700 Synapses Per Second: What Happens in a Baby’s Brain

There is a photograph on the bookshelf in my waiting room. It was there when I moved in, and I have kept it there because of what happens when clients notice it: a black-and-white image of a toddler reaching for something just outside the frame. People glance at it and sometimes stop, and occasionally they ask whose child it is. I tell them I don’t know, that it came with the office. What I do not always say, though I think it every time, is that I keep it because of what that toddler’s brain was doing in the moment the shutter clicked. A rate of growth so extreme that we have no equivalent for it anywhere else in human experience.

Seven hundred synaptic connections per second. Not per minute, not per hour. Per second. For the first several years of life, the human brain forms new neural connections at a rate that will never be approached again, not in adolescence, not in young adulthood, not in any subsequent period of learning or recovery or growth. The development that happens between birth and age five is, by any reasonable measure, the most intensive construction project in the known universe, happening inside a structure the approximate size of a cantaloupe.

I want to spend some time inside what that actually means, because most people know, in an abstract way, that early childhood is important. What they are less clear on is why , what is actually happening in that window that gives it the particular weight it carries. And understanding the mechanism changes how you understand yourself.

A brain built to be finished outside the womb

Human infants are, by the standards of mammalian biology, extraordinarily helpless at birth. A foal can walk within hours. A human newborn cannot lift its own head. This is not a flaw in the design. It is the design, chosen by evolution, in exchange for something else: a brain large enough, eventually, to do everything the human brain can do. The trade-off was this: the human brain has to be born before it is complete, because a completed human brain would not fit through the birth canal. What this means is that the majority of neural development , the wiring that will eventually govern how the person regulates emotions, understands relationships, responds to threat, makes meaning of experience , happens after birth, outside the womb, in the context of whatever relational environment the infant is born into.

The implication is radical. Other animals arrive in the world with instinct mostly pre-loaded, the critical instructions encoded in the genes. The human animal arrives relatively unscripted, designed to be completed by experience. The brain is not a finished instrument at birth. It is closer to clay that has just been placed in a mold. And the mold, in those critical early years, is the relational environment: the people the child is closest to, the quality of their care, the emotional texture of the thousands of ordinary interactions that make up a childhood.

What the 700 connections are building

At birth, the brain has roughly one hundred billion neurons, which sounds like an enormous number and is. But neurons without connections are like cities without roads. The neurons are in place. The infrastructure that allows them to communicate has barely begun. What those first years of development are building, at seven hundred connections per second, is the communication network , the system by which neurons talk to each other, the pathways along which thoughts and feelings and responses will eventually travel.

Each connection is created by experience. Specifically, by repeated experience. The neuropsychologist Donald Hebb’s foundational principle , neurons that fire together wire together , describes the mechanism precisely. When two neurons are activated at the same time, repeatedly, the connection between them grows stronger. The pathway becomes more efficient, less effortful to travel. With enough repetition, it becomes automatic. The baby who is held when it cries is not just receiving comfort. It is building a neural pathway that links the experience of distress to the arrival of relief, that links reaching out to something coming. The baby who is ignored when it cries is building a different pathway, one that links distress to absence, reaching to nothing, need to the self-management that becomes necessary when management is all that is available.

Neither of these is a conscious process. The baby is not deciding what to learn. The baby is absorbing, without discrimination, without the capacity to evaluate what it is being taught, the emotional logic of the environment it finds itself in. This is what makes early experience so weighty and so persistent: the lessons go in before there is any mechanism for questioning them. They are received as fact. As simply the way things are.

The still-face experiment

In the 1970s, the psychologist Edward Tronick designed a procedure that has since become one of the most replicated and most affecting demonstrations in developmental psychology. It is called the still-face experiment, and if you have never seen it, I recommend it, though I want to prepare you for it.

A mother sits face to face with her infant. They interact normally , the ordinary warm exchange of sounds and expressions and gestures that looks, to a casual observer, like simple play. Then, at a signal, the mother is asked to make her face completely still. No expression. No response to the baby’s attempts to engage. Just a neutral face, looking at the child, offering nothing.

