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THE INSIDE MIRRORS THE OUTSIDE

THE CASE OF THE SILVER SPRING MONKEYS

In 1951, the neurosurgeon Wilder Penfield sank the tip of a fine electrode into the brain of a man undergoing surgery.¹ Along the brain tissue just beneath where one might wear headphones, Penfield discovered something surprising. If he gave a small shock of electricity at a particular spot, the patient would feel as though his hand were being touched. If Penfield stimulated a nearby spot, the patient would feel the touch on his torso. A different spot, the knee. Every spot on the patient’s body was represented in the brain.

Then Penfield made a deeper realization: neighboring parts of the body were represented by neighboring spots on the brain. The hand was represented near the forearm, which was represented near the elbow, which was represented near the upper arm, and so on. Along this strip of the brain, there was a detailed map of the body. By moving from spot to spot slowly along the somatosensory cortex, he could find the whole human figure.²

And this wasn’t the only map he found. Along the motor cortex (the strip just in front of the somatosensory cortex), he discovered the same kind of result: a little zap of electricity caused muscles to twitch in specific, neighboring areas of the body. Again, it was laid out in an orderly manner.

Maps of the body are found where inputs enter the brain (somatosensory cortex, top) and outputs leave the brain (motor cortex, bottom). Areas with more detailed sensation, and those that are more finely controlled, command more real estate.

He named these maps of the body the homunculus, or “little man.”

But the existence of the maps is strange and unexpected. How do they exist? After all, the brain is locked in total darkness within the skull. These three pounds of tissue don’t know what your body looks like; the brain has no way to directly see your body. It has access to nothing but a chattering stream of electrical pulses racing up the thick bundles of data cables we call nerves. Stowed away in its bony prison, the brain should have no idea what limbs are connected where, or which are next to which others. So how is there a depiction of the body’s layout in this lightless vault?

A moment of thought will likely lead you to the most straightforward solution: the map of the body must be genetically preprogrammed. Good guess!

But wrong.

Instead, the answer to the mystery is more fiendishly clever.

A clue to the map mystery came decades later, in an unexpected turn of events. Edward Taub, a scientist at the Institute of Behavioral Research in Silver Spring, Maryland, wanted to understand how victims of brain injury could recover movement. To that end, he obtained seventeen monkeys and studied whether severed nerves could regenerate. In each, he carefully cut a nerve bundle that linked the brain to one of the arms or one of the legs. As expected, the unfortunate monkeys lost all sensation from the affected limbs, and Taub set about studying whether there was a way to get the monkeys to regain use.

In 1981, a young volunteer named Alex Pacheco began to work in the lab. Although he presented himself as an intrigued student, in fact he was there to spy for a budding organization, the People for the Ethical Treatment of Animals (PETA). At night Pacheco took photographs. Some of the shots were apparently staged to exaggerate the suffering of the monkeys,³ but in any case the effect was achieved. In September 1981, the Montgomery County police raided the laboratory and shut it down. Dr. Taub was convicted of six counts of failure to provide adequate veterinary care. All of the charges were overturned on appeal; nonetheless, the events led to the creation of the Animal Welfare Act of 1985, in which Congress defined new rules for animal care in research environments.

Although this provided a watershed moment for animal rights, the importance of the story is not only about what happened in Congress. For our purposes here, it’s about what happened to the seventeen monkeys. Immediately after the accusation, PETA swept in and absconded with the monkeys, leading to charges of theft of court evidence. Incensed, Taub’s research institution demanded the return of the monkeys. The legal battle grew increasingly heated, and the battle for possession of the monkeys ascended to the U.S. Supreme Court. The Court rejected PETA’s plea to keep the monkeys, instead granting custody to a third party, the National Institutes of Health. While humans barked at each other in distant courtrooms, the disabled monkeys enjoyed an early retirement by eating, drinking, and playing together for ten years.

Near the end of this period, one of the monkeys became terminally ill. The court agreed that the monkey could be put to sleep. And here’s where the plot turned. A group of neuroscience researchers made a proposal to the judge: the monkey’s severed nerve would not have been in vain if the researchers could be allowed to perform a brain-mapping study on the monkey while it was under anesthesia, just before being euthanized. After some debate, the court granted permission.

