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Patterns of Brain Activity in Children with ADHD
One important current theory about the biological basis for ADHD is that it results from underactivity in those brain regions that are most directly involved in behavioral inhibition (i.e. behavioral inhibition refers to the ability to refrain from acting immediately on behavior impulses - i.e. to "inhibit" one's behavior). This theory of ADHD has been discussed at great length and eloquence by Dr. Russell Barkley and a summary of Dr. Barkley's ideas were presented in a prior issue of ADHD RESEARCH UPDATE. The brain areas believed to be most important for behavioral inhibition are the regions of the frontal cortex.
Although there has been some support for this idea found in several studies in which brain activity in individuals with and without ADHD have been directly examined, this work has not yet been conducted in relatively young children. There has also been little work to compare these brain activation patterns in boys and girls with ADHD. Finally, some have questioned the findings of earlier studies by noting that many of the subjects with ADHD also had co-occurring behavior disorders like Oppositional Defiant Disorder (ODD) or Conduct Disorder (CD). These critics noted that any unusual patterns of brain activity that were found could just as likely be associated with behavior problems in general rather than to ADHD specifically.
A recently published study does a nice job of beginning to address some of these important issues (Baving, L., et al. (1999). Atypical frontal brain activation in ADHD: Preschool and elementary school boys and girls. Journal of the American Academy of Child and Adolescent Psychiatry, 38, 1363-1371. The neurobiology of ADHD is something that I do not know that much about, and there are technical details in this study that are well outside my own area of expertise. I know that many subscribers have a keen interest in this type of work, however, so I wanted to include this interesting study in the newsletter. This type of work is important in that it may ultimately be quite useful in developing more effective treatments for ADHD. It is also important, however, to note the limitations of such neurobiological research in regards to more immediate assessment and treatment needs. These limitations will be discussed below.
Participants in this study were 66 children who were 4.5 years old and 51 8-year-old children. Forty-seven of these children had been diagnostic with ADHD, Combined Type, using a structured diagnostic interview. The remaining 70 children had never displayed any type of disorder and were used as controls. Among the 4.5-year-olds with ADHD, 20 were boys and 8 were girls. Among the 8- year-olds, there were 12 boys with ADHD and 7 girls. About two/thirds of the children with ADHD were also diagnosed with ODD while the others did not have this additional behavior disorder.
All children participated in a non-invasive EEG procedure to assess their patterns of brain activity. None of the children were on stimulant medication at the time the EEG recordings were made, and only 3 of the children had ever received stimulants at all. (The study was conducted in Europe where the use of stimulant medication for treating ADHD is much less common. In fact, stimulant medication is not even available in a number of European countries.) In making these recordings, the researchers were specifically interested in the relative amounts of neural activity occurring in frontal brain area in the right vs. left hemispheres.
Results
The analysis of the results of these EEG recordings revealed clear and significant differences in brain activity between children with and without ADHD.
Boys with with ADHD showed significantly reduced neural activity in the right hemisphere compared to boys without ADHD. This was true for both 4.5 and 8-year-old boys. It was also true for boys who had ADHD alone, as well as for boys who had been diagnosed with both ADHD and ODD.
Interestingly, for girls the pattern was exactly reversed. That is, girls with ADHD showed significantly reduced neural activity in the left frontal region than girls without ADHD. In fact, brain activity in girls with ADHD was very similar to boys without ADHD while brain activity in boys with ADHD was very similar to girls without ADHD.
The reasons for these gender differences is apparently not well understood, although they do point towards the importance of including both boys and girls in studies of ADHD so that potentially important gender differences are not missed or obscured.
Implications
The authors suggest that the differential pattern of brain activity that they observed between children with and without ADHD suggests that "..ADHD is the final stage of a disorder in the development of regulation." In other words, they are hypothesizing that brain regions that are involved in the regulation of behavior and emotional arousal work differently in individuals with ADHD, and that this is the neurological basis of the condition.
Although I am not able to fully understand some of the nuances conveyed in this paper, the study seems important to me for several reasons.
