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