Attention deficit hyperactivity disorder (ADHD) is one of the most common neurodevelopmental disorders, affecting around 5% of children and adolescents worldwide. It is characterized by inattention, hyperactivity and impulsivity that interfere with daily functioning.
While the exact causes of ADHD are still unclear, research shows that there are differences in brain structure and function in people with ADHD compared to those without. Advances in neuroimaging techniques over the past few decades have allowed scientists to get a closer look at the ADHD brain and identify some of the key brain regions and networks involved.
In this article, we provide an overview of what we currently know about the neurobiology of ADHD and how ADHD brains differ from neurotypical brains in terms of structure, function and neurochemistry. We also discuss how neuroimaging is helping us better understand ADHD and develop more targeted treatments.
Structural Differences
Structural neuroimaging techniques like magnetic resonance imaging (MRI) have revealed several anatomical differences in the brains of people with ADHD compared to unaffected individuals.
Overall Brain Volume
One of the most consistent findings is that children and adolescents with ADHD tend to have a 3-5% smaller total brain volume on average. The reduced volume is thought to reflect a delay in brain maturation.
Frontal Lobe
Several studies have found decreased volume in frontal lobe regions like the prefrontal cortex in ADHD groups. The prefrontal cortex is involved in important executive functions like attention, planning, organization and impulse control. Its reduced volume may underlie some of the symptoms of ADHD.
Basal Ganglia
The basal ganglia, a group of structures linked to motivation, habitual behaviors and motor control, also show structural differences in ADHD. For example, children with ADHD tend to have smaller caudate nucleus and globus pallidus volumes.
Corpus Callosum
The corpus callosum connects the brain’s two hemispheres and facilitates communication between them. It appears to be smaller in cross-sectional area and less well-myelinated in those with ADHD.
Cerebellum
On average, people with ADHD have a smaller cerebellum than unaffected individuals. Since the cerebellum is involved in motor control, balance and coordination, this could impact hyperactivity.
Other Regions
Structural abnormalities have additionally been noted in the occipital lobes, parietal lobes, hippocampus, amygdala and other subcortical structures in ADHD groups. The changes are relatively subtle, about 3-8% smaller volume than in control groups.
Functional Differences
Beyond brain structure, neuroimaging has also revealed differences in brain functioning between ADHD and neurotypical groups during cognitive tasks.
Prefrontal Cortex
During tasks that require attention control or executive functions, those with ADHD often show decreased activation in prefrontal regions relative to controls. This suggests impairments in activating the prefrontal cortex to focus attention.
Brain Networks
Two large brain networks where people with ADHD exhibit altered patterns of activation are:
- The frontal-striatal network involved in executive functions, motivation and impulse control.
- The default mode network involved in daydreaming, self-reflection and mind wandering.
In ADHD, these two networks show abnormal interactions and failed suppression of the default mode network during tasks requiring focused attention.
Reward Processing
fMRI studies show those with ADHD have exaggerated activation in reward-related regions like the ventral striatum and prefrontal cortex in response to stimuli like food or gaming rewards. This may relate to their impulsivity symptoms.
Inhibition
Regions like the right inferior frontal cortex involved in inhibiting responses tend to be underactivated in ADHD groups during tasks that require inhibitory control like the Go/No-Go task.
Timing
People with ADHD exhibit more variable responses in motor and cognitive regions from trial to trial. Their sense of timing also differs from neurotypical controls.
Neurotransmitter Differences
ADHD brains additionally show differences in key neurotransmitters like dopamine and norepinephrine that modulate attention, motivation, arousal and impulsivity.
Dopamine
Dopamine signaling is impaired in multiple ways in ADHD:
- Lower density of dopamine receptors and transporters
- Altered dopamine release patterns
- Faster dopamine breakdown
These molecular differences lead to an overall hypodopaminergic state.
Norepinephrine
People with ADHD appear to have lower levels of norepinephrine and abnormalities in norepinephrine transporters. Since norepinephrine promotes alertness, this likely contributes to attention deficits.
Serotonin
Though less is known about serotonin’s role, some studies suggest serotonin signaling deficiencies may also play a part in ADHD.
Neuroimaging Evidence
Here is a summary table of the main structural and functional differences found in ADHD brains versus neurotypical brains:
| Brain Region | Structural Differences in ADHD | Functional Differences in ADHD |
|---|---|---|
| Prefrontal cortex | – Reduced volume in key regions like dorsolateral PFC | – Less activation during attention & executive function tasks |
| Frontal-striatal circuits | – Smaller caudate nucleus and globus pallidus volumes | – Altered connectivity patterns |
| Default mode network | – No major structural differences | – Failed suppression during tasks requiring focused attention |
| Reward regions (e.g. ventral striatum) | – No major structural differences | – Exaggerated responses to rewards |
| Inhibitory control regions (e.g. inferior frontal cortex) | – No major structural differences | – Reduced activation during response inhibition |
| Cerebellum | – Smaller volume | – Alterations in timing & coordination functions |
| Neurotransmitters | – Abnormalities in dopamine, norepinephrine & serotonin systems | – Imbalances in signaling patterns |
Implications
Understanding the neurobiology of ADHD has several key implications:
Better Diagnosis
Neuroimaging markers and patterns could one day act as more objective diagnostic measures alongside clinical evaluations.
Monitoring Treatment Effects
Scans could also monitor how medications and behavioral interventions impact brain structure and function in ADHD.
New Treatment Targets
The brain differences spotlight specific networks and chemical signaling deficiencies to target with new treatments.
Reduced Stigma
Visual evidence of brain differences may help reduce stigma and emphasize that ADHD is a legitimate neurodevelopmental disorder.
Earlier Intervention
If clear neurobiological markers can be identified, diagnosis and treatment could potentially start much earlier in childhood.
Future Research
While our understanding of the ADHD brain has advanced considerably, there is still much we don’t know. Areas for future neuroimaging research include:
- How the brain differences progress over time from childhood to adulthood.
- The interaction between genetic factors and brain structure/function.
- Differences between ADHD subtypes.
- Gender-specific patterns.
- The neural effects of common comorbid conditions like anxiety, depression and learning disabilities.
- Response to medications and behavioral treatments at the neural level.
Larger longitudinal studies tracking subjects’ brains from a young age will help clarify the neurodevelopmental timeline. Analyzing connections between imaging, genetics, cognitive testing and clinical symptoms may uncover new ADHD subtypes and personalized treatment approaches.
As neuroimaging methods keep evolving, we are gaining more insight than ever into the underlying biology of ADHD. While no single brain scan can diagnose ADHD yet, neuroimaging research is a pivotal part of solving the ADHD puzzle.
Conclusion
In summary, current research indicates ADHD brains differ from neurotypical brains in:
- Smaller total brain volume and reduced size in key regions like the prefrontal cortex, basal ganglia and cerebellum.
- Altered activation and connectivity patterns in large-scale brain networks involved in executive function, motivation, inhibition and attention.
- Imbalances in dopamine, norepinephrine and possibly serotonin neurotransmission.
Thanks to advances in neuroimaging technology, we are gaining a deeper understanding of the structural and functional brain signatures of ADHD. These insights are helping to improve diagnosis, monitor treatments, identify new therapy targets and reduce stigma. While more research is still needed, unveiling the neurobiology of ADHD brings us one step closer to better outcomes for the millions affected worldwide.