On January 14, 1990, the research team put recording electrodes in the monkey’s somatosensory cortex. Exactly as Wilder Penfield had done with his human patient, the researchers touched the monkey on its hand, arm, face, and so on while recording from neurons in the brain. In this way, they revealed the map of the body in the brain.

The findings sent ripples through the neuroscience community. The body map had changed over the years. Unsurprisingly, a gentle touch on the monkey’s nerve-severed hand no longer activated any response in the cortex. But the surprise was that the little bit of cortex that used to represent the hand was now excited by a touch to the face.⁴ The map of the body had reorganized. The homunculus still looked like a monkey, but a monkey without a right arm.

This discovery ruled out the possibility that the brain’s map of the body is genetically preprogrammed. Instead, something much more interesting was going on. The brain’s map was flexibly defined by active inputs from the body. When the body changes, the homunculus follows.

The same brain-mapping studies were done later in the year on the other Silver Spring monkeys. In each one of them, the somatosensory cortex had dramatically rearranged: the areas once representing the nerve-severed limbs had been taken over by neighboring areas in the cortex. The homunculi had transformed to match the monkeys’ new body plans.⁵

What does it feel like when the brain reorganizes like that? Unfortunately, monkeys can’t tell us. But people can.

THE AFTERLIFE OF LORD HORATIO NELSON’S RIGHT ARM

The British naval commander Admiral Lord Horatio Nelson (1758–1805) is the hero mounted high on a pedestal overlooking London’s Trafalgar Square.⁶ The statue stands in towering testimony to his charismatic leadership, his tactical strength, and his inventive stratagems, which together led to decisive victories on waters from the Americas to the Nile to Copenhagen. He died heroically in his final showdown—the Battle of Trafalgar—one of Britain’s greatest maritime victories.

Beyond his naval impact, Admiral Nelson also contributed to neuroscience—however, this was totally accidental. His involvement began during his attack on Santa Cruz de Tenerife, when at eleven o’clock at night on July 24, 1797, a musket ball exited a Spanish rifle barrel at a thousand feet per second and ended its trajectory in Lord Nelson’s right arm. His bone shattered. Nelson’s stepson tied a piece of his neck scarf tightly around the arm to stop the bleeding, and Nelson’s sailors rowed vigorously back to the main ship, where the surgeon tensely awaited. After a rapid physical exam, the good news was that Nelson was likely to survive. The bad news was that the risk of gangrene demanded amputation. Nelson’s right arm was surgically removed above the elbow and thrown overboard into the water.

Over the following weeks, Nelson learned to function without his right arm—eating, washing himself, even shooting. He came to jokingly refer to the stump of his amputation as his “fin.”

But some months after the event, strange consequences began. Lord Nelson started to feel—literally feel—that his arm was still present. He experienced sensations from it. He was certain that his missing fingernails from his missing fingers were digging, painfully, into his missing right palm.

Nelson had an optimistic interpretation of this sensation of his phantom limb: he concluded that he now possessed incontrovertible proof of life after death. After all, if an absent limb could give rise to conscious feeling—an ever-present ghost of itself—then an absent body must as well.

Although paintings and sculptures of Lord Horatio Nelson festoon British museums, most visitors don’t notice that Nelson is missing his right arm. Its amputation in 1797 led to an early clinical case of phantom limb sensation and an interesting, but incorrect, metaphysical interpretation by Nelson himself.

Nelson was not the only one to notice these strange sensations. Across the Atlantic some years later, a physician named Silas Weir Mitchell documented numerous Civil War amputees at a hospital in Philadelphia. He was mesmerized by the fact that many of them insisted they still felt sensations from their missing limbs.⁷ Was this proof of Nelson’s corporeal immortality?

As it turns out, Nelson’s conclusion was premature. His brain was remapping itself, exactly as happened with the Silver Spring monkeys. Over time, as historians followed the shifting borders of the British Empire, scientists discovered how to track the shifting borders in the human brain.⁸ With modern imaging techniques, we can see that when an arm is amputated, its representation in the cortex is encroached upon by neighboring areas. In this case, the cortical areas that surround the hand and forearm are the territories of the upper arm and the face. (Why the face? It just happens that’s where things lie when the body has to be represented on a linear map.) So these representations move to take over the land where the hand used to be. Just as with the monkeys, the maps come to reflect the current form of the body.

