Once hidden and enclosed in a great rock, I came down, against my will, from a great ravine In the high mountains to this lower place, to be revealed within this little stone.
—from Michelangelo’s Sonnet 275
Though many forms of visual art are additive—oil paint is spread over a canvas, charcoal is scraped onto paper—marble sculpture is subtractive. When Michelangelo approached a raw block of marble, he envisioned his task not as creating something entirely new, but as removing everything unessential. He chiseled away superfluous material bit by bit until only the sculpture remained.
Like the marble dust surrounding the sculptor’s creations, the connections between the neurons in his brain—called synapses—were also being chipped away. By the time he was an adult, the artist had lost nearly half of these connections.
This massive reduction in synapses was no crisis. It happens in every healthy human brain—even yours.
From before birth until age 2, infant brains experience an explosion of synaptic development. Each of the 100 to 200 billion neurons in a child’s brain reaches out to form connections with other brain cells. At birth, a baby has about 2,500 synapses in its visual cortex for every neuron. But by age 2, that number has grown by 600 percent. If the connections in a newborn’s brain look like the country roads in West Texas, those in a 2-year-old’s brain are a map of downtown New York City.
So far so good, right? It’s no surprise that toddlers’ brains develop at an astonishing rate. Between the time a child utters her first word and her second birthday, she adds an average of 17 new words to her vocabulary every week. Yet while children keep learning more and more words (around 8,000 by age six), their synapse development tapers off and takes a surprising turn—straight down.
Like a shoe store that overstocked on sneakers, our brains start liquidating assets. Neuronal branches withdraw in a process called synaptic pruning, reallocating resources to the connections that remain. By adolescence, we lose an average of 100,000 synapses per second. Is it any wonder that adolescence is such a tumultuous time?
How do our brains decide which connections to keep and which to lose? Monkey faces give us a clue.
Use It or Lose It
Scientists who study Barbary macaques sometimes have trouble telling one macaque face from another. But according to a team of psychologists at England's University of Sheffield, babies have no such difficulty. They showed 6-month-olds and 9-month-olds a picture of a Barbary macaque. Children from both age groups’ eyes opened wide and their faces brightened at the sight of this unusual primate face.
When the researchers showed these children the face of a different Barbary macaque, the 6-month-olds were still fascinated by this new face. The 9-month-olds, on the other hand, appeared uninterested in the new monkey. Why? Because these older infants thought they were seeing the same monkey as before. At some point between these two ages, our ability to distinguish between monkey faces is pruned away. But if 6-month-olds are shown pictures of these primates regularly after their first viewing, they’ll still be able to differentiate between macaques at nine months.
As this study demonstrates, synapses used regularly get stronger, while those neglected during a critical period get pruned. The vast majority of human beings have no need to differentiate between the faces of monkeys, but it’s absolutely crucial that we’re able to distinguish between faces of our fellow humans. So our brains prune to stay efficient. Resources wasted on monkey differentiation are reallocated to pathways used to tell one person from another. Aristotle wasn’t too far off when he wrote, “We are what we repeatedly do.”
This same “use it or lose it” principle guides our ability to learn second languages. Children taught a second language before the age of 2 pick the language up quickly and retain it for most of their lives. But this sensitive period fades quickly. By six months, children raised in a single-language culture start losing the ability to distinguish sounds and syllables unique to other languages. By nine months, children tune out foreign words altogether. As any college student studying for a foreign language exam would attest, if we miss this window, languages take longer to learn and, without regular use, are quickly forgotten.
So it turns out synaptic pruning is a bittersweet process. While few people would grieve the loss of their ability to play a simian version of the game Guess Who?, many would relish an opportunity to learn and retain foreign languages with the ease of a 2-year-old. So should we begin researching ways to halt synaptic pruning? Is it a form of neural self-sabotage, depriving us of amazing abilities and greater intelligence?
When Synaptic Pruning Stalls
Janelle was, by all appearances, a healthy 2-year-old girl. She learned quickly and focused on activities longer than other children her age. Janelle’s father was especially proud of her ability to stack blocks. While other 2-year-olds could only stack two or three before losing interest, Janelle showed remarkable dexterity, easily stacking as many as 10 or 11 blocks. As she stacked, her father would count the blocks out loud, offering praise with each successful block placement and encouragement whenever the stack fell over. But these setbacks seldom fazed the 2-year-old girl. She would jump right back into stacking without so much as a glance toward Dad for reassurance. In fact, during these play sessions, Janelle rarely engaged her father at all.
Unbeknownst to her father, Janelle’s actions are common early signs of autism. Children with this developmental disorder repeat actions over and over, form attachments to one particular toy or object, have difficulty interacting socially, and become upset by changes in routine.
Autism has no known cause, but researchers are beginning to understand some of its symptoms. Imaging studies show that autistic children have more neural connections than those without the disorder. Synaptic pruning begins early in the development of most children, but those with autism are left with hyper-connected brains. This may explain why autistic symptoms often don’t appear until the age of 2 or 3.
While most people habituate to peripheral events—like distant traffic or a television playing in another room—people with autism can’t shut them out. This constant barrage of sights and sounds drives autistic children to seek coordination and sameness, a reprieve from the static. According to Ralph-Axel Müller, a neuroscientist at San Diego State University, “If all parts of the brain talk to all parts of the brain, all you get is noise.”
Researchers from Columbia University Medical Center recently studied a group of mice with an overabundance of synapses and symptoms similar to autism. When these mice were given a drug that restored synaptic pruning, it eased their symptoms. They began socializing with other mice and lost some of their repetitive behaviors.
Though this drug cannot be given to humans due to its terrible side effects, it goes a long way toward revealing the role excess synapses play in autism. So while we may be tempted to delay synaptic pruning to retain skills lost with the destruction of those connections, Lisa Boulanger, a molecular biologist at Princeton, says, “More is not better when it comes to synapses, for sure, and pruning is absolutely essential.”
More, then, is not always better. While we may be tempted to believe that all creation is additive, synaptic pruning reveals the importance of subtraction in God’s ongoing creative process. Jesus’ words in the Gospel of John may not be only metaphor: “I am the true vine, and my Father is the gardener. He cuts off every branch in me that bears no fruit, while every branch that does bear fruit he prunes so that it will be even more fruitful” (15:1–2). From infancy, our unrefined brains are honed until a fine-tuned network of neural circuitry emerges, more powerful than a supercomputer and more elegant than the finest Renaissance sculpture.
Kyle Rohane is editor at LeaderTreks in Carol Stream, Illinois.
