New insights into their unique neural profiles

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

A new meta-analysis, published in American Journal of Psychiatrysought to investigate the neural correlates underlying these overlaps and distinctions by examining 243 task-oriented functional MRI (fMRI) studies. The results reveal that while ADHD and ASD share some patterns of brain activity, the unique differences in brain function in each disorder are much more significant. This suggests that ADHD and ASD should be considered separate conditions, as their patterns of brain activity 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, in contrast, is characterized by difficulties in social communication and interaction, as well as restricted interests and repetitive behaviors.

Despite their distinct diagnostic criteria, people with ADHD often have symptoms typically associated with ASD and vice versa. In addition, genetic studies have revealed common genetic factors for both 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 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 results. The researchers aimed to overcome these limitations and gain a clearer picture of the neural mechanisms underlying ADHD and ASD by conducting a meta-analysis.

A meta-analysis is a statistical technique that combines the results of multiple scientific studies to obtain a more complete understanding of a particular research question. This method allows researchers to pool data from multiple individual studies, thereby improving the overall statistical power and reliability of the results. 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 different methodologies.

The meta-analysis included data from 243 original task-based fMRI studies involving either individuals with ADHD, individuals with ASD, or both, as well as typically developing controls. Studies were selected using a rigorous search of multiple databases, including PubMed and Web of Knowledge, and were reviewed based on strict inclusion and exclusion criteria.

The final sample included 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 common 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 the existence of common neural pathways involved in the cognitive and behavioral symptoms of both disorders.

However, disorder-specific activations were greater. For ASD, higher than normal 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 results indicate that ASD is associated with specific neural dysfunctions in regions related to social processes, cognitive flexibility, and emotional processing.

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

In an editorial on the study, Philip Shaw, a senior investigator in the National Human Genome Research Institute’s Neurobehavioral Clinical Research Division, wrote that the findings underscore the need for more fMRI studies in which people 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 point out, there are only a few fMRI studies that include individuals with ADHD and individuals with ASD performing the same task. Although these comparative studies also found that diagnostic differences outweigh similarities, the brain regions identified do not overlap with those from the meta-analysis. To resolve this discrepancy, we need more fMRI studies in which 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 that we should separate rather than lump these conditions together. A third option is to collect more data. Specifically, by collecting data from a common set of core tasks in a transdiagnostic manner, we could obtain the large datasets needed to fully capture the functional architecture of the brain 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.