Children with hyperactivity disorder have a faulty "off-switch" when it comes to their minds wandering, scientists have found.
Brain scans of youngsters with attention-deficit/hyperactivity disorder (ADHD) have shown for the first time why they may have difficulty concentrating.
They suggest that children with ADHD require either much greater incentives or medication (methylphenidate, often known as Ritalin) to focus on a task compared to children without the condition.
If the incentive is low, then those with ADHD fail to "switch off" brain regions involved in mind-wandering.
But when there are strong incentives, or when youngsters are taking their medication, their brain activity is the same as for a child without ADHD, according to University of Nottingham researchers.
The study, published in the Journal of Child Psychology and Psychiatry, and funded by the Wellcome Trust, involved 18 children with ADHD aged nine to 15.
Their brains were compared with those of 18 similar children without ADHD.
All the children played a computer game that involved hitting green aliens as quickly as possible while avoiding black ones.
The reward for avoiding black aliens was then increased to study the effect of incentives.
Previous studies have shown that children with ADHD have difficulty controlling the part of their brain which gives rise to wandering thoughts or daydreaming.
This part of the brain is normally suppressed when people are focussed on a specific task.
The latest research suggests that children with ADHD have difficulty suppressing this part of their brain unless they are on medication or unless incentives are high.
ADHD is thought to affect between 3% and 7% of school-age children.
Co-author on the study, Dr Martin Batty, said: "Using brain imaging, we have been able to see inside the children's heads and observe what it is about ADHD that is stopping them concentrating.
"Most people are able to control their 'daydreaming' state and focus on the task at hand.
"This is not the case with children with ADHD.
"If a task is not sufficiently interesting, they cannot switch off their background brain activity and they are easily distracted.
"Making a task more interesting - or providing methylphenidate - turns down the volume and allows them to concentrate."
Professor Chris Hollis, who led the study, added: "The results are exciting because for the first time we are beginning to understand how, in children with ADHD, incentives and stimulant medication work in a similar way to alter patterns of brain activity and enable them to concentrate and focus better.
"It also explains why in children with ADHD their performance is often so variable and inconsistent, depending as it does on their interest in a particular task."