The brain adapts to the body plan. When a hand is amputated, neighboring cortical territories move in to usurp the hand’s previously held territory.

However, there’s another mystery buried in here. Why did Nelson still have a sense of his hand, and why, if you were to touch Nelson on his face, would he say that his phantom hand was being touched? Didn’t the neighboring areas take over the hand representation? The answer is that touch to the hand is represented not only by cells in the somatosensory cortex but also by the cells they talk to downstream, and the cells they talk to. So although the map modified itself rapidly in the primary somatosensory cortex, it shifted less and less in downstream areas. In a child born without an arm, the map would be entirely different—but in an adult, like Lord Nelson, the system has less flexibility to rewrite its manifest. Deep in Lord Nelson’s brain, the neurons downstream of the somatosensory cortex did not shift their connections as much, and therefore they believed that any activity they received was due to touch on the hand. As a result, Nelson perceived the ghostly presence of his missing limb.⁹

Monkeys and admirals and civil war veterans tell the same story: when inputs suddenly cease, sensory cortical areas do not lie fallow. Instead, they are invaded by their neighbors.¹⁰ With thousands of amputees now studied in brain scanners, we see the degree to which brain matter is not like 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.

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.

Now we understand how competition leads to takeover. But all this leads to a question: When a sense captures more area, does that give it greater capabilities?

THE MORE THE BETTER

A young boy named Ronnie was born in Robbinsville, North Carolina. Soon after he was born, it became clear he was blind. At one year and one day old, his mother abandoned him, citing that his blindness was her punishment from God. He was raised in poverty by his grandparents until he was five, then sent off to a school for the sightless.

When he was six years old, his mother came by, just once. She had another child now, a girl. His mother said, “Ron, I want you to feel her eyes. You know, her eyes are so pretty. She did not shame me the way that you did. She can see.” That was the last time he ever had contact with his mother.

As hard as his childhood was, it became clear that Ronnie had a gift for music. His instructors spotted his talents, and Ronnie began to formally study classical music. One year after he picked up the violin, his teachers declared him a virtuoso. He went on to master piano, guitar, and several other string and woodwind instruments.

From there he went on to become one of the most popular performers of his day, locking down both pop music and country-western markets. He secured forty country hits at the number-one slot. He earned six Grammy Awards.

Ronnie Milsap is just one of many blind musicians; others include Andrea Bocelli, Ray Charles, Stevie Wonder, Diane Schuur, José Feliciano, and Jeff Healey. Their brains have learned to rely on the signals of sound and touch in their environment, and they become better at processing those than sighted people.

While musical stardom is not guaranteed for blind people, brain reorganization is. As a result, perfect musical pitch is overrepresented in the blind, and blind people are up to ten times better at determining whether a musical pitch subtly wobbles up or down.²⁷ They simply have more brain territory devoted to the task of listening. In a recent experiment, participants who were sighted or blind had one ear plugged up and were then asked to point to the locations of sounds in the room. Because pinpointing a sound requires a comparison of the signals at both ears, it was expected that everyone would fail miserably at this task. And that’s what happened with the sighted participants. But the blind participants were able to generally tell where the sounds were positioned.²⁸ Why? Because the exact shape of the cartilage of the outer ear (even just one ear) bounces sounds around in subtle ways that give clues to location—but only if one is highly attuned to pick up on those signals. The sighted have less cortex devoted to sound, and so their ability to extract subtle sound information is underdeveloped.

This sort of extreme talent with sound is common among the blind. Take Ben Underwood. When he was two years old, Ben stopped seeing out of his left eye. His mother took him to the doctor and soon discovered he had retinal cancer in both eyes. After chemotherapy and radiation failed, surgeons removed both his eyes. But by the time Ben was seven years old, he had devised a useful, unexpected technique: he clicked with his mouth and listened for the returning echoes. By this method, he could determine the locations of open doorways, people, parked cars, garbage cans, and so on. He was echolocating: bouncing his sound waves off objects in the environment and listening to what returned.²⁹

A documentary about Ben kicked off with the statement that he was “the only person in the world who can see with echolocation.”³⁰ The statement was erroneous in a couple of ways. First, Ben may or may not have seen the way a sighted person would think of sight; all we know is that his brain was able to convert sound waves into some practical understanding of the large objects in front of him. But more on that later.

