Brain Wiring Goes Off-Balance in Adult ADHD, Advanced MRI Reveals

Attention-deficit/hyperactivity disorder has long been framed as a condition of childhood, but clinicians now know it persists into adulthood for millions of people worldwide. What has remained stubbornly elusive is a clear picture of what, precisely, differs inside the adult ADHD brain. A new study published in BMC Psychiatry by Jinguang Li, Xiaosong Li, Yuqiong He, Jianjun Ou and colleagues at the Second Xiangya Hospital of Central South University in Changsha, China, offers a striking clue: the disorder appears to be accompanied by a breakdown in the normal left-right asymmetry of the brain’s white matter, the insulated cabling that carries signals between distant regions. Using an advanced diffusion MRI technique called neurite orientation dispersion and density imaging, or NODDI, the team found that adults with ADHD show abnormal lateralization in two specific fiber tracts, each with its own distinct microstructural signature.

The concept of lateralization is central to the finding. In healthy brains, many structures are deliberately asymmetric: language leans left, spatial attention leans right, and the underlying wiring follows suit. These left-right differences are not quirks but functional design features, allowing specialized processing to occur on each side of the brain. When that asymmetry is reduced, reversed, or exaggerated, the efficient division of labor between hemispheres may be compromised. Previous research on ADHD has documented widespread white-matter microstructural abnormalities, but most studies compared overall measures between groups without asking whether the normal pattern of hemispheric asymmetry itself is disrupted. The new work addresses that gap directly by computing lateralization indices for every major white-matter tract.

Technically, the study’s power comes from NODDI’s ability to decompose the diffusion signal in ways that older methods cannot. Conventional diffusion tensor imaging, or DTI, summarizes water movement with metrics such as fractional anisotropy, which conflates many different biological changes: inflammation, demyelination, axonal loss, and swelling can all lower the same number. NODDI instead fits a biophysical model that separates the signal into distinct compartments. It estimates the free water fraction, the proportion of the signal attributable to unconstrained water moving in the extracellular space, which often rises with edema, inflammation, or widening of the spaces around fibers. It also estimates the neurite density index, a measure of how tightly packed the axons and dendrites are within a voxel, and the orientation dispersion index, which captures how fanned out or aligned those fibers are. This separation allowed the researchers to ask not just whether asymmetry is abnormal, but what kind of tissue change drives the abnormality.

The study enrolled 54 adults with ADHD and 48 healthy controls, all of whom underwent clinical assessment and diffusion-weighted MRI. The clinical battery included the Wender Utah Rating Scale, which probes childhood ADHD symptoms retrospectively, the Adult ADHD Self-Report Scale for current symptoms, the Patient Health Questionnaire-9 for depression, the Generalized Anxiety Disorder-7 scale for anxiety, and the Athens Insomnia Scale for sleep difficulties. From the imaging data, the team computed NODDI metrics across the white matter and then derived a lateralization index for each tract, calculated as the difference between hemispheres normalized by their sum. Group differences were tested with independent-samples t-tests and multivariate analyses, and the researchers then examined how any lateralization differences related to symptom scores through correlation and regression analyses, adjusting for age and sex.

Two findings stood out. First, adults with ADHD showed significantly increased leftward lateralization of the free water fraction in the sagittal stratum, a broad sheet of association fibers running along the lateral wall of each hemisphere that connects occipital, parietal, and temporal cortex with frontal regions and carries some of the brain’s most important long-range traffic, including pathways critical for attention and visual processing. The excess leftward asymmetry was statistically robust, with a p-value of 0.002. Second, the patients showed increased rightward lateralization of the neurite density index in the cingulum cingulate gyrus portion, a curved bundle arcing over the corpus callosum that links the cingulate cortex, a hub of attentional control and emotional regulation, with other limbic and association areas. That asymmetry difference reached significance at p = 0.004. Crucially, the two abnormalities pointed in opposite directions and involved different tissue compartments, suggesting they are dissociable processes rather than a single nonspecific change.

