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Felix Maier, Arne Ohlendorf, Siegfried Wahl, Frank Schaeffel; Lack of oblique peripheral astigmatism in the chicken eye, comparison to the human eye and possible consequences for emmetropization. Invest. Ophthalmol. Vis. Sci. 2014;55(13):3597.
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Primates display considerable off-axis astigmatism. In humans, its magnitude can be described by a parabolic function: astigmatism relative to the pupil axis = 3.28*10-3 * angle2 (Howland, 13th IMC in Tuebingen 2010). Howland also proposed that peripheral astigmatism may represent a cue for emmetropization, since either its tangential or radial axis are in better focus, depending on the spherical refractive error. It is known that the chicken eye emmetropizes independently in the center and the periphery but not whether it uses peripheral astigmatism.
Infrared photorefraction was used to map out the refractions over the horizontal visual field, both in the vertical and horizontal meridians, in three 43 day old chicks (6 eyes) and three near emmetropic male human subjects (6 eyes, age 34.7 ± 6.8 years). Chicks were trained to accept that the operator turned their heads as desired by holding their beak. Videos were recorded and eye orientation and refractions were determined from the position of the first Purkinje image relative to the pupil center and the brightness slopes in the pupil. Human subjects were measured with a scanning photorefractor (Tabernero et al. 2009).
There was no significant difference in the amount of astigmatism in chicks and humans in the center of the visual field (0 deg: chicks -0.35±0.79 D, humans 0.65±0.60 D, p = 0.30). Similar to what was found by Howland, astigmatism increased in our subjects’ eyes to the periphery with the function: astigmatism = 2.21*10^-3*angle^2-0.0245*angle+1.589. Strikingly, in the chick eye astigmatism did not increase in the periphery. Highly significant differences were found in the peripheral astigmatism in humans and chicks (at 40 deg in the temporal visual field: humans 4.21±2.39 D, chicks -0.63±0.80 D, p < 0.001 (unpaired t-test) and at 20 deg in the nasal visual field: humans 3.69±1.16 D, chicks 0.74±0.44 D, p < 0.001; the trend is similar at 20 deg in the temporal visual field: humans 1.68±1.40 D, chicks 0.30±0.29 D, p = 0.07 and in the nasal visual field at 40 deg humans 5.17±3.71 D, chicks 0.78±0.11 D, p = 0.12).
The chick eye is perhaps the first vertebrate eye without oblique peripheral astigmatism. While the optical design of the crystalline lens in the chick eye must be fascinating, the lack of peripheral astigmatism suggests that emmetropization cannot rely on it.
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