Second, and more important, Ben was not the only one using echo-location: thousands of blind people have done so.³¹ In fact, the phenomenon has been discussed since at least the 1940s, when the word “echolocation” was first coined in a Science article titled “Echolocation by Blind Men, Bats, and Radar.”³² The author wrote, “Many blind persons develop in the course of time a considerable ability to avoid obstacles by means of auditory cues received from sounds of their own making.” This included their own footsteps, or cane tapping, or finger snapping. He demonstrated that their ability to successfully echolocate was drastically reduced by distracting noises or earplugs.

As we saw earlier, the occipital lobe can be taken over by many tasks, not just those of hearing. Memorization, for example, can benefit from the extra cortical real estate. In one study, blind people were tested to see how well they could remember lists of words. Those with more of their occipital cortex taken over could score higher: they had more territory to devote to the memory task.³³

The general story is straightforward: the more real estate, the better. This sometimes leads to counterintuitive results. Most people are born with three different types of photoreceptors for color vision, but some people are born with only two types, one type, or none, giving them diminished (or no) ability to discriminate among colors. However, color-blind people don’t have it all bad: they are better at distinguishing between shades of gray.³⁴ Why? Because they have the same amount of visual cortex, but fewer color dimensions to worry about. Using the same amount of cortical territory available for a simpler task gives improved performance. Although the military excludes color-blind soldiers from certain jobs, they have come to realize that the color-blind can spot enemy camouflage better than people with normal color vision.

And although we’ve been using the visual system to introduce the critical points, cortical redeployment happens everywhere. When people lose hearing, previously “auditory” brain tissue comes to represent other senses.³⁵ Thus, it won’t come as a surprise that deaf people have better peripheral visual attention, or that they can often see your accent: they can tell what part of the country you’re from, because they’re so good at lip-reading. Similarly, after an amputee loses a limb, the sensation on the stump becomes finer. Touch can now be sensed with lighter pressure, and two touches close together can be sensed as separate touches rather than a single one. Because the brain is now devoting more territory to the remaining, undamaged areas, sensing becomes higher resolution.

Neural redeployment replaces the old paradigm of predetermined brain areas with something more flexible. Territory can be reassigned to different tasks. There is nothing special about visual cortex neurons, for example. They are simply neurons that happen to be involved in processing edges or colors in people who have functioning eyes. These exact same neurons can process other types of information in the sightless.

The old paradigm would assert that the North American acreage labeled as Louisiana was predetermined for French people. The new paradigm is not surprised when the Louisiana Territory gets sold and citizens from around the globe set up shop there.

Given that the brain has to distribute all its tasks across the finite volume of the cortex, it may be that some disorders arise from suboptimal distributions. One example is autistic savantism, in which a child who has severe cognitive and social deficits might be a virtuoso at, say, memorizing the phone book, or copying down visual scenes, or solving the Rubik’s Cube with stunning speed. The pairing of cognitive disabilities with outstanding talents has attracted many theories; one of relevance here is an unusual distribution of cortical real estate.³⁶ The idea is that atypical feats can be accomplished when the brain devotes an unusually large swath of its real estate to one task (such as memorization, or visual analysis, or puzzles). But these human superpowers come at the expense of other tasks among which brains normally divide their territory, such as all the subtasks that add up to reliable social skills.

BLINDINGLY FAST

Recent decades have yielded several revelations about brain plasticity, but perhaps the biggest surprise is its rapidity. Some years ago, researchers at McGill University put several adults who had just recently lost their sight into a brain scanner. The participants were asked to listen to sounds. Not surprisingly, the sounds caused activity in their auditory cortex. But the sounds also caused activity in their occipital cortex—activity that would not have been there even a few weeks earlier, when the participants had sight. The activity wasn’t as strong as that seen in people who had been blind for a long while, but it was detectable nevertheless.³⁷

This demonstrated that the brain can implement changes rapidly when vision disappears. But how rapidly?

