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Herbert Jägle, Emanuela Luca, Ludwig Serey, Michael Bach, Lindsay T. Sharpe; Visual acuity and X-linked colour-blindness. Journal of Vision 2004;4(11):59. doi: https://doi.org/10.1167/4.11.59.
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X-linked or red-green colourblindness usually implies disadvantages. However, by allowing for its diverse molecular genetics, we demonstrate an advantage. Visual acuity was measured with the monitor-based Freiburger Visual Acuity Test in age- and refraction-matched groups of normals (N=8) and X-linked dichromats (N=13), classified according to whether they carried a single (N=6) or multiple (N=7) visual pigment genes. Each eye, after being corrected with the appropriate spherical and cylindrical lenses for a 5-m viewing distance, was tested separately, several times in a random order and the results were averaged. The monitor (Sony 21″ GDM F500) was spectrally calibrated (CAS 140, Instrument Systems, München, Germany) and adjusted by using a photometer (Minolta CA-100) for maximum Michelson contrast (99%) and to produce equal photon catches in the L- and M-cones of the different types of observers: 114 cd/m2, for the normals and deuteranopes, and 121 cd/m2, for the protanopes. The mean log visual acuities did not differ between trichromats and the dichromats subdivided by phenotype. However, the mean log visual acuities of the multi-gene dichromats (0.297 ± 0.022 (OD) and 0.302 ± 0.034 (OS)) are significantly higher than those of the single-gene dichromats (0.225 ± 0.058 (OD) and 0.222 ± 0.043 (OS)) and the trichromats (0.257 ± 0.032 (OD) and 0.249 ± 0.036 (OS)). Their advantage is attributed to a reduction in chromatic noise in the luminance/high acuity visual channel. Single-gene dichromats may not share in the advantage because the missing cone or photopigment type may not be replaced in their retinal mosaics or because of other molecular/developmental differences.
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