Neurodiversity advocates have long argued that autistic brains are differently wired. Now, new evidence shows they are right. Researchers have found atypical numbers of brain cells in two key areas of the brain in autistic children, according to a new study published in the journal Autism Research. These findings contribute to our understanding of how the brain works in autism, continuing a shift away from seeing autism as a group of undesirable behaviors to a uniquely structured brain that provides a unique experience. The researchers hope that investigations like theirs could generate new diagnostic insights and therapies for autism.
Autistic brains show differences in neuron density
Past research into brain structure in autism has faced limitations because it relied on post-mortem investigations. In contrast, for this study researchers at the Del Monte Institute for Neuroscience at the University of Rochester made use of recent advances in processing magnetic resonance diffusion weighted images (DWI). By drawing on the DWI imaging data from the brains of children ages 9-11 collected by the Adolescent Brain Cognitive Development (ABCD®) study, the researchers were able to gain a more accurate view of the brain. They compared images from 142 children with an autism diagnosis to those of 8971 controls in order to study neuron architecture in living children.
The researchers found that certain regions of the cerebral cortex showed lower neuron density in autistic children than in controls. That means there were fewer brain cells in regions associated with memory, learning, reasoning, and problem-solving.
On the other hand, the study found that the brains of autistic children had higher neuron density in the amygdala. A part of the brain’s limbic system, the amygdala is a small but critical structure with multiple functions. The amygdala processes emotions and connects those emotions to tasks like learning, memory formation, and sensory processing. It’s also the home of the fight or flight response.
The neuron density differences are unique to autism
Next, the researchers wanted to find out whether the differences in neuron density they were finding were unique to autism or a shared feature of other neurodevelopmental or psychiatric diagnoses. The researchers pulled images from 1404 individuals out of the original 8971 who had other non-autism psychiatric conditions. When they compared these images to those from the autistic children, the researchers found that their results were the same. They had confirmed that the variations in neuron densities were specific to autism.
Future directions for research and therapies
“These findings mean we now have a new set of measurements that have shown unique promise in characterizing individuals with autism,” said Zachary Christensen, MD/PhD candidate at the University of Rochester School of Medicine and Dentistry, and first author of the paper in a press release. “If characterizing unique deviations in neuron structure in those with autism can be done reliably and with relative ease, that opens a lot of opportunities to characterize how autism develops, and these measures may be used to identify individuals with autism that could benefit from more specific therapeutic interventions.”
Neuron density and brain function
Neuron density, or the number of brain cells in a given volume of the brain, has long been considered a proxy for brain function. Dense clusters of neurons are thought to reflect the connectivity of the brain, and more is thought to be better.
Given this assumption, we might conclude that the lower neuron density this paper found in cortical areas involved with memory, learning, reasoning, and problem-solving means those areas don’t work as well in autistic brains. Or we might think that increased neuronal density in the amygdala means that this brain area works better or more strongly. Indeed, many autistic people have more easily triggered and stronger fight or flight responses than the general population.
However, neuron density does not tell the whole story. How the brain functions also has a lot to do with how many connections there are between neurons, among other factors. A key process in the development of a child’s brain is synaptic pruning, where the brain actually prunes away connections between neuronal cells at the synapse. Too little pruning means too many connections, which means the brain can’t function efficiently. In the brains of children with autism, the process of synaptic pruning slows down during development, leaving their brains with many more synaptic connections than neurotypical peers. This also occurs in the brains of children with ADHD, who have differences in synaptic pruning and connectivity as compared to neurotypical peers.
In addition, neuroscientists increasingly view models of the brain that rely on specific brain areas as outdated. Instead, what drives brain function is how those areas work together, in systems. But rather than simply looking at which brain areas are connected to each other at a structural level, the new field of neural connectonomics seeks to map out the individual connections of every neuron to every other neuron in the brain. Understanding brain function at that level will surely generate findings that surprise us.
Autistic brains are wired differently
In context, neuronal density helps us explore the autistic brain but does not give us obvious conclusions. Consider the low neuron density found in problem-solving regions of the cortex. Now juxtapose that finding with the fact that many autistic people show superior problem-solving abilities. In one Harvard study, autistics were 40% faster at problem-solving than non-autistics. This tells us that the story of brain function in autism is far more complex than simply assuming that lower or higher neuronal density equates to quality of function.
This study’s findings of unique patterns in neuronal density specific to the autistic brain are still exciting, for the simple reason that every increase in our understanding of autism brings opportunities to better support our autistic community.
