ADHD and autism: New insights into their unique neural profiles

Attention deficit hyperactivity disorder (ADHD) and autism spectrum disorder (ASD) are two of the most prevalent neurodevelopmental disorders. Despite their distinct diagnostic criteria, there is a notable clinical and genetic overlap between the two disorders.

A new meta-analysis, published in The American Journal of Psychiatry, sought to investigate the neural correlates underlying these overlaps and distinctions by examining 243 task-based functional MRI (fMRI) studies. The findings reveal that while ADHD and ASD share some brain activity patterns, the unique differences in brain function for each disorder are much more significant. This suggests that ADHD and ASD should be considered distinct conditions, as their brain activity patterns are more different than similar.

The motivation for this study stems from the observed clinical and genetic overlap between ADHD and ASD. ADHD is characterized by persistent patterns of inattention, hyperactivity, and impulsivity that interfere with daily functioning. ASD, on the other hand, is marked by difficulties in social communication and interaction, along with restricted interests and repetitive behaviors.

Despite their distinct diagnostic criteria, individuals with ADHD often exhibit symptoms typically associated with ASD and vice versa. Additionally, genetic studies have revealed shared genetic factors between the two disorders, further blurring the lines between them.

Previous research has attempted to understand these overlaps by using task-based functional MRI (fMRI) studies to identify the neural correlates of ADHD and ASD symptoms. However, these studies often used specific tasks designed for each disorder, which could introduce bias and limit the generalizability of the findings. The researchers wanted to overcome these limitations and gain a clearer picture of the neural mechanisms underlying ADHD and ASD by conducting a meta-analysis.

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A meta-analysis is a statistical technique that combines the results of multiple scientific studies to derive a more comprehensive understanding of a particular research question. This method allows researchers to pool data from various individual studies, enhancing the overall statistical power and reliability of the findings. By aggregating data, a meta-analysis can identify patterns, trends, and effects that might not be apparent in individual studies due to limited sample sizes or varying methodologies.

The meta-analysis included data from 243 original task-based fMRI studies that involved either individuals with ADHD, individuals with ASD, or both, alongside typically developing controls. The studies were selected through a rigorous search of multiple databases, including PubMed and Web of Knowledge, and were screened based on strict inclusion and exclusion criteria.

The final sample consisted of 3,084 participants with ADHD, 2,654 participants with ASD, and 6,795 control subjects. The studies used a variety of neuropsychological tasks, such as go/no-go and n-back tasks for cognitive control, as well as tasks focusing on social processes, reward responsiveness, and attention.

The results highlighted the existence of both shared and disorder-specific neural activations in ADHD and ASD. Shared activations included greater activation in the right-lateralized lingual gyrus and rectal gyrus, as well as lower activation in the left middle frontal gyrus and superior temporal gyrus. These shared activations suggest some common neural pathways involved in the cognitive and behavioral symptoms of both disorders.

However, disorder-specific activations were more prominent. For ASD, greater-than-typical activations were observed in the left middle temporal gyrus, inferior parietal lobule, right hippocampus, and left putamen. Lower activations were noted in the left middle frontal gyrus, right middle temporal gyrus, left amygdala, and right hippocampus. These findings indicate that ASD is associated with specific neural dysfunctions in regions related to social processes, cognitive flexibility, and emotional processing.

For ADHD, greater-than-typical activations were found in the right insula, posterior cingulate cortex, right amygdala, and putamen. Lower activations were seen in the right middle temporal gyrus, left inferior frontal gyrus, right globus pallidus, and left thalamus. These results suggest that ADHD involves distinct neural abnormalities in areas related to attention, inhibition, and reward processing.

In an editorial about the study, Philip Shaw, an Earl Stadtman Senior Investigator at the Neurobehavioral Clinical Research Section of the National Human Genome Research Institute, wrote that the findings highlight the need for more fMRI studies where individuals with ADHD and ASD perform the same tasks. By conducting such studies, researchers can obtain clearer and more consistent data on the unique and shared neural features of these conditions.

“As the authors stress, there are only a handful of fMRI studies that include individuals with ADHD and individuals with ASD performing the same task. Although these head-to-head-studies have also found that diagnostic differences exceed similarities, the brain regions identified did not overlap with those emerging from the meta-analysis. To resolve this discrepancy, we need more fMRI studies where individuals with ADHD, ASD, or both diagnoses perform the same task,” Shaw explained.

“Tamon et al. found largely distinct neural landscapes in ADHD and ASD, suggesting we should split apart rather than lump together these conditions. A third option is to collect more data. Specifically, by collecting data from a common core set of tasks transdiagnostically, we could obtain the large data sets needed to capture fully the brain’s functional architecture in these complex neurodevelopmental conditions.”

The study, “Shared and Specific Neural Correlates of Attention Deficit Hyperactivity Disorder and Autism Spectrum Disorder: A Meta-Analysis of 243 Task-Based Functional MRI Studies,” was authored by Hiroki Tamon, Junya Fujino, Takashi Itahashi, Lennart Frahm, Valeria Parlatini, Yuta Y. Aoki, Francisco Xavier Castellanos, Simon B. Eickhoff, and Samuele Cortese.