User:Drdonigan/sandbox

Previous studies have been conducted in order to determine the possible cause of ADHD. One was performed by Herbert Quay. Quay viewed the main problem as an imbalance between behavioral activating and inhibiting systems, arguing that ADHD was caused by under-activity of the behavioral inhibition system  Quay (1988) put forward a model of childhood mental disorders based on Gray's (1982) theory that there exists within the brain, a behavioral inhibition system (BIS), which processes signals related to aversive or punishing stimuli such as parent or teacher commands. According to this model, children with attention deficit hyperactivity disorder (ADHD) show lower than optimal levels of activity in this system, which leads to less responsiveness at a physiological level to signals related to punishment. Quay hypothesized that children with anxiety or depression would be expected to show increased activity in the BIS and thus be more sensitive to signals of punishment. There were no differences between the groups, suggesting that, in terms of classical conditioning, ADHD children are equally responsive to signals related to punishment as controls.

In recent years, new studies have shown that dopamine, a brain chemical, may play a role in attention deficit hyperactivity disorder. These studies do not claim to be a direct cause of ADHD, however, a direct link has not been confirmed. One study, performed by Phillip Shaw, MD, PhD. focuses on the genetics of ADHD specifically, on a particular variation of the DRD4 gene, which makes a dopamine receptor in the brain. Dopamine is an important chemical messenger used between nerve cells in the brain. It is linked to many functions including movement and mental health. The study was performed by scanning the brains of 105 kids with ADHD and 103 without ADHD, 67 of which returned six years later for follow up brain scans. "The central finding was that the most important known genetic risk factor for ADHD (the 7-repeat form of the dopamine D4 receptor gene) was associated with having a better clinical outcome and higher intelligence in a large group of children with ADHD," Shaw tells WebMD (Hitti, 2007). Kids with ADHD who had the DRD4 gene variation tended to have a thinner brain cortex in regions related to attention when the study started, but their cortex thickened during the follow-up period. "The study thus links genes with clinical outcome and brain development," says Shaw. He notes that most of the children took ADHD medications at some point during the study, but that didn't explain the study's findings. Shaw's team isn't saying that the DRD4 gene is the only gene involved in ADHD. Many genes probably influence ADHD, the researchers note.