The researcher Alvaro Pascual-Leone began to wonder about the speed at which these major brain changes can take place. He noted that aspiring instructors at a school for the blind were required to blindfold themselves for seven full days to gain firsthand understanding of their students’ living experiences. Most of the instructors become aware of enhanced skills with sounds—orienting to them, judging their distance, and identifying them:

Several describe becoming able to identify people quickly and accurately as they started talking or even as they simply walked by due to the cadence of their steps. Several learned to differentiate cars by the sounds of their motors, and one described the “joy of telling motorcycles apart by their sound.”³⁸

This got Pascual-Leone and his colleagues considering what would happen if a sighted person were blindfolded in a laboratory setting for several days. They launched the experiment, and what they found was nothing short of remarkable. They discovered that neural reorganization—the same kind seen in blind subjects—also happens with temporary blindness of sighted subjects. Rapidly.

In one of their studies, sighted participants were blindfolded for five days, during which time they were put through an intensive Braille-training paradigm.³⁹ At the end of five days, the subjects had become quite good at detecting subtle differences between Braille characters—much better than a control group of sighted participants who underwent the same training without a blindfold.

But especially striking was what happened to their brains, as measured in the scanner. Within five days, the blindfolded participants had recruited their occipital cortex when they were touching objects. Control subjects, not surprisingly, used only their somatosensory cortex. The blindfolded subjects also showed occipital responses to sounds and words.

When this new occipital lobe activity was intentionally disrupted in the laboratory by magnetic pulses, the Braille-reading advantage of the blindfolded subjects went away—indicating that the recruitment of this brain area was not an accidental side effect but a critical piece of the improved behavioral performance.

When the blindfold was removed, the response of the occipital cortex to touch or sound disappeared within a day. At that point, the participants’ brains returned to looking indistinguishable from every other sighted brain out there.

In another study, the visual areas of the brain were carefully mapped out using more powerful neuroimaging techniques. Participants were blindfolded, put in a scanner, and asked to perform a touching task that required fine discrimination with their fingers. In these conditions, investigators could detect activity emerging in the primary visual cortex after a blindfolding session of a mere forty to sixty minutes.⁴⁰

The shock of these findings was their sheer speed. The shape shifting of brains is not like the glacial drifting of continental plates, but can instead be remarkably swift. In later chapters, we’ll see that visual deprivation causes the unmasking of already-existing nonvisual input into the occipital cortex, and we’ll come to understand how the brain is always sprung like a mousetrap to implement rapid change. But for now the important point is that the brain’s changes are more brisk than even the most optimistic neuroscientist would have dared to guess at the beginning of this century.

Let’s zoom back out to the bigger picture. Just as sharp teeth and fast legs are useful for survival, so is neural flexibility: it allows brains to optimize performance in a variety of environments.

But the competition in the brain has a potential downside as well. Whenever there’s an imbalance of activity in the senses, a potential takeover can happen, and it can happen rapidly. A redistribution of resources can be optimal when a limb or a sense has been permanently amputated or lost, but the rapid conquest of territory may have to be actively combated in other scenarios. And this consideration led me and my former student Don Vaughn to propose a new theory for what happens to brains in the dark of night.

WHAT DOES DREAMING HAVE TO DO WITH THE ROTATION OF THE PLANET?

One of the unsolved mysteries in neuroscience is why brains dream. What are these bizarre nighttime hallucinations about? Do they have meaning? Or are they simply random neural activity in search of a coherent narrative? And why are dreams so richly visual, igniting the occipital cortex every night into a conflagration of activity?

Consider the following: In the chronic and unforgiving competition for brain real estate, the visual system has a unique problem to deal with. Because of the rotation of the planet, it is cast into darkness for an average of twelve hours every cycle. (This refers to 99.9999 percent of our species’ evolutionary history, not to the current, electricity-blessed times.) We’ve already seen that sensory deprivation triggers neighboring territories to take over. So how does the visual system deal with this unfair disadvantage?

By keeping the occipital cortex active during the night.

