We don’t tend to look at autism and attention deficit hyperactivity disorder as having much in common, but new science offers a compelling reason to consider some of the striking similarities between the two disorders.
Autism Spectrum Disorder, or ASD, and ADHD are different in many respects, to be sure. However, they have several important things in common: Both are common neurodevelopmental disorders, they run in families, and they’re characterized by symptoms like difficulty paying attention and impaired social interactions.
And as scientists are increasingly finding, autism and ADHD actually occur together in a large percentage of patients ― something that’s rarely been recognized until recently. Despite the fact that anywhere from 30 to 80 percent of patients with autism also have ADHD, until two years ago the Diagnostic and Statistical Manual of Mental Disorders stated that a person couldn’t have both disorders at the same time.
Now, scientists have discovered more about the brain mechanisms underlying the interaction of these two disorders. In a new study published online on Nov. 2 in the journal PLOS One, a team of Dutch and Swiss researchers analyzed MRI scans from over 500 volunteers to identify the brain structures involved in the overlap of ADHD and autism. The findings revealed that autistic traits in individuals with ADHD could be predicted by the interaction of certain regions that play a role in the brain’s reward processing system.
The findings revealed that the ADHD group had higher overall scores of autism spectrum disorder and autistic traits than the control group.
“ADHD participants in our study had increased difficulty with social understanding and reduced interest in social contact,” Laurence O’Dwyer, a neuroscientist at Radboud University in the Netherlands and the study’s lead author, said. “They also showed increased levels of repetitive behavior, as well as greater resistance to change, which may translate as an above average preference for stability or constancy in the world around them.”
This aligns with what we know about the prevalence of autism-like traits among people who do not suffer from the disorder. Symptoms of autism are not only more common in people with ADHD, but also in creative people and scientists. Mild, sub-clinical ASD symptoms are also present across the general population, as the study’s authors note.
Abnormalities in reward processing ― the way that brain processes desire, potential rewards, and motivation ― are common to both autism and ADHD, and the new study revealed a closer look at how this plays out on a neurological level. The researchers found that one particular brain region, the caudate nucleus ― a mass of grey matter, which is involved in things like planning, directed movement and goal-directed behavior ― was able to predict the level of autistic traits in patients with ADHD.
But the caudate nucleus doesn’t act on its own to determine autistic traits. It engages in a complex interaction with another part of the brain, the globus pallidus, which is involved in functions such as the regulation of voluntary movement.
Both of these structures are located in a section of the brain called the striatum, which acts as a monitoring system for rewards and plays a critical role in planning, decision-making and motivation. Within this system, the caudate nucleus guides the selection of goals while the globus pallidus updates the reward value based on the outcome of an action.
Abnormal functioning of the reward circuits of the striatum function can result in a lowered motivation to respond to social input, such as the sound of someone’s voice or the sight of a frown or smile. This is commonly seen in autism and can also occur in cases of ADHD.
These findings offer compelling evidence that the overlap between autism and ADHD should be taken seriously if we want to better understand both disorders.
“The current findings highlight specific brain areas that are of critical importance in predicting autistic traits in people with ADHD,” O’Dwyer said. “We are beginning to understand some of the common biological pathways and specific anatomical differences in the brain where autism and ADHD overlap and this greater understanding will be essential for the future development of novel treatment approaches.”