CHAMPAIGN, Ill. -- Impulsive people, such as children with attention deficit hyperactivity disorder (ADHD), are quick to act. What sets them apart from people with normal behavior, however, is not speed. It's the inability to brake, researchers say.
That picture of impulsivity is becoming clearer as scientists from the University of Illinois and the Hospital for Sick Children in Toronto work to refine their understanding of ADHD and to develop an objective diagnostic technique.
"This deficiency -- the inability to inhibit one's actions -- seems to be unique to hyperactive children," said Gordon D. Logan, a U. of I. psychology professor. "Inhibition is important, both in controlling your interaction with the world and in stopping to allow other processes to work, such as stopping to think. If inhibition goes wrong, then a lot of other things can go wrong. Our studies suggest that slow stop-signal reaction time is responsible for poor impulse control."
Logan has worked with psychiatrist Russell J. Schachar and Rosemary Tannock, a special education researcher, both affiliated with the Toronto hospital, for more than a decade. They described much of their work on inhibitory control and impulsivity in children with ADHD in the Clinical Psychology Review in 1993.
In January, they reported in the journal Psychological Science that similar to the ADHD children they had written about in previous studies, young impulsive adults were slower to inhibit their actions than were non-impulsive adults. While the differences were not as striking as those with ADHD children, the findings about adults clearly strengthen theories that an inhibition deficit is at work in impulsive people, Logan said.
The study involved measuring the reaction times of 80 male and 56 female students on a test developed by Logan. The task requires subjects to respond quickly to a letter on a computer screen but to inhibit that response if they hear a tone. The letter is like a stimulus that generates an impulse in a child at school, and the tone is like a teacher calling the child's attention back to class.
The recent research, funded by the National Science Foundation, resulted from improvements to Logan's technique. Eventually the test, which takes about 20 minutes to administer, could become an objective test for hyperactivity, Logan said. Diagnosing ADHD -- one of the most common childhood neurological disorders -- is now based heavily on subjective observations of parents and teachers.
The researchers currently are analzying data gathered from visitors ages 5 to 80 at the Ontario Science Center in Toronto in an effort to document normal stop-reaction times. Such a baseline should improve the diagnostic capabilities of the stop-and-go technique, Logan said.
The scientists also have studied ADHD children as appropriate dosages of methylphenidate hydrochloride (Ritalin) were being adjusted. They've found that the commonly used drug's effectiveness appears to be in normalizing a child's stopping behavior, and, in turn, their everyday behavior.