We suggest that dreaming exists to keep the visual cortex from being taken over by neighboring areas. After all, the rotation of the planet does not affect anything about your ability to touch, hear, taste, or smell; only vision suffers in the dark. As a result, the visual cortex finds itself in danger every night of a takeover by the other senses. And given the startling rapidity with which changes in territory can happen (remember the forty to sixty minutes we just saw), the threat is formidable. Dreams are the means by which the visual cortex prevents takeover.

To better understand this, let’s zoom out. Although a sleeper looks as though he is relaxed and shut down, the brain is fully electrically active. During most of the night, there is no dreaming. But during REM (rapid eye movement) sleep, something special happens. The heart rate and breathing speed up, small muscles twitch, and the brain waves become smaller and faster. This is the stage of sleep in which dreaming occurs.⁴¹ REM sleep is triggered by a particular set of neurons in a brainstem structure called the pons. The increased activity in these neurons has two consequences. The first is that the major muscle groups become paralyzed. Elaborate neural circuitry keeps the body frozen during dreaming, and its elaborateness supports the biological importance of dream sleep; presumably, this circuitry would be unlikely to evolve without an important function behind it. The muscular shutdown allows the brain to simulate world experience without actually moving the body around.

The second consequence is the really important one: waves of spikes travel from the brainstem to the occipital cortex.⁴² When the spikes arrive there, the activity is experienced as visual. We see. This activity is why dreams are pictorial and filmic, instead of conceptual or abstract.

During dream sleep, waves of activity begin in the brainstem and end in the occipital cortex. We suggest this infusion of activity is necessitated by the rotation of the planet into darkness: the visual system needs special strategies to keep its territory intact.

This combination crafts the experience of dreaming: the invasion of the electrical waves into the occipital cortex makes the visual system active, while the muscular paralysis keeps the dreamer from acting on the experiences.

We theorize that the circuitry behind visual dreams is not accidental. Instead, to prevent takeover, the visual system is forced to fight for its territory by generating bursts of activity when the planet rotates into darkness.⁴³ In the face of constant competition for sensory real estate, an occipital self-defense evolved. After all, vision carries mission-critical information, but it is stolen away for half of our hours. Dreams, therefore, may be the strange love child of neural plasticity and the rotation of the planet.

A key point to appreciate is that these nighttime volleys of activity are anatomically precise. They begin in the brainstem and are directed to only one place: the occipital cortex. If the circuitry grew its branches broadly and promiscuously, we’d expect it to connect with many areas throughout the brain. But it doesn’t. It aims with anatomical exactitude at one area alone: a tiny structure called the lateral geniculate nucleus, which broadcasts specifically to the occipital cortex. Through the neuroanatomist’s lens, this high specificity of the circuit suggests an important role.

From this perspective, it should be no surprise that even a person born blind retains the same brainstem-to-occipital-lobe circuitry as everyone else. What about the dreams of blind people? Would they be expected to have no dreaming at all because their brains don’t care about darkness? The answer is instructive. People who have been blind from birth (or were blinded at a very young age) experience no visual imagery in their dreams, but they do have other sensory experiences, such as feeling their way around a rearranged living room or hearing strange animals barking.⁴⁴ This matches perfectly with the lessons we learned a moment ago: that the occipital cortex of a blind person becomes annexed by the other senses. Thus, in the congenitally blind, nighttime occipital activation still occurs, but it is now experienced as something nonvisual. In other words, under normal circumstances, your genetics expect that the unfair disadvantage of darkness is best combated by sending waves of activity at night to the occipital lobe; this holds true in the brain of the blind, even though the original purpose is lost. Note also that people who become blind after the age of seven have more visual content in their dreams than those who become blind earlier—consistent with the fact that the occipital lobe in the late-blind is less fully conquered by other senses, and so the activity is experienced more visually.⁴⁵

As an interesting side note, two other brain areas, the hippocampus and the prefrontal cortex, are less active during dream sleep than during the waking state, and this presumably accounts for our difficulty remembering our dreams. Why does your brain shut down these areas? One possibility is that there is no need to write memory if the central purpose of dream sleep is to keep the visual cortex actively fighting off its neighbors.

