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David H. Foster, Sérgio M. C. Nascimento, Kinjiro Amano, Iván Marín-Franch; Near-optimal tuning of trichromatic vision for constant surface identification in natural scenes. Journal of Vision 2011;11(11):357. doi: https://doi.org/10.1167/11.11.357.
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© ARVO (1962-2015); The Authors (2016-present)
There have been several partial attempts to explain the number and spectral locations of the cone photoreceptor pigments of the normal human eye; e.g. that specifically the medium- and long-wave pigments of trichromatic primates are optimal for discriminating fresh leaves from mature foliage. The aim of this study was to test a more general hypothesis, namely, that long-, medium-, and short-wave pigments are optimal for identifying objects within the natural world despite changes in the color of the illumination. To determine the theoretical limits on identification performance, computer simulations were performed in which the number of cones, pigment spectral locations (on a log-wavelength axis), and post-receptoral interactions were allowed to vary. Stimuli were generated from high-resolution hyperspectral images of 50 close-up and distant images of natural scenes. Of these scenes, 29 were classed as predominantly vegetated and 21 predominantly nonvegetated. From each scene, 1000 points were chosen randomly and the cone signals at each point were calculated for two illuminants selected randomly from combinations of direct sunlight and blue skylight and these same lights filtered by a leafy canopy. Allowance was made for absorption in the ocular media. The information retrieved, derived from an estimated probability mass function for identification performance, was calculated for one, two, three, and four cone pigments. The average information retrieved increased with increasing pigment number, but it approached an asymptote with three pigments. Optimal spectral locations differed according to whether the scenes were predominantly vegetated or nonvegetated, but, for both types, spectral locations were generally shifted towards longer wavelengths with respect to their normal positions, although little additional information was gained by the shift. The number and tuning of normal cone pigments seem to be nearly optimal for surface identification in the natural world.
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