August 2000
By Anne Pycha
Imagine a busy city. Downtown, in an old brick building that serves as city hall, a mapmaker named Cervella sits in her cluttered office on the fifth floor, laboring to keep the city maps up to date. It's no easy task. Sometimes the population swells and new neighborhoods spring up on the periphery, swallowing the surrounding farmland. In difficult times, though, people flee elsewhere, buildings are abandoned, and restaurants close. She's been at this job for forty years, and she's seen it all.
Today, Cervella bends over an aerial photograph of a sprawling suburb. The residents have voted to formally annex the suburb into the city, so now she must redraw the borders. "People behave in the funniest ways. Everything changes, but I don't mind," Cervella chuckles to herself and takes a sip of coffee. "It keeps me employed."
If she ever did look for a new line of work, Cervella's attitude might serve her well in the field of neuroscience. The cortex contains maps, too - but instead of city neighborhoods, these maps represent our skills and our knowledge of the world. And the brain's mapmakers are kept very busy, indeed. When a skill develops or changes, the cortical maps also change, and neuron populations may be annexed for specific purposes, later abandoned, and sometimes annexed again.
Maps in the Brain
Let's take an example. Suppose you learn a new manual skill, such as playing the guitar. After months of steady practice, you take a look at your hands. It's obvious that they have not grown or shrunk, except for maybe a new callus or two. But something else has changed: your brain has been quietly recruiting new neuron populations to support your guitar-playing skill. In particular, the cortical maps of your hands have grown, reflecting the adeptness with which you can now manipulate the strings of the guitar. In other words, your brain has changed.
The brain is plastic: it can and does remodel itself, sometimes within a remarkably short period of time.
Not long ago, many neuroscientists believed that the connections among neurons firmly established themselves within the first few weeks of life, and that cortical maps were fixed and unchangeable. Cervella sighs when she hears this: "Well, not long ago, even the best mapmakers never dreamed they'd have to draw a unified Berlin." But times have changed. Thanks to twenty years of research, we now know that the brain is plastic: it can and does remodel itself, sometimes within a remarkably short period of time.
Adult rats and monkeys have provided some of the most concrete evidence of brain plasticity. Rats, for example, are heavily reliant on their whiskers to send sensory information to the brain. When a rat learns to use his whiskers to discriminate the roughness of different surfaces (is it a sewer grate? is it a banana peel?), the cortical map of the whiskers can change within a matter of hours. Similarly, the cortical maps in a monkey's brain can expand within a matter of days as the monkey learns a new task, such as picking up a tiny ball, discriminating between sounds of different frequencies, or tracking a moving object with her eyes.
The Impact Of Behaviour
These biological changes in the adult brain aren't driven by developmental timelines or inherited traits. Instead, they are driven by behavioral experience. Just as the migratory behavior of residents can change the map of a city, so can our learning behavior change the maps in our brain, causing neurons populations to synchronize their actions, respond to new inputs, and support new skills.
But the brain doesn't rewire itself just for kicks. If your cortical map of auditory sounds changed every time you heard a new voice, you might not recognize your mother the next time she calls. And if the cortical map of your hands changed each time you tried to thread a needle or knead bread dough, your hands might become too specialized too quickly, leaving them unable to perform other important tasks. So what differentiates expert seamstresses and bakers from the rest of us? They don't just practice their trade every now and again: instead, they have paid special attention to their chosen skill, and have perfected that skill with intensive, repetitive practice.
Just as the migratory behavior of residents can change the map of a city, learning behavior can change the maps in our brain.
Let's go back to our guitar example. You can't really learn how to play the guitar if you pick it up once or twice a month, strum for a while, and then wander into the kitchen for a snack. In fact, it's pretty hard to learn anything this way, as your school teachers probably pointed out. When we approach learning casually, we're unlikely to become experts, and our brain is unlikely to rewire itself. When we approach learning seriously, however, something else happens: we attend to a task, we practice it over and over again, and we become emotionally involved. Under these conditions, brain plasticity happens - the winemaker can sharpen her taste buds, the blind person can learn to read Braille, the musician can perfect his pitch, and you can become an honest-to-goodness guitar player.
Practice Makes Perfect
Why are attention, repetition, and intensive practice the prerequisites of brain plasticity? Do we really have to listen to our teachers, go to class every day, and do homework every night? In 1890, philosopher and psychologist William James offered his thoughts to those of us who might have preferred a lazier route: "Millions of items of the outward order are present to my senses which never properly enter into my experience," he wrote. "Why? Because they have no interest for me. My experience is what I agree to attend to. Only those items which I notice shape my mind - without selective interest, experience is an utter chaos."
When we approach learning casually, we're unlikely to become experts, and our brain is unlikely to rewire itself.
When we notice a part of our experiential world or take a selective interest in a new skill, we analyze it - specifically, we take the trouble to examine how it works in space and time. For example, a person learning to read Braille analyzes which patterns of raised dots tend to occur next to one another on the page. A person learning music analyzes which notes tend to occur after one another in time. "Things juxtaposed in space impress us, and continue to be thought, in relation in which they exist there," observed James. "Things sequent in time, ditto."
The crucial role played by the dimensions of space and time doesn't end with our behavioral experience. As we've seen, brain maps change spatially by taking over neighboring neuronal populations on different parts of the cortex. But brain maps can also change in time, by synchronizing the actions of neurons more tightly so that a specific group of neurons may provide near-simultaneous responses to the same input. These timing relationships may actually help support the plasticity of existing cortical maps and the generation of new ones, because a single neuron can participate in the representation of several different sensory or motor representations at different times.
Timing is Everything
If we take a closer look at a single neuron and its synaptic connections, we see that timing is everything. Suppose a neuron sends weak, sporadic chemical messages to the another neuron. This situation is a bit like receiving postcards once every few years from a long-lost acquaintance - the messages aren't always effective enough to cause a sustained reaction in the second neuron. But now suppose that a neuron sends frequent and strong chemical messages, and these messages just happen to arrive when the other neuron is already activated. This situation is more like receiving love letters every day, from someone that you are really excited about. The letters help to cement your budding relationship, while the chemical messages help to create a lasting increase in the connection strength between the neurons. This strengthened connection can last for days or weeks (which amounts to a long-term commitment for cells accustomed to operating in millisecond timeframes), so scientists refer to it as long-term potentiation, or LTP.
It seems likely that changes at the synaptic level, such as LTP, contribute directly to changes in cortical maps, although scientists do not know exactly how this happens (neither does Cervella know exactly how the dynamics of individual households contribute to population changes in her city). We do know, however, that plasticity has a darker side: when a cortical map grows, another map often shrinks. The cortex has a limited supply of cells, so maps must compete with one another for neurons and synaptic space. And while long-term potentiation between neurons sounds like a happy romantic relationship, long-term depression also occurs, inhibiting synaptic communication.
Because plasticity comes at a certain expense, it makes sense that the brain protects itself from random, whimsical change by requiring a real investment from us. Without our attention, without our willingness to practice intensively, the brain just won't budge. It already possesses too many valuable skills, either built-in or learned, to change without a good reason. "Plasticity," said James, "means the possession of a structure weak enough to yield to an influence, but strong enough not to yield all at once."
