Excellent Underwater Vision Examined by Scientists from Lund

We humans are poorly adapted for underwater vision. However, the Moken peoples of south-east Asia manage to collect shells, clams and sea cucumbers using no visual aids when diving to a depth of 3 or 4 metres. Scientists from Lund University in Sweden have now measured the visual acuity of these children and have found that their ability to see well underwater is not a myth: their acuity in this environment is indeed superior to that of European children. The scientists have also found an explanation for this phenomenon.

The results are presented in the new issue of the prestigious scientific journal Current Biology. Anna Gislén and Marie Dacke from the Department of Cell and Organism Biology have travelled several times to the Surin Islands (Thailand) where the Moken tribe live. These people are the so-called sea-gypsies, who for thousands of years have lived on their boats and collected food from the ocean. Some of them have settled down in houses built on three metre high stilts by the shore. At high tide the Moken can dive directly into the water from their houses.

Gislén and Dacke have performed a series of visual tests on the Moken children. Using an experimental apparatus placed under the surface, the children viewed striped patterns that were presented either horizontally or vertically. By using thinner and thinner stripes they could determine the resolution limit of the children.

”Underwater, Moken children can resolve stripes that are twice as fine as the finest seen by European children”, says Anna Gislén. ”We asked ourselves if this was due to a biological, possibly genetic, difference or if it was due to learning. For instance, we examined their eyes in detail, but could find no evidence that their corneas had a different curvature than the corneas of European children. Neither was there any evidence that the Moken children had better acuity on land.”

Was there something that these children did underwater that gave them better acuity? Under normal circumstances the pupil opens underwater to let in more light. But the pupils of Moken children constrict under water. The effect is the same as choosing a smaller aperture in a camera: focal depth is increased and resolution is improved. The Moken children also accommodate maximally, that is, the muscles controlling the lens are constricted and the lens changes shape, thus increasing the refraction of light.

”We are currently doing a follow-up study on Swedish children. Even though the study is not yet finished, it does seem that children can learn to adapt their eyes for better underwater vision”, says Marie Dacke.