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A. Gislen, M. Dacke, R.H. Kröger, M. Abrahamson, D. Nilsson, E.J. Warrant; Improved Underwater Vision in a Human Tribe of Sea-gypsies . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2789.
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Purpose: Sea-gypsies live all over Southeast Asia. One of these groups of people, the Moken, lives in the archipelago of Burma and in the Northwest of Thailand. These human populations spend a considerable time in water and the children collect food items from the sea floor using no visual aids. As the human eye normally produces severely blurred images underwater, we suspected that the Moken children might be adapted for improved underwater vision. Methods: Underwater visual acuity and contrast sensitivity was tested in six Moken children using achromatic sinusoidal gratings of different contrasts and spatial frequencies. As a control group we used 28 European children in the same age range, on holiday with their families in nearby areas. We also examined corneal curvature, refractive state, accommodative power, and pupil diameter in both groups of children. Results: The Moken children had significantly better underwater acuity (Mann-Whitney test: p<0.001) being able to resolve gratings (100 % contrast) more than twice as fine as those resolved by the European children (Moken children: 6.06±0.59 cycles/degree, European children 2.95±0.13 c/deg). The Moken children also had higher contrast sensitivity underwater. There were no differences in corneal curvature and accommodative range between the two groups, and the majority of the Moken children was slightly hyperopic with refractive errors ranging from +2 to -0.5 D. There was, however, a difference in pupil size underwater. The Moken children constricted their pupils when diving and the European children did not (pupil diameter Moken children: 1.96±0.05 mm, n=6; European children: 2.50±0.05 mm, n=15; Mann-Whitney: p<0.001). Conclusions: The improved underwater vision in the Moken children can be explained by the reduction in pupil size followed by heavy accommodation. As the European children do not show this behaviour, it must be either a genetic trait or a learned behaviour.
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