A brain area that helps orchestrate mental activity works overtime in children with attention deficit hyperactivity disorder, reflecting the internal struggle to hold more than one thing in mind at a time, neuroscientists reported Sunday.

The scientists used a functional magnetic imaging scanner to track signs of neural activity among 19 affected children and 23 other children who were asked to remember a simple sequence of letters. The scientists discovered that a criticalmental control area, called the dorsal anterior cingulate cortex, worked much harder and, perhaps, less efficiently among children with attention problems.

This fundamental difference in brain function might be an underlying cause of the inattentiveness, impulsivity and focus problems that make it hard for ADHD children to concentrate in the classroom, the scientists said during an annualgathering of 31,000 brain researchers in Washington, D.C.

'Our findings suggest that the function as well as the structure of this brain area is different in children with ADHD,' said Wayne State University biologist Tudor Puiu, who reported the team's findings Sunday at a conference held by the Society for Neuroscience. 'It might explain the cognitive problems we see in the classroom.'

All told, about two million U.S. children have been diagnosed with attention problems. No one yet understands the basic neurobiology responsible for the mental ailment, which has grown more common since 2003, according to a survey by the U.S. Health Resources and Services Administration.

The portion of those with the most severe symptoms who are treated with prescription stimulants, such as methylphenidate (Ritalin) and amphetamine (Adderall), also has continued to rise, the National Institutes of Health reported in September.

The finding reported Sunday adds to growing biomedical evidence that those diagnosed with the attention disorder