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Human eye evolved to see dark world
Despite being creatures of the day, new research has found our eyes are built to see a world that contains more darkness than light.

The discovery by Dr Charles Ratcliff from the University of Pennsylvania helps answer a question that's been puzzling scientists for years: Why do we have more retinal cells that respond to the dark than to light?

Their findings appear this week in the Proceedings of the National Academy of Sciences.

Retinal ganglion cells are located in the back of the eye and form the optic nerve, which transports messages from the photo-receptors lining the retina to the visual cortex of the brain.

In humans, cells that respond to a dark spot on a light background, are called 'OFF' cells, while their counterparts are called 'ON' cells. Previous research has shown OFF cells are smaller, but more densely clustered than ON cells. It is thought this clustering allocates more neural processing to dark regions of an image.
Perfect imbalance

Ratcliff and colleagues reasoned this imbalance of cells might reflect what we see in the world around us.

To test their hypothesis, they measured the statistical distribution of light and dark areas in a number of images. They found most contain more dark contrasts than light, regardless of the spatial scale used.

Ratcliff and colleagues then constructed artificial images that matched the statistical characteristics of natural ones and computed the optimal configuration of OFF and ON cells for visual processing.

They found the best configuration of cells contained densely clustered regions of OFF cells, similar to the layout found in the human retina.

Ratcliff says the findings suggest human vision has evolved to efficiently represent visual information in the natural world.
Metabolic limit

Associate Professor Ian Trounce from the Centre for Eye Research Australia, says it's the first time scientists have compared retinal ganglion OFF and ON cell ratios with pictures of the real world.

He says the results highlight how organisms respond and adapt to their environment.

"It's a natural evolutionary response to the images we see," says Trounce.

But he says the human eye and retina are at the very edge of what is metabolically possible.

"It's the most metabolically active tissue in the body," he says. "The neurones are so metabolically active they just can't pack any more mitochondria into the cells to do the job."

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