Researchers of the brain have long believed that once fully grown, it no longer changes. Only the young brain was thought to be “plastic”, as the ability to adapt is called in technical terms, and it was believed to lose this plasticity with time. However, research has shown that this theory is not completely correct.
Mapping monkey brains
When scientists first began to investigate in detail which areas of the brain control movement in individual parts of the body, at the beginning of the 20th century, they found that their results were not alike in all the monkeys they tested. The researchers thought at first that the discrepancies might be due to a lack of precision in their experiments, but it soon turned out that, although mapping brain functions was by no means a simple task, their puzzling findings were not caused by methodological errors.
Researchers wanted to find which area of the brain controls movement in various parts a monkey’s body. To do this they stimulated one region of the brain after another, moving methodically through specific points within each region and recording physical movements as they occurred. . By stimulating one point, for example, they triggered the movement of a finger; another point caused the entire hand to move. Looking back, it is hard not to reflect upon the monkey’s side of this purely mechanical investigation. As brain tissue is not susceptible to pain, the procedure was not painful for the animals, but it cannot have been very comfortable either.
More accurate measurements have since confirmed that these early brain-maps were correct in identifying discrepancies between individual brains. The point that triggered, for example, movement of the hand was not located in exactly the same place in all test subjects. It turned out that the maps were to some extent specific to every individual monkey, much as fingerprints are unique to every individual person.
The maps also revealed that the movements these animals performed together, in sequence, were controlled by neurons (brain cells) located close together. Scientists also discovered that the tissue directing the movements most characteristic of monkeys in general took up a larger proportion of the brain than tissue coordinating less typically “monkey-like” actions. If we were to illustrate this with an example from the human world, this would mean that the brain of a violinist has a substantially larger and more developed area controlling the fingers of the left hand than someone who has never played a musical instrument, an activity requiring great dexterity. Similarly, professional dancers possess a much larger area responsible for foot movement than people who only use their feet for walking.
Do brain maps change?
A question that soon arose from this research was how these areas in the brain were formed and parceled out; and consequently whether they could change during an individual’s lifetime. The more or less generally accepted belief of scientists for the greater part of the 20th century was that these areas took shape in early youth and could not undergo significant changes later. However, some skeptics decided to try and find out for themselves whether it was really true that the map of a monkey’s brain would not change with time.
It was during the period between the two world wars that scientists discovered that brain maps do indeed change, most likely in proportion to the use of individual muscles. The movements executed more frequently were represented to a greater extent within the brain than less frequent movements. However, these experiments went more or less unnoticed because of the already established view that a fully grown brain could not undergo any further structural changes.
Despite this consensus, in the seventies American neurologist Michael Merzenich and his colleagues decided to investigate more thoroughly how brain structure might change under different external influences. At first, Merzenich wanted to find out what effect there would be on a monkey’s brain if sensory information was cut from a specific part of its arm. He first accurately determined which areas of the monkey’s brain were responsible for processing the sensory stimuli from specific parts of the body. He then performed an operation that caused the monkey to lose feeling in one of its palms. After some time he mapped the monkey’s brain again and discovered that the area which had previously been responsible for the sensations in the thumb and its surroundings was not blank, but was now processing information from another part of the arm which still sent signals to the brain, because the nerves from that part of the arm had not been disconnected.
After this discovery he directed his research into less invasive experiments. He was curious whether the brain maps would also change if he taught the monkeys a new skill. He and his colleagues conducted a rather elaborate experiment in which they taught adult monkeys to use their fingers with great precision. The monkeys would get a reward if they used the correct amount of pressure while handling a device. For them this was not at all a simple task, because they had to invest a lot of effort into training their fingers in order to win their prize. It was no surprise that at the same time the researchers also discovered that the area in the monkey’s brain which controlled the movement of the fingers grew substantially with use.
How can the blind read?
The same characteristics that had been discovered in monkeys were later confirmed in humans. One interesting piece of research focused on how the brains of blind people could process Braille. Scientists first discovered that the area of the brain which processes the sensations in the reading finger of a blind person grows with practice. This was hardly surprising; what was more fascinating, though, was that the area responsible grows at the expense of the surrounding region, which interprets sensation in the Braille reader’s other fingers. This adjacent tissue diminishes as the “reading” cells become more developed.
Researchers stumbled on an even greater surprise when, using more modern methods, they discovered that during sleep, even in blind people, the area which is otherwise responsible for processing visual signals becomes active. They expected this part of the brain to be dormant in blind people, as they do not receive any signals from their eyes. In their dreams, however, it would seem that the blind still see; or still at least use the visual dimension of their minds.
Moreover, it also seems the brain is so adaptable or, technically speaking, plastic, that an area not being used adjusts and becomes specialized in other tasks. As recently as the mid-nineties, however, this principle would not have been widely accepted. The scientists who observed the brain activity of blind people reading Braille encountered great difficulties publishing their findings. Science magazine refused to print their work, because the editorial staff could not come to terms with how completely different parts of the brain could interconnect and work together, performing new tasks. In time, the article was published in the rival journal, Nature.
Later, new research revealed that the visual part of the brain in blind people is in fact maintained by the essential role it assumes in the fluent reading of Braille. Someone who can read Braille really well does not feel the dots on the paper at all, but directly perceives entire words, just as someone reading ordinary writing is not conscious of individual letters, but takes in words and sentences as visual units.
In the year 2000 there was even a case described in scientific literature of a woman who had been blind since early childhood and had become an extremely accomplished Braille reader at school. When she was 26, though, she suffered a stroke in the area otherwise responsible for processing visual information. It would seem reasonable that such a stroke should not have significant consequences for a blind person, but this case strongly suggested the opposite. For although the woman could still feel the dots of Braille under her fingers, she suddenly no longer understood their meaning. Her ability to read Braille was stored in the area that normally processes visual information and which had had been damaged by her stroke.
It has only recently become clear that even the older brain is much more plastic or adaptable than it was first presumed to be. Nevertheless, the fact does remain that the ability to adapt diminishes with time. Younger brains are simply much more plastic than older brains.
Michael Merzenich and his team, already mentioned pioneers in researching brain plasticity, set up two companies offering professional help in overcoming difficulties in brain function. Scientific Learning helps children with learning disabilities. Using special computer games which are designed to develop specific parts of the brain that cause these children to have problems, the company has achieved impressive results, success which has more recently led the group to create their Posit Science company, an organization focused on helping the older population. This firm has developed various “mental fitness” techniques to aid older people keep their brains in trim, and go some way to recovering their mental prime.