There are a lot of theories about what ADHD is.

Could you define for me what ADHD is? ADHD is defined as symptoms of hyperactivity, impulsivity, and inattention beyond what's usual for a developmental age. A 6-year-old is expected to be more hyperactive than a 10-year-old. But a 6-year-old with ADHD can be much more hyperactive than a 6-year-old without ADHD in certain settings; likewise inattention; likewise impulsivity. Those are observed in the home, in school, in play situations. But we don't have a test. We don't have an objective way of definitively saying, "This person has ADHD, or does not," in part, because we don't really understand what it is. ...

How does ADHD work on the brain? What do we know about it?

We don't yet know what's going on in ADHD. We've approached it in a number of different ways, and one way has been to look at what brain regions are smaller, or different. What we've found is that there are a few regions that are smaller in kids who have ADHD. There are other groups that are looking at functional MRI, or at SPECTA (Single Photon Emission Computomography), usually in adults who have ADHD. And most of the evidence converges and suggests that regions that are rich in dopamine are involved. And that's interesting, because dopamine is one of the chemicals that Ritalin boosts.

... What does dopamine have to do with any of this?

... Dopamine is an important signal that certain parts of the brain use to regulate movement. If you don't have dopamine in those brain regions, you develop Parkinson's disease. But it's also apparently used to send a signal that something important is happening. ...

It used to be said that dopamine was the reward chemical--that if something was rewarding, then you would release dopamine. It turns out to be more complicated than that. It's not just whether something's going to feel good, or be rewarded; it's more if there's a possibility that something would feel good.

If an animal knows that they're going to be rewarded when they correctly do a task, then dopamine is no longer involved. But when the animal thinks that maybe this is the way to solve the task, dopamine is leading the way, saying, "This, try this, try this." So it's a very important signaling molecule. And it's classified officially, I guess, or technically, as a neurotransmitter. But it's probably more appropriately described as a neuro-modulator, which means that it sets or modulates the tone for these complex systems.

So we think that it's important to have optimal levels of dopamine in various brain regions; if they're not optimal, then things don't work as well. So that's a very sort of cut-and-dried, simplistic possible explanation of what's going on in dopamine and ADHD.

... If I have ADHD, what do I have in my brain? A deficiency of dopamine? A small cerebellum?

The leading theory is probably that, effectively, your brain doesn't save its dopamine and use it as well as it might. Now, that's not been conclusively shown. There are two studies in adults with ADHD that suggest that that's true in the basal ganglia. And because they use radiation, it's going to be very difficult to repeat those studies in children. But it is sort of exciting to have that new information.

In fact, in one of the studies, they took individuals who had never been treated for their ADHD, tested them, and then gave them Ritalin and re-tested them--and found that the Ritalin, in fact, did affect the molecule that they were trying to have an effect on. And the result of that should be an increase in dopamine in the basal ganglia--we know in the basal ganglia, because we can measure it. We don't know what's going on in the frontal lobes; we don't know what's going on in the cerebellum, because there's much less dopamine there, and so it's much more difficult to measure.

So it's only a partial answer. If you're an adult with ADHD, I can make some tentative conclusions. But if you're a child with ADHD, I don't yet know what's going on in your brain.

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Written 2000. Reviewed: 02/2006