Causes

Brain Structures. Increasingly, research using advanced imaging techniques shows there is a difference in the size of certain parts of the brain of ADHD children compared to children who do not have ADHD. The areas showing change include:

• The prefrontal cortex. The prefrontal cortex is located in the front of the brain. It is thought to be the brain's command center. It regulates the brain’s ability to block certain responses. A number of imaging studies have indicated that the prefrontal cortex of the brain in people with ADHD may be less active than in those without the disorder.
• The caudate nucleus and the globus pallidus. The caudate nucleus and globus pallidus, located near the center of the brain, speed up or stop orders coming from the prefrontal cortex. A major 2002 study reported that it was smaller than average in young children with ADHD, but tended to normalize as children got older. Abnormalities in these areas may impair a person's ability to stop certain actions, resulting in the impulsivity typical of people with ADHD.
• The cerebellum. The cerebellum is the area above the brain stem. This area helps control muscle tone and balance, and synchronizes muscle activity. An important 2002 study reported that this area tends to be smaller in children with ADHD compared to those without the condition.

It is important to note that such abnormalities do not get worse and are unrelated to intelligence.

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Brain Chemicals. Abnormal activity of certain brain chemicals in the prefrontal cortex may contribute to ADHD. The chemicals dopamine and norepinephrine are of special interest. Dopamine and norepinephrine are  neurotransmitters, or chemical messengers, that affect both mental and emotional functioning. They also play a role in the "reward response." This response occurs when a person experiences pleasure in response to certain stimuli (such as food or love). Studies now suggest that increased levels of the brain chemicals glutamate, glutamine, and GABA, collectively called Glx, interact with the pathways that transport dopamine and norepinephrine.

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