Children with autism may have slower development of some neurons although their brains are often larger than normal, researchers say.

Previously, some scientists thought the brain abnormalities in autism resulted from faster development.

Brain scans confirmed children with autism tend to have brains that are enlarged by about 10 per cent, Stephen Dager of the University of Washington School of Medicine and his colleagues found.

Although their brains were larger, the children with autism appeared to lack the neuronal development of healthy children, the team reports in Tuesday's issue of the journal Neurology.

The scientists used magnetic resonance imaging to scan the brains of 60 autistic children, 16 children with developmental delay and 10 children showing normal development.

Dager's team focused on measuring water properties of brain tissue.

As the brain develops, water becomes bound in neurons. Normally, this happens quickly in the first six months of life and then slows until 18 months of age.

In autistic brains, water moved more, and there was a delay in development of grey matter found on the surface of the brain, the researchers found.

By eliminating the faster development idea, the researchers say they are narrowing down potential causes of the disorder.

The mechanism behind the findings remains unknown, but Dager's team plans to study whether inflammation early in development contributes to autism.

The results cannot be used as a diagnostic tool to determine if a child has autism, Dager cautioned.