Separate Networks Govern Impulsivity in ADHD, Drug Use

Article excerpt

FROM NATURE NEUROSCIENCE

Distinct brain networks seem to control impulsivity traits in adolescents with symptoms of attention-deficit/hyperactivity disorder and those with a history of substance use, even though both groups share the same deficit in impulse control, according to the largest single imaging study of either adolescents or adults.

The study, which involved obtaining functional MRI scans of 1,896 adolescents aged 14 years, does not support the viewpoint that ADHD symptoms represent a predictive endophenotype for subsequent drug abuse.

Evidence from the study also suggests that among adolescents who used drugs, the differences in the activation pattern of brain networks actually preceded drug use, meaning that heredity plays a significant role, according to lead investigator Robert Whelan, Ph.D., and his colleagues in the IMAGEN Consortium. In particular, variations in the gene encoding the norepinephrine transporter seemed to predispose adolescents toward impulsivity.

Investigators with the IMAGEN study, a European research project investigating mental health and risk-taking behavior in teenagers, measured the adolescents' performance on the stop-signal task (SST). It requires the participant to cancel an already initiated motor response; their subsequent stop-signal reaction time (SSRT) serves as a clinical index of impulse control. People with ADHD and those with substance abuse are known to have longer SSRTs.

Because of the large sample size, Dr. Whelan and his colleagues analyzed their data using a factor analysis approach instead of the mass univariate approach commonly used in functional MRI studies. By doing so, they could identify individual differences in activation levels and "biologically relevant networks of interdependent, rather than single, regions of interest." Dr. Whelan is with the departments of psychiatry and psychology at the University of Vermont, Burlington, and the Institute of Neuroscience, Trinity College Dublin.

Overall, the investigators identified seven networks involved in successful inhibition and six involved in failed inhibition. In general, participants with faster SSRTs (defined by the median split SSRT) had significantly higher brain activity in the "stop fair bilateral frontal network, the "stop success" right frontal network, and the stop success basal ganglia network. …