Autism and Motor Skills: A Matter of White Matter?

by Faith Hickman Brynie

November 19, 2007

Walk on your heels. Dance on your tiptoes. Tap your foot and fingers in rhythm. Easy, right?

Not for children with autism. They have difficulty with motor tasks, such as tying their shoes, riding a bike, or playing baseball. New research suggests that differences in the white matter of the brain may explain why.

“When we think about individuals with autism,” says Stewart Mostofsky, a pediatric neurologist at the Kennedy Krieger Institute in Baltimore, “we don’t think about them excelling in athletic pursuits. They tend to be motorically clumsy and awkward in a similar way to how they are socially clumsy and awkward.”

Mostofsky says studies of motor skills and autism offer two advantages to researchers. First, motor skills are easier to observe, measure and assess than are social and communicative skills. Second, the brain’s motor cortex is a well-defined region. “We know where to look,” Mostofsky says.

He also knows what kind of brain tissue to examine. Young children with autism have a greater volume of white matter in their brains than typically developing children have (the difference disappears by adolescence). White matter is composed of the fibers that connect neurons. It is different from the gray matter, which is made of neuronal cell bodies.

Mostofsky and a team of researchers recently looked at the relationship between white matter in the brain’s motor cortex and motor skills in children. They studied 76 children ages 8 to 12. Twenty of the children had been diagnosed as high-functioning children with autism. Twenty more had been diagnosed with attention-deficit/hyperactivity disorder (ADHD). Children with ADHD often also show poor motor skills. The other 36 children were classed as typically developing (TD) children.

The researchers used a standardized test, the Physical and Neurologic Examination of Subtle Signs (PANESS), to measure the children’s motor skills. The test includes both gross and fine movements such as walking a line, hopping on one foot and patting the hand rhythmically. The higher the score, the poorer the motor performance. To measure the volume of white matter in the brain’s motor regions, the scientists used anatomic magnetic resonance imaging (aMRI).

Comparing white matter volume to test scores revealed a strong relationship, but the relationship differed among the groups. In the typically developing children, “bigger was better,” Mostofsky says. “The larger the white matter volume in the motor cortex, the better the children performed.” The children with ADHD, while performing worse on motor skills tests than typically developing children, showed the same "bigger is better" white-matter pattern as the typically developing children.

The children with autism also had poorer motor skills on average than children in the typically developing group. But in their case, the larger their white matter volume, the worse they performed. The researchers reported their results in the August issue of the journal Brain.

Mostofksy plans to use other imaging approaches to examine white matter structure in greater detail. “The white matter volume that we looked at in the motor cortex was the localized or radiate white matter,” he explains. The radiate white matter forms connections within the motor cortex itself or to nearby regions. It is different from white matter that connects more distant brain regions.

He sees his research on motor skills as a window of opportunity for finding out more about the brain basis of the deficits in social and communicative skills that define autism. “One prevailing hypothesis [about autism] is that there is an overgrowth of localized connectivity and a relative undergrowth of more distant connectivity,” Mostofsky says. He plans to use an imaging technique called diffusion tensor imaging (DTI) to test that hypothesis.

 “If you think about what underlies the development of social skills, it really is a learning process similar to what underlies the acquisition of motor skills. You don’t learn social skills by memorizing them; you learn them in the same way that you learn motor behaviors," he says. "It may be that the deficits that contribute to motor impairment in autism are parallel to those that contribute to impairments in social functioning."

Mostofsky also plans to study children as young as four to see if the association is detectable at an early age. He believes this research may lead to better methods for early detection and treatment of autism, as well as a better understanding of the deficits in social skills and communication seen in children with autism.