ADHD and the brain: why looking at the left and right hemispheres may reveal something new. 

When scientists study the brains of people with ADHD, they often begin by asking a familiar question: how does the ADHD brain differ from the brain of someone without the diagnosis? Over the years, researchers have examined brain volume, cortical thickness, functional connectivity and the organisation of white matter, searching for patterns that might help explain difficulties with attention, inhibition and executive functioning.

Yet one potentially important aspect of brain organisation receives less attention in discussions of ADHD: the relationship between the two hemispheres.

The human brain is not perfectly symmetrical, nor is it supposed to be. Differences between the left and right hemispheres are a normal feature of its organisation. Some cognitive functions show stronger specialisation in one hemisphere, while others depend on coordinated activity across both sides. These asymmetries are not simply anatomical curiosities. They are part of how neural systems are organised.

A new study published in BMC Psychiatry on 12 October 2026 suggests that examining these left–right differences in the brain's white matter may reveal patterns associated with adult ADHD that are not immediately apparent when researchers look at the two hemispheres separately.

The findings are preliminary, but they offer an interesting perspective on a familiar problem in ADHD neuroscience: perhaps understanding the condition requires us to pay closer attention not only to individual brain structures, but also to how those structures are organised in relation to one another.

Looking beyond the usual MRI measures

The researchers examined 54 adults with ADHD and 48 adults without ADHD using diffusion-weighted magnetic resonance imaging. Unlike conventional structural MRI, which provides detailed images of brain anatomy, diffusion MRI measures the movement of water molecules through tissue. Because water diffusion is influenced by the organisation of neural fibres and their surrounding environment, it can provide indirect information about white-matter microstructure.

The study used a technique known as neurite orientation dispersion and density imaging, or NODDI. This approach attempts to separate different components of the diffusion signal, producing estimates related to neurite density, fibre orientation and the fraction of relatively unrestricted water within tissue.

These distinctions matter because conventional diffusion measures can reflect several underlying tissue properties at once. A difference in a diffusion MRI signal does not necessarily tell us which biological feature is responsible.

NODDI offers a more detailed model, although its measures remain indirect estimates rather than direct observations of individual axons, myelin or cellular pathology.

Instead of comparing only the absolute values of these measures, the researchers calculated lateralisation indices, allowing them to examine whether the balance between corresponding white-matter regions in the left and right hemispheres differed between groups.

This is a subtle but important change in perspective. Two groups might show relatively modest differences in a particular brain region when each hemisphere is examined independently, yet differ in the relationship between the two sides.

What the researchers found

Two findings stood out.

First, adults with ADHD showed greater leftward lateralisation of the free-water fraction in the sagittal stratum, a white-matter region containing pathways connecting posterior cortical areas with other parts of the brain. The group difference was statistically significant, and the degree of lateralisation was positively associated with ADHD symptom scores.

Second, the ADHD group showed greater rightward lateralisation of the neurite density index in the cingulum, a white-matter pathway associated with networks involved in cognitive and emotional processing.

Interestingly, this second measure was associated with lower depressive symptom scores rather than greater ADHD symptom severity.

The distinction is important. The two findings involved different anatomical regions, different diffusion-derived measures and different clinical associations. They should not be treated as evidence of one generalised abnormality affecting the entire ADHD brain.

Nor do they mean that the left hemisphere is overactive, the right hemisphere is underactive or that ADHD can be explained by an imbalance between the two sides.

The researchers identified differences in specific measures of white-matter microstructure. That is a much narrower and more scientifically meaningful conclusion.

Why white matter deserves attention

Much of the popular discussion about ADHD focuses on particular brain regions, especially those involved in attention, motivation and executive control. But cognitive functioning depends on communication between regions, not simply on the activity of individual structures.

White matter contains the long-range fibre pathways that support this communication. Its organisation influences how signals can travel through distributed neural systems.

