Brain Development and What Autism Spectrum Disorder Is

The best way to understand your child's autism is to start by understanding how the brain typically matures. It may sound complicated, but the basic idea is really quite intuitive, and once you grasp it, almost everything your doctor and therapists tell you will start to fall into place. Think of this chapter as the foundation the rest of the book rests on; we'll come back again and again to the ideas introduced here (neuroplasticity, the balance between excitation and calming, the social brain networks) throughout the book.

A Developing Brain: Building Billions of Connections

From the moment of birth, and even before, in the womb, the brain is a remarkably dynamic organ that is constantly under construction. The human brain contains roughly 86 billion neurons (nerve cells), but at birth the connections between these neurons are not yet mature. The first three years of life are when the brain grows fastest, forming more than a million new neural connections (synapses) every second. This building process continues at varying speeds until the end of adolescence, and in some regions of the brain, into the mid-twenties.

These connections aren't formed at random; they're shaped by experience. Every interaction, a baby looking at their mother's face, responding to her voice, following a toy, strengthens the connections between the relevant brain regions. At the same time, connections that go unused are gradually trimmed away; this is called "synaptic pruning," and it's necessary for the brain to work efficiently. In typical development, this process of building and pruning follows a certain order and timing: first the basic sensory and motor areas mature, then language, and then the areas involved in complex social thinking and executive function (especially the prefrontal cortex) continue to mature.

A simple comparison

You can picture the developing brain as a huge city still under construction. Some streets (connections) are used so much that they widen and speed up; the little side streets that are rarely used gradually close off. In autism spectrum disorder, the "roads" between certain parts of this city are built with a different pattern than in typical development: some connections form more densely than expected, others more sparsely. The result isn't a "broken" city, but a city with a different traffic layout, where some routes carry heavy traffic while others stay weakly connected.

The Social Brain: Networks That Work Differently in Autism

Neuroscience has shown that social behavior doesn't come from a single "center," but from a whole set of networks that talk to one another. Some of the most important parts of this "social brain" network are:

• The amygdala — evaluates stimuli that carry emotional significance (especially facial expressions); in autism, some studies have described differences in its early-childhood growth and different activation patterns during face processing.
• The prefrontal cortex — linked to planning, impulse control, flexible thinking, and the ability to understand another person's point of view, known as "theory of mind."
• The mirror neuron system — a network associated with imitation and empathy that lets us simulate other people's actions and intentions "from the inside."
• The superior temporal sulcus — plays a role in perceiving biological motion (walking, gestures, eye movement).
• The cerebellum — now known to play a role not only in coordinating movement but also in shifting attention and cognitive flexibility; some children with autism show cerebellar differences.

None of these networks is a single "autism center" on its own; autism has to do with how these networks work together in sync, in other words, with connectivity. That's why it's more accurate to understand autism not as "damage" to a single brain region, but as a difference across large-scale networks.

The Excitation–Calming Balance and Sensory Processing

The brain's main excitatory messenger is glutamate, and its main calming (inhibitory) messenger is a substance called GABA. Current research suggests that in autism, this balance between excitation and calming may follow a different course than in typical development, in certain brain regions and during certain developmental periods. This idea of a "balance difference" is a framework used to explain both the sensory over-responsiveness seen in some children (a tendency toward overstimulation) and the sensory under-responsiveness seen in others (an insufficient response to stimuli). It's also thought that this balance may provide a possible neurobiological basis for why epilepsy and autism occur together in some children (see Chapter 13).

What Exactly Is Autism Spectrum Disorder?

Autism spectrum disorder (ASD) is a developmental difference in the brain, present from birth or from very early in life, in which lasting difficulties in social communication and interaction go together with repetitive patterns of behavior and/or narrow, intense interests. The word "spectrum" is the key here: autism doesn't have a single look. Two children with the very same diagnosis can be very different from each other in how they function, in their language development, in their sensory profile, and in the things that come along with it.

This difference affects the way the brain processes information in three main areas: social communication and interaction; repetitive behaviors and narrow interests; and, in most children, the sensory sensitivities that come with them. Each of these three areas will be covered separately and in depth in the second part of the book. An important point: the signs in these three areas aren't independent of one another. For example, a child pulling away from social interaction in a crowded, noisy setting can be the result of both social communication difficulty and sensory overload.

Why Do We Say "Spectrum"? A Short History

The idea of autism was first described in 1943 by Leo Kanner, in a group of children who withdrew socially and insisted on sameness. A year later, Hans Asperger described children with similar social difficulties but with better-preserved language development. For the next forty years, these two descriptions were used as separate and fairly narrow categories; before the 1980s, a diagnosis of autism was usually limited to children with severe language delay.

Over time, it became clear that children with social communication difficulties and repetitive behaviors, but with language that was less affected or not affected at all, shared the same basic neurobiological pattern. With the DSM-5 diagnostic system published in 2013, separate categories such as Asperger's syndrome, autistic disorder, pervasive developmental disorder–not otherwise specified (PDD-NOS), and childhood disintegrative disorder were combined under a single umbrella diagnosis: autism spectrum disorder. (Rett syndrome, whose underlying genetic cause has been identified, continues to be classified separately as its own genetic syndrome rather than under this umbrella; see Chapter 14.)

This merging carries an important clinical message: autism is a continuous range, with no sharp lines between the "mild" and "severe" ends. That's why throughout this book we'll often use the phrase "where on the spectrum"; it's meant to remind you that your child's individual profile matters far more than any general category. You may still hear older terms like "Asperger's syndrome" or "high-functioning autism" in everyday conversation; these are no longer official diagnostic categories, but some families and autistic adults continue to use these terms as a way of describing their identity.

Info Box — A Brief History of the Terms

1943 — Kanner described "early infantile autism."

1944 — Asperger described children with social difficulty but preserved language development.

1980 — The category of "pervasive developmental disorder" was established in DSM-III.

1994 — In DSM-IV, Asperger's syndrome, autistic disorder, and PDD-NOS were listed as separate diagnoses.

2013 — In DSM-5, all these categories were combined under one umbrella: autism spectrum disorder.

A Child's Developing Brain: Why Do the Early Years Matter So Much?

A child's brain is far more flexible than an adult's; this is called neuroplasticity. This flexibility means that supportive interventions carried out in early childhood can have a positive effect on the brain's developing connection patterns. That's why the principle you'll often hear in the autism field, "the earlier, the better," isn't empty optimism but rests on a neuroscientific reality: the brain is more ready to learn certain skills (especially language and social interaction) during certain "sensitive periods," and intensive, well-timed support helps make the most of those windows.

That said, having missed the window for early diagnosis never means "it's too late now." The brain continues to learn and adapt throughout childhood, and even into adolescence and adulthood; neuroplasticity lasts a lifetime, even if it lessens over time. In later chapters we'll go into detail about approaches that show meaningful progress can be made at any age.

Caution / When to Seek Advice

When a child loses skills they had already gained (such as words, eye contact, or a social smile), developmental regression, this is always a sign that calls for seeing a pediatric neurologist or developmental pediatrician without delay. Whether in the context of autism or to rule out another underlying neurological or metabolic condition, this situation requires urgent evaluation.

Chapter Summary

In this chapter we looked at how the brain develops, which networks are involved in social behavior, and how autism arises from these networks working with a different pattern. In the next chapter, building on this understanding, we'll go through the most common false beliefs about autism one by one and make clear what autism "is not."