Child and Brain: The Stages of Development
The Apparent Race
You’re a proud new parent and you’ve spent innumerable hours—months, probably, if not years—poring over parenting books. You’re practically an expert on the subject now, even though your child is only a month old. You wanted to get it all just right, so you’ve read up on “the Mozart Effect” and Attention Deficit Disorder (ADD). You’ve powered through Dr. Spock. You probably have a chart up somewhere by which you’re measuring your child’s development against expectations. When should she grasp her own bottle? When will she say her first words? When will she walk? Run? Climb? These are benchmarks parents inevitably seek, often with much worry and doubt.
Benchmarks of brain development, though, are neither hard nor fast. The age range for some achievements can vary by months—even years. And in some cases, even if a child falls outside the range of “normal,” it’s not necessarily cause for alarm. Speed of development naturally varies; every child grows and adjusts to the world at his or her own pace, oblivious to the fact that the baby boy down the street, born three days later, is already regarding his own hand. Human development is no race, even if we sometimes mistake it for one.
Learning to Walk Before We Run
Inside the developing child, an unimaginable process is in motion, a complicated series of interactions between body and world. Each step is dependent on the succession of previous steps; genetic make-up and environment collide at every juncture, shaping and stretching towards potential. The young brain creates synapses by the thousands, putting to use the hundred billion nerve cells inherited at birth, and bringing to life the first awkward and unpracticed movements of limbs and digits. The twenty hours of sleep most infants require each day is a method of preserving metabolic energy for the enormous task of brain development that is occurring at an incredible rate.
In the words of Richard Barnet and Dr. Ann Barnet, “a child is discovering new things for the first time in virtually every waking moment.” In the first year alone, a child will learn, among hundreds of other things, to vocalize and imitate speech sounds, to reach for and grasp objects, and to turn toward sound. During this period, complex networks take shape in the brain, guiding these functions to categorize and connect with the child’s world. These milestones, by which we might generally mark progress, are not only the key events of early development, but also the very building blocks of skills to come, such as speaking, walking, catching a ball, or playing a concerto.
During the period between one and five months of life, most infants begin to hold a hand away from their body, staring at it in amazement. If they were to have language for the occasion, they’d likely be saying, “I think this is part of me”. To see this happen is to witness a child approaching the first understanding that he or she is alive. Neurologically, this is a major achievement in the process of building a proprioceptive map, or a sense of the body in space. Proprioception is one of the senses that, like hearing, sight, or smell, lets a child explore physical relationships to the world. A proprioceptive map is a significant part of developing the ability to extend an arm to catch a ball, or climb stairs without looking at the steps below.
It isn’t without justification that we have coined the phrase “you’ve got to walk before you run”; in the early stages of life much depends upon the sequence of events. And the sequence of events depends to a staggering degree upon the underlying neurological events unfolding inside.
Piecing Together the World
Consider the physical act of vocalization. Recognizable words are a long journey from the muttering of those first few infant noises. Words are formed gradually, using a process called auditory feedback, a two-part process in which control over the muscles responsible for speech results from the practice of mumbling endless streams of vowel sounds, and, through mimicry, those sounds are gradually matched up with the human speech sounds the child hears. In the language of developmental psychologists Alison Gopnik, Andrew Meltzoff, and Patricia Kuhl, a child is a scientist of sorts, constantly comparing what he or she perceives between the varying worlds of touch, sight, and sound; in short, piecing together the puzzle of a new world. At two or three months, a child is not yet making sense of language, but through auditory feedback becomes familiar with the sounds of language, and how to separate and reproduce them.
These “sounds” are called phonemes, the smallest sounds in language that can change meaning, and they are catalogued at birth. Out of several hundred possible phonemes, the English language uses 44. By two and a half months, an English speaking child is already cooing nearly exclusively in these sounds. Early on, nerves in the brain discern any of the hundreds of known phonemes—this is why a newborn can feasibly learn any language. But at the end of one year a child exposed to just one language will only be able to recognize the phonemes used in that native speech. It is no coincidence that at around this time children begin to produce their first words.
A certain amount of early development is purely physical, purely the gaining of strength. When a child can raise his or her head completely, without assistance (at 2 to 4 months) or sit with no support for the first time (at 5 to 6 months) and, later, can walk (at 11 to 15 months)—these are milestones largely wrought from simple muscle maturation. But what about when, at about 1 month, a child first perceives that one toy is closer than another? Or at 2 to 4 months turns to the sound of a rattle?
The Advent of Depth Perception
Like vocalization, development of depth perception is the result of improved cortical processes. Spatial perception improves when connections in the brain’s primary visual cortex are refined. At birth, all cells in the primary visual cortex receive input from both eyes; only later will some receive input exclusively from the right eye or the left. Before the functional connections are refined, a newborn’s brain cannot distinguish between the images apprehended by each individual eye, and sees without perspective. The experience is the equivalent of listening to a compact disc through an old, mono-radio. Perceiving in “stereo” (binocularity), conversely, separates the different parts of the music, creating space between objects. Though the image a newborn sees is complex, the visual processing system considering the image is not yet developed, and the result is an inability to calculate distance to objects.
Binocularity develops in the first month through a process of synaptic competition as the neurons in the primary visual cortex are eventually assigned to one eye or the other. Through this process, the visual cortex maintains two slightly different images of the object within the field of vision. The brain takes these images and integrates them, resulting in a more complex picture—forming perspective with distance and depth.
Turning to Sound
Similarly, a child first turning to observe a sound exhibits the maturation of a number of different processes. In the first months, there is an assortment of maps concurrently developing in the brain that are independent of one another—including a visual map, an auditory map, and a somatosensory, or tactile map. Only after maturation and integration of these processes can a child hear a sound and, by connecting the visual, auditory, and somatosensory maps, locate its origin.
When experience brings these maps into sync — sometime between 2 and 4 months — the sound sets off a extraordinary series of events all orchestrated by the superior colliculus, the part of the brain responsible for orienting the head. The sound reaches the superior colliculus, and is identified as “to my left” on the auditory map. The visual map, similarly informed by the superior colliculus, knows that the origin of the sound is beyond the field of vision, and from the somatosensory map, exactly how far the head must be turned to locate the sound. Finally, the superior colliculus sends a message to the muscles in the neck to move the head to that position, and the sound is precisely centered within the child’s gaze.
There are really just a handful of processes that a developing brain employs in order to make sense of the world. All generally involve the expansion and elaboration of the nervous system coupled with careful observation and cataloging of environment. It is amazing that a child, who does not yet have a name for memory, is successfully using this complicated, if fundamental, function even before birth.
A child’s daily learning curve is so steep that we surely cannot even grasp its acceleration, let alone try to compare it to a particularly fruitful day in our own lives. The processes unfolding inside the brain catapult learning exponentially in those first crucial years. One might be tempted to set up parameters for when and how important events should transpire. And this is useful, even necessary. But it is important to keep in mind that growth and development, language and physical maturation, intelligence and sense of self in the world, do not emerge in a vacuum, but rather arise in a complex, interwoven, and unique interaction between the child and the environment. A child’s development defies simple textbook descriptions of isolated benchmarks — children develop in the world, not on a growth chart.