Adults with ADHD showed stronger long-lasting patterns in their brain activity than neurotypical adults, and these patterns were linked to the severity of ADHD symptoms across both groups, according to a study published in Translational Psychiatry. The researchers suggest the findings are consistent with the idea that ADHD symptoms reflect individual differences in where each person’s brain operates between order and disorder.

ADHD is associated with difficulties such as inattention, impulsivity and hyperactivity, but symptoms vary widely from one person to another. Researchers have studied brain activity in people with ADHD, yet less is known about how brain patterns relate to differences in symptom severity across both diagnosed and non-diagnosed adults.

The study focused on a framework known as brain criticality, which describes how brain activity may shift between states that are too uncoordinated and states that are overly synchronized. The framework proposes that the brain functions best near the transition point between these states, where incoming activity is neither amplified nor dampened. The balance between excitation and inhibition in the brain is thought to determine where a person’s brain operates, and because ADHD has been linked to greater excitability, the researchers hypothesized that ADHD symptoms would reflect a shift toward the overly synchronized side.

To test this idea, Hamed Haque of the University of Helsinki and colleagues measured long-range temporal correlations, a feature of brain signals in which fluctuations in the strength of brain rhythms at one moment are related to fluctuations seconds or even minutes later. These correlations are strongest near the critical transition point, so researchers use them as an indirect marker of where the brain is operating.

The team studied 70 adults: 36 neurotypical participants (average age of 34 years; 13 women) and 34 adults with a prior ADHD diagnosis (average age of 37 years; 19 women). Participants completed a resting brain recording and two attention tasks while their brain activity was measured using magnetoencephalography, a method that detects magnetic signals produced by brain activity. In one task, they tried to detect faint images, and in the other, they responded quickly to one type of stimulus while withholding responses to another.

Participants also completed questionnaires measuring ADHD symptoms and impulsivity. Three neurotypical participants scored above a screening threshold for ADHD and were excluded from the group comparisons, though they were included in the analyses linking brain activity to symptoms. The researchers excluded people with other psychiatric or neurological conditions, and participants with ADHD stopped taking their ADHD medication 24 hours before testing.

On the tasks themselves, the two groups performed similarly in most respects. However, participants with ADHD more often responded when they should have held back, a pattern seen in earlier studies. The study found no group difference in reaction-time variability, despite this being a measure often associated with ADHD. The authors suggest that the response task may not have been demanding enough to reveal differences, or that the experience of being recorded in the scanner may have raised arousal levels in both groups.

In the brain recordings, the ADHD group showed stronger long-range temporal correlations than the neurotypical group in slower alpha-band rhythms (8 to 13 cycles per second) during the detection task and in faster gamma-band rhythms (35 to 65 cycles per second) during the response task, with moderate effect sizes. These differences were not explained by age or by differences in the overall strength of brain rhythms. The groups also differed in how their brain activity shifted from rest to task: the neurotypical group showed a widespread decrease in alpha-band correlations during the detection task, a change that was weak in the ADHD group, while the ADHD group showed a robust increase in gamma-band correlations during the response task.

When the researchers examined symptom scores across all participants, the relationship with brain activity depended on the frequency range and the task. In the gamma range, stronger long-range temporal correlations accompanied higher symptom scores, most clearly during the response task. In the beta range (22 to 24 cycles per second), the pattern was curved: the strongest correlations appeared among participants with moderate symptom scores, while those with lower or higher scores showed weaker correlations.

The authors interpret these patterns as reflecting differences in each person’s position within the brain’s operating range. Small increases in excitability might move the brain closer to the critical point, where long-range correlations peak, while larger increases might push it past that point toward overly synchronized activity, which could explain why the most severe symptoms were linked to weaker beta-band correlations. The researchers also speculate that moderate ADHD traits might sometimes be beneficial, consistent with research linking ADHD to hyperfocus and novelty-seeking. The results did not show a single brain pattern that neatly separates everyone with ADHD from everyone without it, as long-range temporal correlations varied widely between individuals.

Haque and colleagues concluded: “Our study thus proposes a novel complex systems-level framework to explain the emergence of ADHD symptoms and their continuum across the population.”

The study contributes to the scientific understanding of the brain dynamics underlying ADHD symptoms. However, it was correlational, so it cannot show that these brain patterns cause ADHD symptoms. It also involved a modest number of adults and excluded people with other psychiatric or neurological conditions, so it may not represent the full range of people with ADHD.

The researchers note that other mechanisms could also explain the results, and long-range temporal correlations can occur in systems that are not near a critical point, so the criticality interpretation remains indirect. The findings also varied by task and frequency range, and an earlier study using resting brain recordings found no differences in long-range temporal correlations in adults with ADHD. Finally, participants with ADHD paused their medication for 24 hours before testing, so the results may not reflect their usual functioning.

The study, “ADHD symptoms across the population are explained by individual variability in brain criticality,” was authored by Hamed Haque, Jonni Hirvonen, Sheng H. Wang, Jaana Simola, Isabel Morales-Muñoz, Benjamin U. Cowley, J. Matias Palva and Satu Palva.