proper world experience. Without it, the brain becomes malformed and pathological. Like a tree that needs nutrient-rich soil to arborize, a brain requires the rich soil of social and sensory interaction.
With this background in hand, we now see that the brain leverages its environment to shape itself. But how, exactly, does it absorb the world—especially from inside its dark cave? What happens when a person loses an arm or goes deaf? Does a blind person actually enjoy better hearing? And what does any of this have to do with why we dream?
3
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
