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Brief neuroanatomy of autism

Differences in brain structure

Autism is a neurodevelopmental condition present in both children and adults worldwide. The core symptoms include difficulties understanding social interaction and communication, and restrictive or repetitive behaviours such as strict routines and stimming.

When the term autism was first coined in the 20th century, it was thought of as a disease.

However, it is now described as a cognitive difference rather than a disease; that is, the brains of autistic individuals – along with people diagnosed with dyslexia, dyspraxia, or attention deficit hyperactive disorder – are not defective, but simply wired differently.

The exact cause or mechanism for autism has not been determined; the symptoms are thought to be brought about by a combination of genetic and environmental factors. Currently, autism disorders are diagnosed solely by observing behaviours, without measuring the brain directly. However, behaviours may be seen as the observable consequence of brain activity. So, what is it about their brains that might make autistic individuals behave differently to neurotypicals?

Total brain volume

Back before sophisticated imaging techniques were in use, psychiatrics had already observed the head size of autistic infants was often larger than that of other children. Later studies provided more evidence that most children who would go on to be diagnosed had a normal-sized head at birth, but an abnormally large circumference by the time they had turned 2 to 4 years old. Interestingly, increase in head size has been found to be correlated with the onset of main symptoms of autism. However, after childhood, growth appears to slow down, and autistic teenagers and adults present brain sizes comparable to those of neurotypicals.

The amygdala

As well transient increase of total brain volume, the size and volume of several brain structures in particular seems to differ between individuals with and without autism. Most studies have found that the amygdala, a small area in the centre of the brain that mediates emotions such as fear, appears enlarged in autistic children. The amygdala is a particularly interesting structure to study in autism, as individuals often have difficulty interpreting and regulating emotions and social interactions. Its increased size seems to persist at least until early adolescence. However, studies in adolescents and adults tend to show that the enlargement slows down, and in some cases is even reversed so that the number of amygdala neurons may be lower than normal in autistic adults.

The cerebellum

Another brain structure that tends to present abnormalities in autism is the cerebellum. Sitting at the back of the head near the spinal cord, it is known to mediate fine motor control and proprioception. Yet, recent literature suggests it may also play an important role in some higher other cognitive functions, including language and social cognition. Specifically, it may be involved in our ability to imagine hypothetical scenarios and to abstract information from social interactions. In other words, it may help us recognise similarities and patterns in past social interactions that we can apply to understand a current situation. This ability is poor in autism; indeed, some investigations have found the volume of the cerebellum may be smaller in autistic individuals, although research is not conclusive.

Nevertheless, most research agrees that the number of Purkinje cells is markedly lower in people with autism. Purkinje cells are a type of neuron found exclusively in the cerebellum, able to integrate large amounts of input information into a coherent signal. They are also the only source of output for the cerebellum; they are responsible for connecting the structure with other parts of the brain such as the cortex and subcortical structures. These connections eventually bring about a specific function, including motor control and cognition. Therefore, a low number of Purkinje cells may cause underconnectivity between the cerebellum and other areas, which might be the reason for functions such as social cognition being impaired in autism.

Written by Julia Ruiz Rua

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