hardware, but instead dynamically reallocates.
Although amputations lead to dramatic cortical reorganization, the brain’s shape shifting can be induced by modifying the body in more modest ways. For example, if I were to fasten a tight pressure cuff to your arm, your brain would adjust to the weakened incoming signals by devoting less territory to that part of your body.11 The same thing happens if the nerves from your arm are blocked for a long time with anesthetics. In fact, if you merely tie two fingers of your hand together—so they no longer operate independently, but instead as a unit—their cortical representation will eventually merge from two distinct regions into a single area.12
So how does the brain, confined to its dark perch, keep constant track of what the body looks like?
TIMING IS EVERYTHING
Imagine taking a bird’s-eye view of your neighborhood. You notice that some people take their dogs for a walk every morning at six o’clock. Others don’t get out with their canines until nine. Others stroll their pooches after lunch. Others opt for nighttime walks. If you watched the dynamics of the neighborhood for a while, you’d notice that people in the neighborhood who happen to walk at the same time tend to become friends with one another: they bump into one another, they chat, they eventually invite each other over for barbecues. Friendship follows timing.
It’s the same with neurons. They spend a small fraction of their time sending abrupt electrical pulses (also called spikes). The timing of these pulses is critically important. Let’s zoom in to a typical neuron. It reaches out to touch ten thousand neighbors. But it doesn’t form equally strong relationships with all ten thousand. Instead, the strengths are based on timing. If our neuron spikes, and then a connected neuron spikes just after that, the bond between them is strengthened. This rule can be summarized as neurons that fire together, wire together.13
In the young neighborhood of a new brain, nerves coming from the body to the brain branch out broadly. But they set down permanent roots in places where they fire in close timing with other neurons. Because of the synchrony, they strengthen their bonds. They don’t host barbecues, but instead they release more neurotransmitters, or set up more receptors to receive the neurotransmitters, thus causing a stronger link between them.
How does this simple trick lead to a map of the body? Consider what happens as you bump, touch, hug, kick, hit, and pat things in the world. When you pick up a coffee mug, patches of skin on your fingers will tend to be active at the same time. When you wear a shoe, patches of skin on your foot will tend to be active at the same time. In contrast, touches on your ring finger and your little toe will tend to enjoy less correlation, because there are few situations in life when those are active at the same moment. The same is true all over your body: patches that are neighboring will tend to be co-active more than patches that are not neighboring. After interacting with the world for a while, areas of skin that happen to be co-active often will wire up next to one another, and those that are not correlated will tend to be far apart. The consequence of years of these co-activations is an atlas of neighboring areas: a map of the body. In other words, the brain contains a map of the body because of a simple rule that governs how individual brain cells make connections with one another: neurons that are active close in time to one another tend to make and maintain connections between themselves. That’s how a map of the body emerges in the darkness.14
But why does the map change when the input changes?
COLONIZATION IS A FULL-TIME BUSINESS
At the beginning of the seventeenth century, France began its colonization of North America. Its technique? Sending ships full of Frenchmen. It worked. The French settlers took root in the fresh territory. In 1609, the French erected a fur post that would eventually become the city of Quebec, which was destined to become the capital of New France. Within twenty-five years, the French had spread into Wisconsin. As new French settlers voyaged across the Atlantic, their territory grew.
But New France wasn’t easy to maintain: it was under constant competition from the other powers that were sailing ships that way, mostly Britain and Spain. So France’s king, Louis XIV, started to intuit an important lesson: if he wanted New France to firmly take root, he had to keep sending ships—because the British were sending even more ships. He understood that Quebec wasn’t growing rapidly enough because of a lack of women, and so he sent 850 young women (called King’s Daughters) to stimulate the local French population. The effort helped to lift the population of New France to seven thousand by 1674 and then to fifteen thousand by 1689.
The problem was that the British were sending far more young men and women. By 1750, when New France had sixty thousand inhabitants, Britain’s colonies boasted a million. That made all the difference in the subsequent wars between the two powers: despite their allegiances with the Native Americans, the French were badly outstripped. For a short time, the government of France forced newly released prisoners to marry local prostitutes, and then the newlywed couples were linked with chains and shipped off to Louisiana to settle the land. But even these French efforts were insufficient.
By the end of their sixth war, the French realized they had lost. New France was dissolved. The spoils of Canada moved under the control of Great Britain, and the Louisiana Territory went to the young United States.15
The waxing and waning of the French grip on the New World had everything to do with how many boats were being sent over. In the face of fierce competition, the French had simply not shipped enough people over the water to keep a hold on their territory. As a result, all that now remains of the French presence in the New World are linguistic fossils, as seen in place-names such as Louisiana, Vermont, and Illinois.
Without competition, colonization is easy, but in the face of rivalry holding on to territory requires constant work. The same story plays out constantly in the brain. When a part of the body no longer sends information, it loses territory. Admiral Nelson’s arm was France, and his cortex the New World. It started off with a healthy colonization, sending useful spikes of information up the nerves and into the brain, and in Nelson’s youth it staked out a healthy territory. But then came the musket ball, followed hours later by his tattered arm splashing into the dark water . . . and now his brain received no new input from that part of his body. With time, the arm lost its neural real estate. Eventually, all that remained were fossils of the arm’s former presence, such as a feeling of phantom pain.
These lessons of colonization apply to more than arms: they apply to any system sending information into the brain. When a person’s eyes are damaged, signals no longer flood in along the pathways to the occipital cortex (the portion at the back of the brain, often thought of as “visual” cortex). And so that part of the cortex becomes no longer visual. The ships carrying visual data have stopped arriving, so the coveted territory is taken over by the competing kingdoms of sensory information.16 As a result, when a blind person passes her fingertips over the raised dots of a Braille poem, her occipital cortex becomes active from mere touch.17 If she gets a stroke that damages her occipital cortex, she’ll lose her ability to understand Braille.18 Her occipital cortex has been colonized by touch.
And it’s not only touch, but any sources of information. When blind subjects listen to sounds, their auditory cortex becomes active, and so does their occipital cortex.19
Cortical reorganization: unused cortex is taken over by competing neighborhoods. In this brain scan, sound and touch activate the otherwise unused occipital cortex of the blind (black indicates regions more active in the blind than the sighted).For a better view of the hills and valleys of the cortex, the brain has been computationally “inflated.” Figure adapted from Renier et al. (2010).
Not only can touch and sound activate the previously visual cortex of the blind, but so can smell, taste, the reminiscence of events, or the solving of math problems.20 As with a map of the New World, territory goes to the fiercest competitors.