However, it can also mean children only write or draw on the right-hand side of a page, or that they knock things on their left-hand side over more often.

The condition is seen in adults who have had stroke.

It's not to be expected that treating this would be the answer to the whole problem of ADHD
Professor Eric Taylor, Institute of Psychiatry

"Left neglect" is seen where the right side of the brain is affected.

It means things on someone's left-hand side are simply not noticed, especially if they are doing something they find boring or unstimulating.

Children who do not have ADHD may also show symptoms of the condition, the researchers say.

The research is published in the Journal of Child Psychology and Psychiatry, and in Brain and Cognition.

Fidgeting

Left neglect is a well-known condition in adults who have suffered right-sided brain injury. It means they may act as if half the world has simply disappeared.

Researchers from the MRC's Cognition and Brain Sciences Unit, in Cambridge, found some children with ADHD, who had no brain damage and perfectly normal intelligence, showed left neglect as severe as that seen in some adults with substantial damage to the right side of the brain.

This latest study asked parents and teachers of healthy Nottingham children to assess how much they ran around or fidgeted - potential indications of ADHD.

They then compared the 10% who displayed such habits the least with the 10% who displayed them the most.

It was found that those who fidgeted or ran around the most were more likely to have problems perceiving things on their left, even though their symptoms were not severe enough to merit a diagnosis of ADHD.

Dr Tom Manley, who led the research, said: "The right side of our brain seems to be heavily involved in keeping us awake and alert, particularly when we are bored.

"Because the right side of the brain is interested in what is going on to our left and vice versa, as this alertness declines over time or with boredom, it takes some of our awareness of the left with it.

"All children lose information disproportionately from the left, but children with ADHD appear to reach this point more quickly and to a greater extent than other children unless they are given stimulant medication."

He told the BBC News website: "This condition isn't expected in children, although people who work with adults who have had strokes know it well.

"There are three or four rehabilitative techniques used with adults which work well.

"Our early studies suggest they may work for children, but more research is needed. Nevertheless, improving early assessment in children should be a priority."

Eric Taylor, professor of child and adolescent psychiatry at the Institute of Psychiatry in London, said: "This is the first actual evidence of this link.

"But it's in keeping with research from our unit that there are right-sided brain problems in children with ADHD.

"Clearly, it is only part of the problem. It's not to be expected that treating this would be the answer to the whole problem of ADHD.

"But it may be one particular way of helping children with the condition."