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,