Proceedings: Evolution of visual pigments

Origin and distribution of Daltonism in Italy

A research project was started in Italy in order to map dyschromatopsies in this country. Field data was collected by the authors in Liguria, Veneto, Emilia-Romagna, Apulia, Sicily, and by other authors in Lazio, Calabria, Basilicata, and Sardinia. The samples were all composed of more than 500 male subjects, of homogeneous origin, and assessed using the Ishihara (1973) and the Farnsworth (1947) tests. Statistical analysis identified two groups showing different mean percentages: one, coastal (n = 13,091; 6.50%) and the other, inland (n = 17,881; 5.21%). The results stimulated an ecological-ergonomic hypothesis based on nutrition (i.e., food gathering strategies) and the kind of light prevailing in the habitat. If trichromacy has been brought about by frugivore feeding (Polyach, 1957), dichromacy may have been maintained by giving a selective advantage to fishermen (originally fishing one fish at a time by hand). This is valid for both sea or fresh water habitats, characterized by blue-green prevailing light and with sharp discrimination for blue-gray shapes and colors (usually, 70% of dichromats are deutans). To support this hypothesis a field survey was carried out among 661 Sicilian traditional fishermen, which furnished the highest rates of dyschromatopsia in Italy: 7.90%. In conclusion, we can hypothesize that endogamous fishermen groups bordering the coastal areas of continents, searching for waters abounding in fish, could have determined a greater incidence of color blindness in the coastal regions, while exogamous marriages between fishermen and peasants contributed to the lower diffusion of the genes responsible in inland areas.

Ocular filtering of ultraviolet radiation and the spectral spacing of photoreceptors benefit Von Kries colour constancy

Ocular filters in the eyes of many vertebrates, including humans, absorb wavelengths shorter than approximately 400nm. These filters prevent the β-band of a visual pigment from being exposed to ultraviolet radiation, essentially narrowing the spectral sensitivity of the different photoreceptor classes. A comparison of different hypothetical visual systems is used to show that von Kries colour constancy is improved by ocular filtration of ultraviolet radiation, whilst there is no reduction in colour discrimination. Furthermore, it is shown that the asymmetric spectral spacing of different photoreceptor classes present in the human visual system may benefit colour constancy. The results are interpreted in relation to predictions of von Kries colour constancy for a standard human observer.