Functional Disconnection Identified Between Key Brain Areas in Children With ADHD

Pam Harrison

January 21, 2010

January 21, 2010 — Children diagnosed as having attention-deficit/hyperactivity disorder (ADHD) have a specific deficit in "top-down" attentional control that is likely a sign of a functional disconnection between the brain's frontal and occipital cortex, according to a study published online January 8 in Biological Psychiatry.

Ali Mazaheri, PhD, University of California, Davis, and colleagues found that both typically developing children and children with ADHD benefit from being shown cues signaling the modality of upcoming stimuli (either visual or auditory) because reaction times were significantly faster and discrimination performance more accurate for validly cued visual and auditory targets than for invalidly cued targets.

However, independent of cue-target validity, typically developing children were faster in the target discrimination task. In terms of brain activity, it was found that the cues signaling upcoming visual stimuli induced a decrease in posterior electroencephalographic (EEG) alpha activity in typically developing children but not in children with ADHD.

Moreover, this decrease in posterior alpha activity correlated with the behavioral benefits imparted by the cues in typically developing children but not in ADHD children.

Given that posterior occipital alpha activity was also anticorrelated with frontal theta activity in the typical children, the study authors concluded that a functional disconnection between the frontal and the occipital cortex could be related to the attention deficits exhibited by ADHD patients.

"In order for the brain to operate, you have to be able to shut down certain areas and increase activity in other areas, and the way the brain disengages is through alpha activity," Dr. Mazaheri told Medscape Psychiatry. "In this particular study, we expected alpha activity to do down in the visual cortex during the attentional task, and in typically developing children, it did go down, but in ADHD children, it did not. This suggests that the brains of children with ADHD apparently prepare to attend to upcoming stimuli differently than typically developing children."

Brain Regions Not Communicating

A total of 25 children between the ages of 8 and 12 years — 14 of whom had been diagnosed as having ADHD — underwent EEG monitoring while engaged in a simple attention test. The test consisted of being shown a red or blue image or hearing a high or low sound and then having to react by pressing a button.

Immediately before the test, children were shown either the letter V to alert them to the fact that the test would involve a visual cue or an inverted V (A) to alert them to the fact that they would be responding to an auditory cue. Not all visual or auditory cues were valid, however, because children could be shown a V when the test was not visual and vice versa.

As investigators explain, the pattern of brain activity observed in typically developing children — namely, a decrease in posterior alpha activity, which was anticorrelated with frontal theta activity — suggests that there is a form of functional connectivity between frontal brain systems involved in attentional control and perceptual systems in posterior cortical areas.

"That is, we interpret the anticorrelated theta and alpha activity in the typically developing children as an EEG signature of the top-down influence ... of attentional control systems onto perceptual structures ... which prepares the brain to selectively process the anticipated upcoming stimulus," the investigators write.

In contrast, children with ADHD could not fully use the same top-down attentional control to bias sensory processing, a deficit that was reflected in their slower overall reaction times, the absence of cue-induced posterior ECG alpha reductions, and the absence of the typical pattern of anticorrelation between posterior alpha and frontal theta activity.

"It appears that these 2 brain regions, which are important for attentional tasks, do not appear to be communicating with each other during the task in children with ADHD," said Dr. Mazaheri.

According to current models of how the brain allocates attention, signals from the frontal cortex — the V and A cues, for example — should alert other parts of the brain, including the visual processing areas at the back of the head, to prepare to pay attention.

A drop in alpha activity in the visual cortex would reflect this heightened state of readiness.

The study was funded by grants from the National Institutes of Mental Health, the Netherlands Organization for Scientific Research, the Perry Family Foundation, the Debber Family Foundations, and the Aristos Academy. The authors have disclosed no relevant financial relationships.

Biol Psychiatry. Published online January 8, 2010.

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