Biology in the News Explained

Brain development and ADHD

Here’s a not-too-surprising headline:

Brain development linked to symptoms of ADHD.”

A new paper has taken brain images from 26 four-five year-olds, thirteen diagnosed with ADHD and thirteen undiagnosed for controls. The study confirms previous work showing a correlation between ADHD and a smaller (than controls) brain structure called the caudate nucleus (highlighted on the image), one of the basal “ganglia” of the brain.

Caudate nucleus of the brain

The basal ganglia (or basal nuclei) are the parts of the brain involved with voluntary motion and some forms procedural learning (development of a motor skill through practice, such as playing a musical instrument). The caudate nucleus specifically functions in learning and memory; it tells the cortex (the area of our brain where higher reasoning occurs) to do something based on current conditions. Importantly, the caudate nucleus controls motor skills partly through inhibition of particular behaviors, and disinhibition of others; an overactive caudate nucleus may be implicated in obsessive-compulsive disorder.

Smaller caudate nuclei had been documented before in older children with ADHD, but not before in children so young. The authors point out that previous studies have not been able to sort out what comes first: changes in brain structure or the behavior, which is part of the motivation of looking at younger children. The authors of this study steer clear of this question, preferring to focus on how this information may be used for preventive measures:

…the current findings represent the initial phase of an ongoing longitudinal study of cortical and subcortical development in preschool children who present with symptoms of ADHD. This type of design should allow for characterization of the early developmental changes in preschoolers that contribute to the behavioral phenotype, and ultimately the formal diagnosis of ADHD, as well as those factors (anatomic, cognitive, behavioral) that are associated with ‘‘protection from’’ the disorder.

The chicken-and-egg question remains: is this anomaly genetic, or is there an early feedback loop which causes this to happen during development? As always, the answer is almost certainly in between: an interaction of genes and environment. Thus, their focus on how perhaps this developmental trajectory may be altered is reasonable.

A weakness of the study was that the difference in the right caudate nuclei between ADHD and non-ADHD children was fairly small. The difference was considered statistically significant in the study at p=0.027, using the standard cutoff value of p=0.05, but there were 22 comparisons made of different brain structures; a Bonferroni correction would have put the conservative statistical cutoff of p=0.05/22=0.0023, which would have rendered their result nonsignificant. It is obvious from this figure from the paper (below) that there is a large overlap in right caudate volume of ADHD children versus controls. What the authors describe as a “strong relationship” is at best debatable.

However, the study had the advantage of being small and controlled, and thus any differences found are more likely to be real. Given the other studies that have implicated the caudate nucleus in ADHD, and also given what we know about the function of the caudate nucleus in inhibiting/disinhibiting behavior, the result may be cautiously accepted.

The main problem with trying to determine the source of such conditions as ADHD is that we know so little about how the brain works to begin with. But MRI studies such as these are providing a wealth of fascinating information.

Reference

Mahone, E. M., Crocetti, D., Ranta, M.E., Gaddis, A., Cataldo, M., Slifer, K. J., Denckla, M. B. and Mostofsky, S.H. (2011). A Preliminary Neuroimaging Study of Preschool Children with ADHD, The Clinical Neuropsychologist, First published on: 09 June 2011 (iFirst)

Image generated by Life Science Databases(LSDB). [CC-BY-SA-2.1-jp (www.creativecommons.org/licenses/by-sa/2.1/jp/deed.en)], via Wikimedia

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  1. ADHD Awareness Week: Oct. 16-22 « Sky Dancing - [...] another blog entry on a different study of brain development in children with ADHD. This study found that children ...

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