First, it provides yet additional evidence that there really does appear to be a neurological basis to ADHD, at least in some children who receive this diagnosis. Because there are unfortunately so many people who continue to doubt the reality of ADHD, research that points to real differences in brain functioning in children and adults with ADHD is important in combating this unreasonable opinion. This study makes a useful contribution to this body of literature.
In fact, this study goes further in this regard than earlier work in this area in a variety of ways. First, significant differences in patterns of brain activity were found in children with ADHD as young as 4.5 - this is a younger age group than has been included in much other neurobiological work. This is important because some scientists have noted - with good cause - that finding such differences among adults with ADHD is hard to interpret. This is because differences in brain activity in adults may just as likely reflect a consequence of having ADHD as opposed to something that may have been influential in the development of the individual's ADHD to start with. This argument is more difficult to make when differences are found in such young children, although it still can not be entirely ruled out as a possibility.
This study is also important because almost none of the participants with ADHD had ever been treated with stimulant medication. Again, some have contended that differential brain activity observed in adults or teens who have received medication over extended periods may, perhaps, reflect the long-term consequences of taking the medication. That can not be used as a critique of these results - both because the children were so young and because none of the 4.5 year olds had ever received medication.
Finally, in this study, differential brain activity patterns was found for children with pure ADHD, as well as for children with both ADHD and ODD. Thus, it is hard to argue that the differences observed would be found in children with any type of behavior disorder, rather than being specific to ADHD. It would have been helpful, of course, if children with ODD but not ADHD had been included and no differences between them and control children on the EEG recordings were found. This would have made the case for the findings being specific to ADHD even stronger.
Despite all the strengths of the study noted above, it is important to consider these results in the context of the very real limitations of such work. With this study - as in all studies of this type that I have seen - there is a fair amount of overlap between brain functioning in children with and without ADHD. In other words, some of the children with ADHD showed patterns of brain activity in this study that were characteristic of non-ADHD children and vice versa. Thus, even though the group averages for children with ADHD differ from the group average for non-ADHD children, there is a fair amount of variability that is seen in individual children.
What does this really mean? First, even if it could be clearly established that the differences in brain functioning documented in this study were a "cause" of ADHD, this would only be true for some children with ADHD and not others. This is because - as discussed above - not all children with ADHD showed the characteristic pattern. Thus, at most, the authors may have identified a neurological basis of ADHD for some children but not others. Children can manifest the behavioral symptoms of ADHD regardless of whether or not they show the pattern of brain activity identified in this study, suggesting that that there are likely to be multiple causes of ADHD, as opposed to a single cause.
This is especially important when it comes to the utility of such neurological assessments in the evaluation/diagnosis of ADHD. A great hope - among both parents and professionals - when the initial studies documenting differences in brain functioning in individuals with ADHD were published was that the procedures used to document such differences could also be used to provide an "objective" basis for diagnosing ADHD.
The problem with this - and I think it is likely that this problem will always exist - is that the kind of variability discussed above suggests that these tests may not have tremendous utility for making individual diagnostic decisions. If a child is clearly showing all the behavioral manifestations of ADHD but not the pattern of brain activity thought to be diagnostic of ADHD, does that mean the child "does not really have the disorder"? Similarly, if a child does not show the behavioral symptoms but not the underlying pattern of brain functioning, does this child have ADHD even though the symptoms are not evident?
These are the kinds of issues that are very important to keep in mind when one see reports in the media about new studies demonstrating important differences in brain structure or function in people with ADHD. These differences do not apply to everyone who shows the symptoms of ADHD - there are probably multiple factors that are responsible for the development of ADHD, and multiple factors may be involved even for an individual child.
So, this work can be extremely valuable in helping us to better understand the biological underpinnings of ADHD in some individuals, and, ultimately, may progress to the point where it can help to inform both diagnosis and treatment. At this point, however, we still have a ways to go before getting there.
Note: This article originally appeared in Attention Research Update, an online newsletter written by Dr. David Rabiner, a Duke University psychologist and former member of CHADD's Professional Advisory Board. You can learn more about Attention Research Update and sign up for a free subscription at www.helpforadd.com.