Neurobiology of ADHD

Attention-deficit/hyperactivity disorder (ADHD) is a childhood psychiatric disorder, estimated to affect 3 to 5% of school-aged children (American Psychiatric Association, 1994; Barkley, 1997). Children diagnosed with ADHD vary widely in the type and severity of symptoms that they demonstrate, but the disorder is generally characterized by developmentally inappropriate levels of inattention, impulsivity, and hyperactivity (American Psychiatric Association, 1987, 1994; Barkley, 1997; Cantwell, 1996). The diagnostic criteria for ADHD, specified in the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV: American Psychiatric Association, 1994), divides symptoms into three domains of inattention, hyperactivity, and impulsivity. Inattention symptoms include difficulty concentrating and distractibility, impulsivity symptoms include acting without thinking and taking risks, and hyperactivity symptoms include being constantly “on the go” and excessive restlessness or fidgeting. The DSM-IV describes three subtypes of ADHD—predominantly inattentive type, predominantly hyperactive-impulsive type, and combined type, which describes children who exhibit features of all three symptom domains (American Psychiatric Association, 1994).