The human visual system has evolved to cope with the world around us rather than to provide a perfect picture of everything we see.

The brain and the eye work together to interpret their surroundings, and as long as that is achieved there is little need for them to improve.

Quite why it is that a set of horizontal lines appears shorter than a set of vertical lines of exactly the same length remains a matter of debate, but it is a common fault within nature.

Zebras tend to have horizontal lines around their rumps and hind legs, and this visual trickery is thought to protect them from predators. When the lion sees the zebra’s behind, the lines give the impression of long legs and a thin rump – suggesting a scrawny animal not worth the chase.

Part of the reason for the imperfect interpretation of lines is likely to lie in the processes by which the brain reads two and three-dimensional images.

One possibility is that the brain has a function that enables it to counteract foreshortening when looking at three-dimensional objects but, when applied to a two-dimensional image, it cannot interpret the data correctly.

Peter Thompson, who presented his research yesterday, said that another suspect factor is the switch from three-dimensional information to two-dimentional when an image hits the retina. “A lot of visual illusions may arise because we have two-dimen-sional images on our retinas which are recreated into a three-dimensional view of the world.”

Just as digital cameras compress information into j-pegs, a human eye has 125 million photoreceptors that squash all it sees through 1 million optical nerve fibres – with the potential loss of definition.

Despite the illusions that trick the human visual system Dr Thomson said that, by and large, it does what is required and in many ways is remarkably well developed.

When someone is seen approaching, for example, they take up a greater part of the view as they get closer but the brain is able to interpret the sight to realise the person is merely getting nearer rather than bulging.

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