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As lateralization of visual function has been shown in seven avian species, as well as in a broad range of both lower and higher vertebrates (Rogers and Andrew, 2002), it can be assumed to be a general characteristic of vertebrates even though it may be expressed in different ways in different species. Some evidence, mentioned here, indicates that a lateralized brain enhances cognitive performance; and this may be a reason why it evolved and has been retained across species. This paper discusses the asymmetrical organization of the visual pathways in birds and the role that exposure of the avian embryo to light plays in the development of lateralization of some visual functions. It considers species differences in transmission of light through the egg shell, and the intensity of light required to generate the asymmetry. The development of asymmetry in visual projections to the forebrain in the chicken is prevented not only by incubating eggs in the dark but also by exposing them to light of high intensity. The extent of light exposure received by the embryo depends on the color and construction of the egg shell, as well as on the location of the nest and the incubating behavior of the parent(s). Steroid hormones also affect light-dependent development of lateralization, mediating the influence of maternal stress on the strength of lateralization in offspring. |
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