The spectral input systems of hymenopteran insects and their receptor-based colour vision
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To analyze the human red, green, and red-green hybrid cone pigments in vivo , we studied 41 male dichromats, each of whose X chromosome carries only a single visual pigment gene (single-gene dichromats). This simplified arrangement avoids the difficulties of complex opsin gene arrays and overlapping cone spectral sensitivities present in trichromats and of multiple genes encoding identical or nearly identical cone pigments in many dichromats. It thus allows for a straightforward correlation between each observer’s spectral sensitivity measured at the cornea and the amino acid sequence of his visual pigment. For each of the 41 single-gene dichromats we determined the amino acid sequences of the X-linked cone pigment as deduced from its gene sequence. To correlate these sequences with spectral sensitivities in vivo , we determined the Rayleigh matches to different red/green ratios for 29 single-gene dichromats and measured psychophysically the spectral sensitivity of the remaining green (middle wavelength) or red (long wavelength) cones in 37 single-gene dichromats. Cone spectral sensitivity maxima obtained from subjects with identical visual pigment amino acid sequences show up to a ∼3 nm variation from subject to subject, presumably because of a combination of inexact (or no) corrections for variation in preretinal absorption, variation in photopigment optical density, optical effects within the photoreceptor, and measurement error. This variation implies that spectral sensitivities must be averaged over multiple subjects with the same genotype to obtain representative values for a given pigment. The principal results of this study are that (1) ∼54% of the single-gene protanopes (and ∼19% of all protanopes) possess any one of several 5′red-3′green hybrid genes that encode anomalous pigments and that would be predicted to produce protanomaly if present in anomalous trichromats; (2) the alanine/serine polymorphism at position 180 in the red pigment gene produces a spectral shift of ∼2.7 nm; (3) for each exon the set of amino acids normally associated with the red pigment produces spectral shifts to longer wavelengths, and the set of amino acids normally associated with the green pigment produces spectral shifts to shorter wavelengths; and (4) changes in exons 2, 3, 4, and 5 from green to red are associated with average spectral shifts to long wavelengths of ∼1 nm (range, −0.5 to 2.5 nm), ∼3.3 nm (range, −0.5 to 7 nm), ∼2.8 nm (range, −0.5 to 6 nm), and ∼24.9 nm (range, 22.2–27.6 nm).
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We show that the color matches of normal and anomalous trichromatic observers, taken together, uniquely determine the spectral sensitivities of the normal red- and green-sensitive pigments for wavelengths longer than about 530 nm. The key assumptions are (i) that a visual match is a match for the pigments; and (ii) that one pigment is common to normal and deutan observers, another to normal and protan observers. Calculated spectral sensitivities for these two pigments agree closely with the luminosity curves of protanopes and deuteranopes. We show also that simple protanomalous and deuteranomalous observers may share a third (anomalous) pigment. The spectral sensitivity calculated for this pigment is reasonably consistent with recent measurements on anomalous vision.
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Color Vision Defects
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The pigeon's colour vision was examined, using behavioural and physiological techniques Avian colour vision has aroused interest because of the suggestion that chromatic discrimination in birds is mediated by a single cone pigment, combined with several types of retinal oil-droplets which act as differential colour filters. Using an operant conditioning method, difference thresholds were measured throughout the spectrum (400 - 680 nm) to generate a wavelength discrimination function, which yields information about the type of visual system an animal possesses. Earlier work had suggested that birds are trichromatic, but the finding of three clearly defined regions of optimum discrimination at 595, 530 and 460 nm indicates instead that the pigeon's colour vision is at least tetrachromatic. The pigeon's saturation discrimination abilities were also studied using a similar technique Saturation increased towards the spectral extremes while a point of least saturation occurred at 597 nm. Additional subsidiary saturation minima were found at 443, 496, 536 and 662 nm. These results largely corroborated those of the wave length discrimination experiment but indicated that the pigeon's visual system may be more complex than a tetrachromatic one Preliminary to an extension of the wavelength discrimination study, the pigeon's spectral sensitivity was measured electroretinographically The resulting spectral sensitivity curve peaked at 560 - 580 nm, in agreement with previously reported data. Furthermore, spectral sensitivity extended well into the ultraviolet region (<400 nm), where sensitivity was quite high In a second study of wavelength discrimination, results of the first experiment showing three threshold minima were confirmed and, additionally, pigeons maintained good discrimination between wavelengths within the ultraviolet range. Experimental findings were discussed in terms of the physiological mechanisms underlying visual performance, in particular, the present results, together with other evidence, suggest that the retinal oil-droplets are not basic to avian colour vision The functional significance of the pigeon's colour vision was also considered
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The eye’s optics form an inverted image of the world on the dense layer of light-sensitive photoreceptors that carpet its rear surface. There, the photoreceptors transduce arriving photons into the temporal and spatial patterns of electrical signals that eventually lead to perception. Four types of photoreceptors initiate vision: The rods, more effective at low light levels, provide our nighttime or scotopic vision, while the three classes of cones, more effective at moderate to high light levels, provide our daytime or photopic vision. The three cone types, each with different spectral sensitivity, are the foundations of our trichromatic color vision. They are referred to as long-, middle-, and short-wavelength–sensitive (L, M, and S), according to the relative spectral positions of their peak sensitivities. The alternative nomenclature red, green, and blue (R, G, and B) has fallen into disfavor because the three cones are most sensitive in the yellow-green, green, and violet parts of the spectrum and because the color sensations of pure red, green, and blue depend on the activity of more than one cone type. A precise knowledge of the L-, M-, and S-cone spectral sensitivities is essential to the understanding and modeling of normal color vision and “reduced” forms of color vision, in which one or more of the cone types is missing. In this chapter, we consider the derivation of the cone spectral sensitivities from sensitivity measurements and from color matching data.
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