What happens next is immediate and consistent across thousands of subjects. The baby tries to re-engage the mother. Smiles, gestures, makes sounds. When this does not work, the baby tries harder, escalating, cycling through every strategy available to a very small person trying to restore a connection that has gone dark. When nothing works, when the still face holds, the baby begins to deteriorate. Some cry. Some turn away entirely. Some show signs of what in an adult we would call a kind of collapse , the body losing its postural tone, the expression going flat, the affect shutting down. All of this in less than two minutes.

Two minutes of a blank face, and the baby’s entire regulatory system reorganizes around the loss. This is how sensitive the developing brain is to the relational environment. This is how much the baby’s capacity to manage its own inner experience depends on the responsiveness of the face across from it. The mother’s face is not decoration. It is, for the baby, the primary instrument of nervous system regulation. The infant cannot yet regulate its own emotional states , that capacity develops gradually over years. In the meantime, it borrows the caregiver’s regulated nervous system, using the warmth and responsiveness of the person across from it to manage what it cannot yet manage alone.

What the brain does with what it receives

The construction that happens in those early years is not just about volume. It is about architecture. The brain is not building randomly. It is building in response to information it is receiving about what kind of world this is and what that world is going to require. A baby whose distress is consistently met with warmth is receiving a very specific set of information: when things go wrong, help arrives. When I reach, something comes. The world is, in some fundamental sense, safe enough to navigate. The brain builds itself around that expectation, wiring in pathways that reflect it, creating a nervous system calibrated for a world where distress is temporary and comfort is available.

A baby whose distress is not reliably met , whose cries go unanswered, whose reaching meets absence, whose environment is chronically unpredictable or frightening , is receiving different information. Not wrong information. Accurate information about the specific environment that baby is in. The brain builds accordingly, calibrating for a world where danger is more likely, comfort is less reliable, and management of distress is something you do inside yourself because no one else is going to do it with you. The amygdala, the brain’s threat-detection system, can become enlarged and hyperreactive in children raised in chronically stressful environments. The prefrontal cortex, which regulates the alarm, can be underdeveloped relative to baseline. Both adaptations are rational responses to the information the environment provided. Both carry costs that extend well beyond childhood.

And then the pruning begins

Starting around age two and continuing through adolescence, the brain does something that surprises people when they first hear about it: it deletes connections. Roughly half of the synaptic connections formed in infancy are eliminated during this pruning process. The brain keeps what it has been using and removes what it has not, consolidating the architecture into something more efficient. The connections that survive are the ones that were most active, most repeatedly traveled, most central to the experience of being in this particular environment with these particular people.

By the time a child is five or six, the basic shape of their nervous system is largely set. Not permanently. Not irreversibly. The brain retains meaningful plasticity throughout life, and the research on earned secure attachment is explicit that the patterns laid down in childhood can be genuinely updated in adulthood through the right kinds of relational experience. But the early architecture is influential in a way that later experience is not, because it was built when the construction rate was at its peak, and because the rest of the structure was built on top of it.

The toddler in the photograph on my bookshelf has a brain doing something extraordinary in the moment the shutter clicked. Building. At a rate we will not see again in their lifetime. Building on the basis of what is happening around them, in ordinary moments that nobody thought to record because nobody understood yet how much they mattered. You were that toddler once. What happened around you shaped what got built. And what got built is not a verdict , it is a starting point, a set of roads laid down in conditions that may be very different from the conditions you are in now. Roads can be expanded. New roads can be built alongside old ones. But first, it helps to understand what you are actually working with, and why those original roads feel, even now, like the only way home.

Daniel

Daniel Wethington is a Licensed Professional Counselor and the founder of Arise Counseling Services in York, Pennsylvania, where he specializes in attachment trauma, anxiety, and gaming and technology addiction. He is the author of four books, including Shaped in Silence and Breaking Free, which explore how early attachment experiences shape adult relationships and compulsive behaviors, which you can find here. Dan works with individuals and couples across Pennsylvania, both in person and through telehealth. If something in this article resonated with you, you can schedule an appointment or learn more at http://arise-pa.com.