Why are so many humans near sighted?

Myopia linked to modern life?

Also in this week's column:

Why are so many modern humans near sighted?

Asked by Alan Harper of Oakland, California, USA

The eye receives rays of light and bends them so that an image is resolved on a small point of the retina. But things can go wrong.

If the rays focus in front of the retina, the person has myopia (aka near-sightedness or short-sightedness) and suffers blurred vision of distant objects. But if the rays focus at a point behind the retina, the person has hyperopia (aka hypermetropia, far-sightedness or long-sightedness) and suffers blurred vision of nearby objects.

According to Dr Stephen Miller, director of the clinical care centre of the American Optometric Association in St Louis, "the shape of the eyeball and the focusing power of the lens and cornea help determine focus, but the angle at which light rays hit the eye plays a role".

"Light comes into the eye from all directions. Rays entering the eye at an angle from above or below would tend to focus somewhere before or behind the centre of vision. Those rays coming in essentially perpendicular to the eye, on the other hand, would tend to be focused more directly on the retina, providing a clearer image of what one is looking at."

Myopia occurs in at least seven different forms, occurs in varying degrees of severity, and can first develop in infancy, youth, or adulthood. The prevalence of myopia varies from country to country. Depending upon how it is defined, myopia rates are as high as 70 to 90 per cent in Asia, 30 to 40 per cent in Europe and the US, but only 10 to 20 per cent in Africa.

A 2005 study published in Optometry and Vision Science found that slightly more than 50 per cent of UK first-year university students are myopic.

School myopia appears during the childhood school years. This form of myopia is attributed to use of the eyes for close work at school.

As humans use their eyes more and more in close activities (reading, computers, video games, television, and so on) in our modern world, it's not surprising there is so much myopia.

Stephen Juan, Ph.D. is an anthropologist at the University of Sydney. Email your Odd Body questions to s.juan@edfac.usyd.edu.au

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