, it has been difficult to link this subtle effect to the architecture of the cone mosaic because optical blurring reduces the ability to stimulate single cones and the topography of the underlying cone mosaic in each observer has not been known. Methods: Adaptive optics allows both these difficulties to be addressed. The locations of L, M, and S cones within a 0.5 deg retinal patch at ~1.25 deg eccentricity were determined in 5 color normal subjects with retinal densitometry and high resolution imaging using adaptive optics. Tiny, brief, monochromatic, near-threshold test flashes, imaged on the retina with adaptive optics, were presented randomly throughout the same retinal patch. Subjects reported whether each flash was seen and if so its color appearance using 8 hues and 3 saturation levels. The statistics of the perceptual response were compared with the organization of the underlying mosaic. Results: L:M ratio ranged from 1:3 to 16:1. With adaptive optics, stimuli could be produced with FWHM of only 0.35 minutes of arc, about one third of a cone diameter at 1.25 deg and ~ 2X smaller than possible with conventional imaging. Color fluctuation increased, on average, by a factor of 2 when viewing test flashes produced with adaptive optics than when viewing test flashes without adaptive optics. Contrary to initial expectations, subjects reported a large variety of percepts, using nearly every category of hue and saturation, even for extremely low probabilities of detection. For test wavelengths to which S cones have negligible sensitivity, subjects reported up to 27% of colored flashes blue. The fraction of flashes seen as white increased linearly with the asymmetry in the L:M ratio. Conclusions: While relative cone numerosity may be inconsequential for the color appearance of large stimuli, it does effect color appearance when the size of the stimulus approaches the size of individual photoreceptors. The results confirm suggestions by previous investigators that M cones may contribute to sensations of blueness. The rich variety of color percepts at low frequencies of seeing is inconsistent with the ubiquitous assumption in previous studies that the color appearance of tiny flashes depends solely on the photopigment in the receptors that mediate detection. Instead, the results imply that cones of each class do not generate a single unique percept.
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