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Whatever genetic and/or environmental factors may lead to Autism Spectrum Disorder, one thing is clear: it is the development of the brain that is ultimately impacted. The better we understand what goes amiss with this development, the better we will be able to intervene.

We have only just begun to learn exactly what sets the brain of a person with an ASD apart from that of a “typical” person. Investigations have been undertaken on several fronts, from brain structure, to brain chemistry, to attempts to better understand which neurological processes or connections are normally involved in complex social interaction –the area most glaringly affected in people with ASDs.

The "Experience-Expectant" Brain

As we begin to think about the brain in ASDs, one crucial concept to understand is the accepted psychobiological notion that brain structure and experience are interconnected, especially during infancy and early childhood. 1,2   If something fundamental goes awry in the autistic brain, and so interferes with early social interaction and experience, the reverse will also be true. That is, the continuing development of the “experience-expectant” 3  brain will be negatively impacted when crucial social experiences do not occur. Neurological development, already gone wrong, will be even further derailed when an infant on the autism spectrum fails to focus in on human faces and voices,4  or to seek out their parent’s gaze. 5   Both the “give” and the “take” of parent-infant interaction will not occur as they were meant to, and so the brain –wired and waiting for certain input—will suffer from never having received it.

Early Brain Overgrowth

One striking finding, made via studies using MRI and records of individuals’ head circumference, is that there is an early period of brain overgrowth in people who are eventually diagnosed with an ASD. Although their head circumference –and supposedly their brain—is normal, or even slightly on the small side, at birth, it begins to rapidly increase beyond the norm. It is believed that cerebral gray matter, as well as cerebral and cerebellar white matter, grow at an accelerated rate, creating this larger brain size. Eventually, brain growth slows down, again at an abnormal rate, so that people with ASDs catch up to “typical” people; there is no difference in brain volume between adults with ASDs and other adults. 6

In one study, this abnormal accelerated early brain growth was found to occur in children between the ages of 1 to 2 months and 6 to 14 months 7  –ages so early that, in many cases, other signs of autism might not yet have been noticed by parents or pediatricians. A rapid increase in a baby or toddler’s head circumference may be a useful early indicator of ASD.

It is not yet known exactly what causes the brain overgrowth in autism, or how the increased brain volume is related to actual autistic symptoms. 8  One theory involves a stalled out “synaptic pruning” process. The synapses –that is, the neural connections in the brain—are supposed to multiply vigorously, especially during the first 12 to 24 months of life, and then be thinned out based on experience. Those that are being used are supposed to stay. Those that are not used are supposed to degenerate and disappear. It is thought that in autism this “culling out” may not happen, that the overabundant synapses may not be eliminated, leaving behind “a persistent and abnormal organization of neural structure.” 9

One group of researchers summarized it this way: “Growth and elaboration of neural architecture and connectivity occurs prematurely and without being guided by functional experiences and adaptive learning; that is, in early life the autistic brain exhibits premature ‘growth without guidance’.”  10

It becomes very clear why so much emphasis is put on early intervention in autism. The brain is not fixed at birth, but constantly transforming itself. If it’s transforming in the wrong direction, jumping in and intervening in the process makes a great deal of sense.

Brain Structures, Processing, and Chemistry

Are specific brain structures involved in autism? It is believed that there are because if the entire brain –every structure, cell, and synapse—was affected, there would be no areas of spared ability: no gifts for detail-focused processing, no “high-functioning” people with normal (or better) IQ, no savant skills. 11   Researchers are proposing, based on a variety of MRI-based studies, that specific brain structures, such as the amygdale, the Superior Temporal Sulcus Region, and the Fusiform Gyrus, are implicated in the social cognition problems in autism, including eye gaze processing, voice perception, theory of mind deficits, and difficulties focusing on faces and making sense of facial expressions. 12   All three structures function in parallel; might they represent the “social brain” 13  in humans?

One thing is certain. It is not just brain structures that are important, but connections between various parts of the brain. 14   As advances in measuring connectivity in the human brain are made, scientists will be able to go beyond looking at certain regions or structures to investigating how autism involves “disrupted dynamics in processing.”  15   The abilities most impacted by autism, such as social interaction and language, are precisely those abilities that require a high degree of integration, 16  that is, an array of brain functions that have to work together at the same time. Does this explain why people with ASDs are good at parts, rather than wholes, at seeing the trees, but not the forest? Is it that specific areas of the brain work fine, but that dispersed areas that are supposed to work together are not working together?

In addition to brain structure, researchers are delving into the role that brain chemistry plays, especially that involving neurotransmitters: serotonin, dopamine, and norepinephrine. Although little has been conclusively discovered in this area as of yet, neurochemistry clearly does have some impact on people with ASDs and warrants further investigation. 17   It is interesting to note that two atypical antipsychotic medications called risperidone (brand name Risperdal) and aripiprazole (brand name Abilify), which work by interfering with normal neurotransmitter activity, are the only clinically tested 18,19  and FDA approved 20,21 medications for management of the irritability, tantrums, aggression, and self-injury associated with ASDs in children and adolescents. Clearly, neurotransmitters are important; we just don’t know exactly how.

We still have much to learn about brain structures, connectivity, and chemistry, as well as the “social brain” and how it impacts and is impacted by early experience. Research along all of these avenues will be crucial as we continue to seek a deeper understanding of Autism Spectrum Disorder and how to prevent or treat it.

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