Attention Deficit/Hyperactivity Disorder (ADHD) is the most commonly diagnosed psychiatric disorder in children, and is becoming a big deal in adults as well. ADHD is a pile of related symptoms, most of them dealing with motivation, impulsivity, inattention, and, you know hyperactivity (they call it ADHD for a reason). Right now, we treat ADHD with stimulants such as Ritalin and Adderall, which in low doses and when they act over a long period of time can increase focus and help people with ADHD function better.
But the question remains as to what CAUSES ADHD, what abnormalities are going on in the brain that cause the symptoms. There are several hypotheses as to what's going on. One of them is the dopamine hypothesis, that dysfunctions in dopamine systems are responsible some of the symptoms. But in order to prove this, we have to find evidence for it in humans. There is some evidence that dopamine dysfunction contributes, and now we have a little bit more.
Volkow et al. "Motivation deficit in ADHD is associated with dysfunction of the dopamine reward pathway" Molecular Psychiatry, 2011.
(I should note here that Dr. Nora Volkow is the current head of the National Institute on Drug Abuse, and is also one of the foremost researchers on ADHD in humans).
In this case, the authors wanted to look at how dopamine system function was related to scores of motivation in adults with ADHD. The problem with this is how to measure "motivation". In this case, they looked at people's ADHD scores (compared to non-ADHD controls), and looked at scores on personality tests, particularly those related to "achivement". These are things like
drives self, enjoys working hard, welcomes difficult
and demanding tasks, persists where others give up,
is ambitious, puts work and accomplishments before
many other things, sets high standards, is a perfec-
tionist’ vs ‘does not like to work harder than is strictly
necessary, avoids very demanding projects, sees no
point in persisting when success seems unlikely, is
not terribly ambitious or a perfectionist’.
They compared these scores with PET images of the dopamine systems of the patients, and correlated the results. PET imagine (positron emission tomography). This imaging lets you take a radiolabeled chemical, inject it in (don't worry, it's harmless), and take an image of the results. In this case, they used a radiolabeled chemical that would compete for dopamine receptors, the D2/D3 variety, and one for the dopamine transporter, to look at dopamine system function. These drugs would bind to receptors and give off radiation, allowing them to be seen and produce a picture of receptor availability, and therefore an indirect picture of dopamine system function. The more binding of the chemicals you get, the more glow, and the more receptors are there, and the opposite is true as well.
When they correlated the PET results with the personality tests, this is what they got.
What you can see here is a correlation between "trait motivation" scores and dopamine function in the nucleus accumbens. The more D2/D3 and DAT present, the better the motivation score.
And here is a similar correlation, showing that the better someone's score on the ADHD symptom test (the CAARS), the higher their trait motivation score.
The authors conclude that the dysruption of the dopamine pathway (defined here as a decrease in D2/D3 binding and DAT binding) is associate with decreased motivation as well as higher ADHD scores.
But the authors note, and I agree, that correlation isn't causation. But it does suggest that differences in motivation may be related to dopamine function. This in itself isn't new, dopamine and motivation are known to be related (especially when involving things like drugs of abuse), but the correlation here is a nice bit to add to the current knowledge on ADHD.
But I'm not quite sure what they mean by "motivation". They used an Achievement Scale of a personality test as a surrogate measure for motivation (because we don't really have a scale for that). While that's fine in and of itself, I'm not sure that this is the most accurate measure. Motivation, after all, doesn't exist in a vacuum. And obviously achievement doesn't either. While the results may correlate, I'm not sure if we can conclude cause. There are some caveats. For example, children and adults with ADHD do not do as well in school, they have both educational and social difficulties, not to mention dealing with the difficulties of sitting still in the school while attempting to do work. A lot of these difficulties resolve with treatment, but early problems like this could certainly impact how "motivated" the people end up being rated, especially if they are looking at tests like these and thinking of them in terms of academic and job achievement (which they might very well be doing).
I think in this case it might have been better to test motivation for certain things more directly (the authors agree with me, by the way, it's mentioned in the discussion). You can do this behaviorally in humans using a modified progressive ratio test, having them press a button increasingly for a reward (first M&M costs 2 presses, next costs 4, then 16, then 32, you get the idea). Looking at this kind of persistence, as well as other tasks might help to really look at motivation.
But it could also be something for which you're never going to be able to really suss out the answer. I mean, ADHD is ADHD for a reason. ADHD patients may well fail the tasks due to distractability, and then you have to ask how distractability and persistence is related to motivation. So while a correlation is good (and dopamine has a good bit to do with motivation and so it's certainly plausible), and is certainly interesting, I'm not sure we're at a cause yet. The authors think that this paper helps to establish ADHD as partially a disorder of motivation, but I don't think you can really separate motivation from lack of persistence and distractability as a result of the disorder. I also don't think you can talk about motivation in adults as being part of a disorder, when they have grown up with this disorder all their lives. At this point, their experiences may be impacting their motivation, and the correlation may be related to other ADHD traits. While the correlation itself is interesting, I think the concept of "motivation" may be just too much to separate out.
Volkow, N., Wang, G., Newcorn, J., Kollins, S., Wigal, T., Telang, F., Fowler, J., Goldstein, R., Klein, N., Logan, J., Wong, C., & Swanson, J. (2010). Motivation deficit in ADHD is associated with dysfunction of the dopamine reward pathway Molecular Psychiatry, 16 (11), 1147-1154 DOI: 10.1038/mp.2010.97