Human beings are entitled to the five quintessential senses of taste, smell, sound, touch and sight. If you could improve the performance of one of these senses, which one would you pick?
Spiderman was bitten by a radioactive spider which gave him the spider-like powers of strength and stickiness as well as trusty web slingers. The gods similarly blessed Wonder Woman with her unequivocal strength, power and flight capabilities.
It seems that the "Waterbabies," a term concocted by "Science" magazine writer Constance Holden for the "Moken children," have been blessed by evolution to see clearly underwater.
A group of Swedish scientists led by Anna Gislén published an investigation entitled "Superior Underwater Vision in a Human Population of Sea Gypsies" in the May 2003 edition of "Current Biology." The study compared visual acuity between two groups of children: those of European descent and children of "the Mokens" -- a population of sea gypsies in Southeast Asia. The results of tests for visual acuity conducted both underwater and on land were shocking. The Moken children were able to see twice as clearly underwater as their European counterparts.
According to the "Current Biology" article, human beings experience visual ambiguity underwater due to the loss of refractive power in their eyes. This is typical of a terrestrial eye. Organisms well adapted to underwater vision have flatter cornea and stronger lenses. All light that enters the eye must first pass through the cornea. The dimensions of the average adult human eye are about one inch wide, one inch deep and 0.9 inches long. The lens, which is directly behind the pupil behind the cornea, is able to tweak vision. This is possible because the lens is attached to muscles that adjust its size to focus the object in sight.
Having a flatter cornea and a stronger lens would mean that light could easily enter the eye and could be better-focused. These two traits were discovered in Moken children.
For their experiment, the scientists created an apparatus that was able to fixate the heads of the children underwater with a set visual range. A set of color gratings was shown at different spatial frequencies to the children. Moken children identified the gratings at 6.06 cycles/degree compared to only 2.95 cycles/degree for the European children. The Moken children achieved this increase in cycles/degree by constricting their pupils to around 1.96 millimeters in diameter. European children could only constrict their pupils to as little as 2.50 millimeters in diameter.
The study reported that two forces work in opposition to each other when the eye is asked to focus on an object underwater. The first acts to expand the pupil size to let in more light as the amount of light is reduced. The opposite force, termed an accommodation, acts to constrict the pupil for pattern recognition. This latter force was found to be stronger in the Moken children although scientists were unable to quantify this difference.
Another good indication of this accommodation is suggested by the evidence that both groups of children have similar pupil sizes on land. Therefore, when diving, the Moken children must undergo significant changes to their eye morphology that are reversed when they reemerge. Only the children were observed because adults employ other methods of food acquisition that do not involve the sea, according to the study. Therefore, whether or not this visual acuity disappears with age is unknown.
Gislén hypothesized that "it does disappear as one gets older due to age-related changes in the flexibility of the lens."
According to the study, the Mokens have been dependent on the ocean for food for thousands of generations. The children would collect food from the sea floors without the help of visual aids. In an attempt to explain this adaptive strategy, the scientists have considered various methods of voluntary ocular accommodations.
"Sea gypsies have lived by and off the sea for thousands of years and evolution may have favored those who had intrinsically better underwater accommodative powers," Gislén said.
She has also performed other experiments with Swedish children that test for improvements in visual acuity. Unpublished results suggest that underwater vision can be improved by simply learning. The visual acuity improvement achieved by the Swedish children matched that of the Moken children.
"I think that the eye is just more flexible than we ever thought!" Gislén said. "You simply learn to use the eye to the limit of its ability -- something possibly all children can learn to do."
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