The clinical correlations sharpened the story. Greater leftward free-water lateralization in the sagittal stratum was positively associated with ADHD symptom severity, as measured by both the Wender Utah Rating Scale and the Adult ADHD Self-Report Scale, and the association remained significant after adjustment for age and sex. In other words, the more the free-water asymmetry deviated toward the left hemisphere, the more severe the participants’ ADHD symptoms, both those recalled from childhood and those currently self-reported. This is the kind of dose-response relationship that lends biological plausibility to an imaging finding, because it ties a continuous microstructural measure to a continuous clinical dimension rather than merely separating two diagnostic groups.

The cingulum finding told a different and somewhat counterintuitive tale. Greater rightward lateralization of neurite density in the cingulum was associated with lower scores on the Patient Health Questionnaire-9, meaning that participants whose cingulum asymmetry deviated further from the typical pattern reported fewer depressive symptoms. The association held in both correlation and regression analyses. The authors interpret the two tracts as carrying dissociable signatures: the sagittal stratum free-water change tracks the core ADHD symptomatology, while the cingulum neurite density change appears more closely related to the depressive dimension that frequently co-occurs with adult ADHD. If replicated, this would suggest that NODDI lateralization indices could eventually help disentangle which aspects of a patient’s clinical picture map onto which white-matter systems, a level of biological interpretability that composite DTI metrics have rarely achieved.

Mechanistically, the free-water result is especially intriguing. Elevated free water fraction is often read as a marker of extracellular fluid expansion, which can accompany neuroinflammation, blood-brain barrier leakage, or simply enlarged perivascular and interstitial spaces. An exaggerated leftward free-water asymmetry in the sagittal stratum could therefore indicate a hemisphere-specific vulnerability of this pathway in ADHD, perhaps reflecting chronic low-grade inflammatory processes or developmental differences in how the tract matures. The rightward neurite density shift in the cingulum, by contrast, speaks to the packing of axons themselves, hinting at altered arborization or myelination on the right side of a tract that normally supports attention and mood regulation. Because NODDI separates these compartments, the study avoids the interpretive ambiguity that has plagued fractional anisotropy findings in psychiatry for two decades.

The researchers are careful about the limits of their work. The sample, while well characterized, is modest, and the groups were not perfectly balanced in size. The authors explicitly call for replication in larger and better-balanced cohorts before the lateralization indices can be considered established biomarkers. Cross-sectional data also cannot determine whether the asymmetric changes are a cause of ADHD symptoms, a consequence of living with the disorder, or a shared downstream marker of some earlier developmental event. Longitudinal designs, and ideally studies spanning adolescence into adulthood, would help resolve the direction of the relationship. It is also worth noting that the clinical associations rest on self-report and retrospective instruments, which, although validated, carry inherent measurement noise.

Even with those caveats, the study marks a meaningful step forward in the neuroimaging of adult ADHD. By shifting the question from whether white matter differs to whether the brain’s normal left-right wiring plan is disrupted, and by using a model that separates free water from neurite density, the researchers have produced findings that are both biologically specific and clinically anchored. The sagittal stratum and the cingulum are exactly the kinds of tracts one would expect to be implicated in a disorder of attention, and the dissociation between an ADHD-linked free-water signature and a mood-linked neurite density signature hints at a future in which diffusion MRI could help clinicians parse the heterogeneous presentations of adult ADHD. For now, the message is one of cautious excitement: the brain’s asymmetry, long studied in gray matter, is proving to be an equally rich frontier in its wiring, and ADHD may be one of the clearest windows into what happens when that asymmetry goes awry.

Subject of Research: White-matter lateralization abnormalities in adult ADHD measured with NODDI diffusion MRI

Article Title: Disrupted white-matter lateralization in adult ADHD: dissociable free-water and neurite density signatures revealed by NODDI

Article References: Li, J., Li, X., Shen, Y., Gao, B., Hou, Y., Lin, L., Wang, L., Guo, X., Zhou, C., Liu, Z., Cui, X., He, Y., & Ou, J. (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

Image Credits: AI Generated

DOI: 10.1186/s12888-026-08747-x

Keywords: ADHD, adult ADHD, white matter, lateralization, NODDI, diffusion MRI, free water fraction, neurite density index, sagittal stratum, cingulum, BMC Psychiatry, neuroimaging