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.
The story has grown even more interesting in recent years: when new occupants move into the visual cortex, they retain some of the former architecture—like the mosques in Turkey that used to be Roman cathedrals. As an example, the area that processes visual written language in the sighted is the same area that becomes active when the blind read Braille.21 Similarly, the main area for processing visual motion in the sighted is activated for tactile motion in the blind (for example, something moving across the fingertips or the tongue).22 The main neural network involved in visual object recognition in the sighted is activated by touch in the blind.23 Such observations have led to the hypothesis that the brain is a “task machine”—doing jobs like detecting motion or objects in the world—rather than a system organized by particular senses.24 In other words, brain regions care about solving certain types of tasks, irrespective of the sensory channel by which information arrives.
There’s a side note here that we’ll return to in later chapters: age matters. In those born blind, their occipital cortex is completely taken over by other senses. If a person goes blind at an early age—say, at five years old—the takeover is less comprehensive. For the “late blind” (those who lost vision after the age of ten), the cortical takeovers are even smaller. The older the brain, the less flexible it is for redeployment, just as North American borders now shift very little after settling into place for five centuries.
The same thing we see with the loss of vision happens with the loss of any sense. For example, in the deaf, the auditory cortex becomes employed for vision and other tasks.25 Just as Lord Nelson’s loss of a limb led to cortical takeovers by neighboring territories, so too does the loss of hearing, smell, taste, or anything else. The cartography of the brain constantly shifts to best represent the incoming data.26
Once you begin looking for it, you’ll see this competition for territory everywhere around you. Think of an airport in a major city. If there are a large number of incoming flights from a particular airline (United), and fewer flights from another (Delta), then it would be no surprise to see the number of United counters grow, while those from Delta shrink. United would take over more of the gates, more of the baggage claims, and more space on the monitors. If another airline went fully out of business (think Trans World Airlines), then all of its presence in the airport would be quickly taken over. And so it goes with the brain and its sensory inputs.