This is particularly relevant to ADHD because many of the difficulties associated with the condition involve coordinating multiple processes over time. Maintaining a goal, resisting competing information, updating working memory and adjusting behaviour when circumstances change all require interactions between neural systems.

However, it would be premature to assume that the white-matter differences identified in this study directly explain any particular executive-function difficulty.

The researchers did not demonstrate that these anatomical measures caused problems with attention or behaviour. The observed associations with symptom scores are interesting, but they do not establish a mechanism.

The more defensible interpretation is that patterns of hemispheric white-matter organisation may represent another dimension of variation worth investigating in adult ADHD.

The problem with searching for one ADHD brain

One of the continuing challenges in ADHD neuroscience is the considerable variation between individuals who share the same diagnosis.

Some adults primarily experience difficulties with sustained attention and organisation. Others struggle more with impulsivity, emotional regulation, task initiation or managing competing demands. Cognitive performance also varies across situations, and the same person may function very differently depending on context, fatigue, motivation and environmental demands.

It would be surprising if all of this variation could be reduced to one consistent anatomical difference.

This is one reason I find studies examining more specific features of brain organisation particularly interesting. They move us away from the expectation that there must be one clearly identifiable ADHD brain and towards questions about which neural characteristics are associated with particular dimensions of functioning.

The new study does not resolve that problem, but it adds another potentially useful way of examining it.

Instead of asking only whether a particular pathway differs between people with and without ADHD, we can also ask whether its organisation across the two hemispheres differs, and whether those differences relate to particular symptoms or cognitive characteristics.

Such approaches may eventually contribute to a more nuanced understanding of the neurobiological diversity found within ADHD.

What the findings do not tell us

As with many neuroimaging studies, the limitations are as important as the findings themselves.

The sample was relatively small, involving just over 100 participants. Although the researchers identified statistically significant group differences, larger independent studies are needed to establish whether these patterns are reproducible.

The study was also cross-sectional. It cannot tell us whether the observed white-matter characteristics preceded the development of ADHD symptoms, emerged alongside them or were influenced by other factors over time.

The biological interpretation of diffusion MRI measures also requires caution. An increased free-water fraction does not, by itself, demonstrate inflammation, tissue damage or any other specific pathological process. Similarly, a neurite density index is not a direct count of neural fibres.

Importantly, the findings do not provide a diagnostic test for ADHD. Group-level differences in neuroimaging measures cannot automatically be used to identify the condition in an individual person.

That distinction is particularly important when discussing neuroscience with the wider public. A statistically significant difference between groups may advance scientific understanding without being sufficiently specific, reliable or clinically meaningful for diagnosis.

Why this study is worth following

What makes this research interesting is not the possibility that scientists have finally located the anatomical signature of ADHD. The evidence does not support such a conclusion.

Its contribution is methodological and conceptual.

By examining hemispheric asymmetry using more detailed diffusion MRI modelling, the researchers have identified differences that invite further investigation. Future studies could explore whether these patterns replicate in larger samples, how they relate to cognitive performance and whether they change across development or adulthood.

It would also be useful to examine how structural lateralisation relates to functional brain activity. White-matter organisation tells us something about the architecture supporting neural communication, but it does not directly reveal how those networks operate during attention, working memory or real-world tasks.

For me, this is one of the most interesting directions in ADHD neuroscience: moving beyond isolated group differences towards understanding how structural organisation, functional activity and everyday behaviour relate to one another.

We are still a considerable distance from being able to explain an individual's experience of ADHD from a brain scan.

But studies like this remind us that the organisation of the brain may be just as informative as the characteristics of its individual parts.

And sometimes, understanding a difference requires looking not only at what is happening on one side of the brain, but at the relationship between both.

Reference

Li, J., Li, X., Shen, Y., et al. (2026). Disrupted white-matter lateralization in adult ADHD: dissociable free-water and neurite density signatures revealed by NODDI. BMC Psychiatry. https://doi.org/10.1186/s12888-026-08747-x