“When the tide came in, these kids started swimming. But not like I had seen before. They were more underwater than above water, they had their eyes wide open – they were like little dolphins.”
Deep in the island archipelagos on the Andaman Sea, and along the west coast of Thailand live small tribes called the Moken people, also known as sea-nomads. Their children spend much of their day in the sea, diving for food. They are uniquely adapted to this job – because they can see underwater. And it turns out that with a little practice, their unique vision might be accessible to any young person.
In 1999, Anna Gislen at the University of Lund, in Sweden was investigating different aspects of vision, when a colleague suggested that she might be interested in studying the unique characteristics of the Moken tribe. “I’d been sitting in a dark lab for three months, so I thought, ‘yeah, why not go to Asia instead’,” says Gislen.
Gislen and her six-year old daughter travelled to Thailand and integrated themselves within the Moken communities, who mostly lived on houses sat upon poles. When the tide came in, the Moken children splashed around in the water, diving down to pick up food that lay metres below what Gislen or her daughter could see. “They had their eyes wide open, fishing for clams, shells and sea cucumbers, with no problem at all,” she says.
Gislen set up an experiment to test just how good the children’s underwater vision really was. The kids were excited about joining in, says Gislen, “they thought it was just a fun game.”
The kids had to dive underwater and place their heads onto a panel. From there they could see a card displaying either vertical or horizontal lines. Once they had stared at the card, they came back to the surface to report which direction the lines travelled. Each time they dived down, the lines would get thinner, making the task harder. It turned out that the Moken children were able to see twice as well as European children who performed the same experiment at a later date.
What was going on? To see clearly above land, you need to be able to refract light that enters the eye onto the retina. The retina sits at the back of the eye and contains specialised cells, which convert the light signals into electrical signals that the brain interprets as images.
Light is refracted when it enters the human eye because the outer cornea contains water, which makes it slightly denser than the air outside the eye. An internal lens refracts the light even further.
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