ADHD and Rewar-- Hey, look! A squirrel!

ADHD and Dopamine
The current, generally accepted belief is that ADHD (attention deficit/hyperactivity disorder) is caused by a dopamine imbalance within the brain. However, there is controversy as to whether ADHD is caused by hypodopaminergic functioning or hyperdopaminergic. There are several reasons for this theory. First of all, amongst other things, dopamine is often related to attention and motivation. Another reason is due to the effectiveness of methylphenidate based medication (a.k.a. meth salts). Methylphenidate acts as an dopamine reuptake inhibitor by blocking dopamine transporters. Thus there is more dopamine in the neurons, but the dopamine fires less often.

CD and ADHD
Conduct disorder (CD) is characteristic of socially disruptive, aggressive and usually destructive behavior. Destructive not only to property and others, but to one's own daily life. ADHD is very similar to CD, except there tend to be far less violent tendencies, if any. However, like CD, ADHD can be inhibit one's academic career, occupation, or even social ability. It is believed that CD and ADHD are in essence the same on a neurobiological level, and the only difference comes in upbringing. Individuals who grow up in a high-risk environment are more likely to develop CD rather than ADHD, and visa-versa for a low risk environment. Indeed, the two are so similar that CD is classified as a type of ADHD.

Objective
The objective of Gatzke-Kopp et al's experiment was to try and determine whether ADHD/CD and thus other externalizing disorders such as antisocial personality disorder and borderline personality disorder. The original objective was to compare neurological differences between people with CD and ADHD when dealing with rewards and reward anticipation, however it rather quickly became apparent that this would not be possible since ADHD and CD were so similar. A reward based approach was used because both ADHD and CD are often characterized by impulsivity, that is, the inability to defer gratification, even if doing so will result in a greater reward. In the end, it was decided to use a control group of individuals with no mental disorders. Only males were used because of the small sample size.

Procedure
A sample of twenty-one males between the ages of twelve and sixteen with ADHD/CD were chosen as the experimental group. This was later reduced to nineteen because two of the participants were unable to stay still enough for long enough. A control group of eleven males of the same ages with no mental disorders was also chosen. Any individuals taking any sort of stimulant medication were asked to dis continue use at least thirty-six hours before the test. Previous experiments have shown that there is no appreciable difference between individuals with ADHD who have never taken stimulants, and one who has stopped taking them for thrity-six hours.
Both groups were asked to play three rounds of a simple game. A green square was flashed for 1.2 seconds on either the left or right side (chosen psuedorandomly). The participant would then have to press the appropriate button. If the answer was correct, and a monetary reward was given, then a tone was played for half a second; if the answer was correct, and no reward was given, then a different tone was played for half a second. If the answer was incorrect, then there was no adverse affect.
The game had three rounds. The first round any and all correct answers resulted in a forty cent reward. In the second round, no money was ever rewarded, and in the final round, money was rewarded only part of the time.

Results
The results from the fMRI showed that during the first round when a monetary reward was given, both the control group and the experimental group showed increased activity in their striatum in both the left and right hemispheres. The striatum is a part of the basal ganglia, and is most often associated with motor functions (e.g. movement planning), however, it also deals with executive functioning, reward anticipation, and the processing of unique or novel stimuli. The area is also comprised mainly of medium spiny neurons, which are heavy with dopamine (figure 1).
However, during the non-monetary portion, the control group's anterior cingulate cortex (ACC) was highly activated, however, in the experimental group, the striatum continued to be highly activated. Amongst other things the ACC is used for motivation, task anticipation and error detection. (figure 2 and figure 3)

Conclusions
Ultimately, the experiment did not resolve the hypodopaminergic versus hyperdopaminergic debate. While is is true that dopamine is far more prevelant in the striatum than the ACC, there is no garantee that the neurotransmitter being used is indeed dopamine, since fMRI's can only detect brain activity and not which neurotransmitter is being used.
However, this experiment can give us a great deal of insight into other symptoms of ADHD/CD. The activation of the striatum may help explain the impulsivity of individuals with ADHD/CD. Because the striatum is involved with reward anticipation, its continued activation could mean that the anticipation in someone with ADHD/CD is much greater than in a normal individual, and thus much harder to resist.
It is also interesting to note the lack of activation of the ACC. The ACC is important for error detection and error correction, and thus reinforcement learning (i.e. learning from ones mistakes). This could explain, at least to some degree, why people with ADHD are often disturbing, since they would not have learned to control themselves despite being scolded; perhaps more obviously, though, it may help explain the aggressive tendencies, usually socially related, of individuals with CD in a similar, but more pronounced, way it explains the ADHD disruptive behavior.

References
Gatzke-Kopp, Lisa et al. Neurological Correlates of Reward Responding in Adolescents With and Without Externalizing Behavior Disorders. Journal of Abnormal Psychology. Vol. 18 N.1(2009)

Wikipedia. "File:Dopamineseratonin.gif". http://en.wikipedia.org/wiki/File:Dopamineseratonin.gif

Wikipedia. "Methylphenidate". http://en.wikipedia.org/wiki/Methylphenidate

American Psychiatric Association: Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision. Washington, DC, American Psychiatric Association, 2000.