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What Does ADHD do to the Brain?

written by: Daniel P. McGoldrick • edited by: Paul Arnold • updated: 11/30/2010

Neurologists and neuroscientists are working to uncover the underlying neural mechanisms that are associated with ADHD. Advances in technology, especially the results of the brain scan imaging discussed below, helps them learn increasingly more about what ADHD does to the brain.

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    So what does ADHD do to the brain? Scientists and researchers have carried out extensive research over the last decade or so to find out. They have scanned the brains of individuals with ADHD and compared them to scans of people who don’t have the disorder. Since the brain is such an intricately complicated organ with so many interconnected parts and overlapping functions, even the scientists can’t agree conclusively on all the findings. But let’s start with some generally accepted conclusions suggesting that the brain of someone with ADHD is structurally different to the brain of someone without it.

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    ADHD PET Scans Pathophsyiolgy is defined as the study of the physical and biologic manifestations of disease (or in the case of ADHD, a disorder) as they correlate with the underlying abnormalities and physiologic disturbances. As this relates to ADHD, we’re going to discuss exclusively what it does to the brain. Let’s start with what scientists know (which can also be stated as what they think they know). First, we’ll start with the more obvious characteristics such as what’s smaller or what’s missing, and then we’ll move onto more complex issues about what that means and the interplay of complex functions.

    Brain scanning is the manner in which this evidence is created so that researchers can make their conclusions based on what they see. Previously, brain scans could only take a snapshot of the brain at a given time but now the technology has advanced significantly to allow for brain imaging to see it as it functions. These functional imaging machines such as fMRI (Functional Magnetic Resonance Imaging) and CT (Computerized Tomography) can track brain activity when it's in progress, and see what areas "light up" when tested.

    Unfortunately, just because they can see this doesn’t mean they know exactly what it means. There are competing theories, just as there are about what causes ADHD; but rather than getting bogged down in complex arguments, we’ll try and focus on what’s generally accepted at this point. We look forward to following this issue as it develops and more research unfolds to prove a given hypothesis.

    A person with ADHD has a smaller prefrontal lobe, which is the part of the brain responsible for planning, reasoning, and problem solving (what is known as executive functions). It's generally about 10 % smaller. The anterior temporal lobes, caudate nucleus, and globus pallidus are typically smaller too. The smaller right frontal lobe may be a cause of inattentive behavior in ADHD since it's generally considered that the right side of the brain is involved in the attention process. Impulsivity and stimulus processing are also believed to be modulated there. An excessive amount of grey matter in the inferior parietal cortices has been noted in ADHD individuals as well.

    PET Scan Images courtesy of Wikimedia Commons

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    Less Activity in the Brain

    The more we learn about brain chemicals and the workings of neurotransmitters, the closer we get to answering the question: what does ADHD do to the brain? As far as brain chemicals are concerned, one study found that children with ADHD had excessive amounts of the brain stimulant glutamate and conversely, a lower than average amount of another brain chemical that is known to help control impulses.

    The neurotransmitters dopamine (DA) and norepinephrine (NE) are certainly involved in the pathophysiology of ADHD. Dopamine is associated with mood, risk taking, impulsivity, and reward. Norepinephrine is believed to moderate attention, arousal and mood. Brain studies have suggested that defects in receptors genes and transporter proteins for both of these neurotransmitters plays a part in the abnormal functions of an ADHD brain.

    Research has also indicated that the frontal lobe, basal ganglia, caudate nucleus, cerebellum, and other areas of the brain, play major roles in ADHD since they are all involved in complex behavior regulating processes (what we pointed out as the executive functions above). These functions also include such processes as self-monitoring, internal dialogue, inhibition, memory, motor control, and emotional regulation. And according to a popular theory in the ADHD research community, the core deficit in the disorder relates to problems in response inhibition. The effects and problems with the other executive functions stem from there.

    Although all these studies aren’t necessarily conclusive, the best interpretation of the research is that brain scan images have clearly demonstrated to the world that ADHD is a very real disorder. As more is discovered and understood, doctors will be better able to treat ADHD patients effectively.

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    Zametkin AJ, Liotta W The neurobiology of attention-deficit/hyperactivity disorder. J Clin Psychiatry. 1998;59 (suppl 7): S17-S23