By Shari Roan, Times Staff Writer

The brains of children with attention deficit hyperactivity disorder are clearly different from those of normal children, a new brain-imaging study has shown, providing further evidence that the problem is biologically based.

The study of 152 children casts doubt on lingering theories that ADHD is caused by food allergies or isn't a real illness. Instead, the research shows, children with the disorder have anatomical differences in several parts of the brain, which probably accounts for the symptoms of restlessness, poor concentration, distractibility and talkativeness they typically experience.

"It looks like the whole brain is involved with ADHD, but some parts are more affected than others," said Dr. Francisco X. Castellanos, director of the Institute for Pediatric Neuroscience at the New York University Child Study Center and lead investigator of the study.

Using brain scans taken over 10 years, the study compared the children with ADHD with 139 normal children in the same age range. Although Castellanos calls brain studies on ADHD "primitive," the study produced several findings that should enhance the understanding of a disorder that affects about 5% of children.

Previous studies suggested that the front part of the brain is altered in ADHD children, but this study showed differences in several regions of the brain.

The brains of ADHD children were 3% smaller in volume compared with normal children.

The brain differences remained the same through a child's development, which suggests the differences are fixed.

The brain differences among ADHD children who took medication, such as Ritalin, and those who did not were similar. That suggests that it's not the medication that causes brain differences, as some researchers have suggested, Castellanos said.

Although medication helps resolve the symptoms of ADHD in some children, "medication didn't change the brain," Castellanos said.

The study did show, however, that children with ADHD who took medication were similar to normal kids in terms of the brain's white matter, which contains nerve fibers. Although far from proven, that could suggest that medication enhances the normal development of that part of the brain, he said.

The fact that the brain differences remain largely unchanged suggests that the cause of ADHD occurs prenatally -- it could be genetic -- or early in life, giving researchers more avenues to explore regarding the causes of ADHD and variations of the disorder, he said.

The research also should reassure parents, teachers and children with the disorder that no one is to blame for difficulties children encounter.