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 canienes 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 thoughsand. 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.

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 strenthen 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.

But why does the map change when the input changes?

-- Page33-35 LIVEWIRED by David Eagleman