We can learn a great deal from a cross-species perspective. Some mammals are born immature—meaning they’re unable to walk, get food, regulate their own temperature, or defend themselves. Examples are humans, ferrets, and platypuses. Other mammals are born mature—such as the guinea pig, sheep, and giraffe—all of whom come out of the womb with teeth, fur, open eyes, and an ability to regulate their temperature, walk within an hour of being born, and eat solid food. Here’s the important clue: the animals born immature have much more REM sleep—up to about eight times as much—and this difference is especially clear in the first month of life.⁴⁶ In our interpretation, when a highly plastic brain drops into the world, it needs to constantly fight to keep things balanced. When a brain arrives mostly solidified, there is less need for it to engage in the nighttime fighting.

Moreover, look at the falloff in REM sleep with age. All mammalian species spend some fraction of their sleep time in REM, and that fraction steadily decreases as they get older.⁴⁷ In humans, infants spend half of their sleeping time in REM, adults spend only 10–20 percent of sleep in REM, and the elderly spend even less. This cross-species trend is consistent with the fact that infants’ brains are so much more plastic (as we will see in chapter 9), and thus the competition for territory is even more critical. As an animal gets older, cortical takeovers become less possible. The falloff in plasticity parallels the falloff of time spent in REM sleep.

This hypothesis leads to a prediction for the distant future, when we discover life on other planets. Some planets (especially those orbiting red dwarf stars) become locked into place, such that they always have the same surface facing their star: they thus have permanent day on one side of the planet, and permanent night on the other.⁴⁸ If life-forms on that planet were to have livewired brains even vaguely similar to ours, the prediction would be that those on the daylight side of the planet might have vision like us but would not have dreams. The same prediction would apply for very fast-spinning planets: if the nighttime is shorter than the time of a cortical takeover, then dreaming would also be unnecessary. Thousands of years hence, we might finally know whether we dreamers are in the universal minority.

AS OUTSIDE, SO INSIDE

Most visitors to Admiral Nelson’s statue in London’s Trafalgar Square have probably not considered the distortion of the somatosensory cortex in the left hemisphere of that elevated head. But they should. It exposes one of the most remarkable feats of the brain: the ability to optimally encode the body it is dealing with.

We’ve seen so far that changes to sensory inputs (as with amputation or blindness or deafness) lead to massive cortical reorganization. The brain’s maps are not genetically pre-scripted but instead molded by the input. They are experience-dependent. They are an emergent property of local border competitions rather than the result of a pre-specified global plan. Because neurons that fire together wire together, co-activation establishes neighboring representations in the brain. No matter the shape of your body, it will naturally end up mapped on the brain’s surface.

Evolutionarily, such activity-dependent mechanisms allow natural selection to quickly test out innumerable varieties of body types—from claws to fins, wings to prehensile tails. Nature does not need to genetically rewrite the brain each time it wants to try out a new body plan; it simply lets the brain adjust itself. And this underscores a point that reverberates throughout this book: the brain is very different from a digital computer. We’ll want to abandon our notions of traditional engineering and keep our eyes wide open as we move deeper into the neural terrain.

The shape shifting around the body plan illustrates what happens in all sensory systems. We saw that when people are born blind, their “visual” cortex becomes tuned to hearing, touch, and other senses. And the perceptual consequence of the cortical takeover is increased sensitivity: the more real estate the brain devotes to a task, the higher resolution it has.

Finally, we discovered that when people with normal visual systems are blindfolded for as little as an hour, their primary visual cortex becomes active when they perform tasks with their fingers or when they hear tones or words. Removing the blindfold quickly reverts the visual cortex so that it responds only to visual input. As we’ll see more in upcoming chapters, the brain’s sudden ability to “see” with the fingers and ears depends on connections from other senses that are already there but not used so long as the eyes are sending data.

Collectively, these considerations led us to propose that visual dreams are a by-product of neural competition and the rotation of the planet. An organism that wishes to keep its visual system from takeover by the other senses must devise a way to keep the visual system active when the darkness sets in.

So now we’re ready for a question. We’ve painted the picture of an extremely flexible cortex. What are the limits of its flexibility? Can we feed any kinds of data into the brain? Would it simply figure out what to do with the data it receives?