The precise reason why a cartoon caused hundreds of children to have seizures may have been uncovered by experts.

Approximately 685 Japanese children suffered the fits after watching a Pokemon television cartoon in 1997.

It triggered fears that modern cartoons and even video games could be putting children at risk of fits.

But an Italian study, published in the March issue of Nature Neuroscience, found that children prone to a particular type of epilepsy nearly all had problems in one particular area of the brain.

And its findings may pave the way for programme makers and game designers to eliminate dangerous flickering from the screen.

Photosensitive epilepsy is thought to affect between 0.5% and 0.8% of children aged between four and 14.

The team from the University of Pisa found that these children's brains were less able to distinguish between the black and white in flickering images.

Researchers attached electrodes to the heads of 23 children, just over half of whom had photosensitive epilepsy.

They were exposed to flickering images with different levels of contrasts between light and dark.

Among the non-epileptic children, brain activity in an area called the cortex increased when the contrast in the flicker was 20% - the difference between grey and white.

However, when the contrast increased, the brain activity levelled off.

However, among those with epilepsy, this activity persisted even when the contrast was 90% - stark black and white.

Reducing the risk

The frequency of the flashing had to be correct to trigger this response, and did not work when colours were substituted for black and white.

The authors suggest that in photosensitive epilepsy, the normal mechanisms which help stop the brain overreacting to sudden and repeated changes in contrast are defective.

If cartoon designers avoid the particular frequencies involved, they suggest, then the risk would be much reduced.

Flashing lights in discotheques have long been associated with triggering epileptic fits in some people.

However, individual patients have different triggers, such as smells, sounds and changes in natural hormone levels.

A spokesman for the British Epilepsy Association stressed that UK guidelines meant that the risk of cartoon or video game-induced seizures was low.

She added: "It's only a small proportion of people with epilepsy who are sensitive to flashing or flickering light.

"It normally first starts when quite young, and can be well-controlled by drugs."

The association recommends that parents worried about the issue should take the following steps:

• Don't allow children to sit too close to the television or computer screen
• Restrict computer use or television if the child is tired
• Watch television or use the computer in a well-lit room
• Make sure the child takes frequent breaks