For years scientists thought most brain development stopped after a "critical period" in the first few years of life. Recent research on monkeys and other animals shows that the brain continually and dynamically reorganizes itself, even in adulthood. This finding helps explain how learning occurs and may lead to ways of improving recovery from learning disabilities, stroke, and other brain disorders through drug treatments or special "brain exercises."

      Old brains can learn new tricks. For years, scientists believed that connections between the brain's nerve cells, or neurons, develop by early childhood and then become fixed throughout life. In the last decade, however, animal research has revealed that brain areas routinely adjust the way they process information and retain the ability to take on new functions during adulthood.

The new findings:

·                 Reveal how human experiences and physical disorders affect the brain.

·                 Provide insights into the development of dyslexia and other learning disorders.

·                 Offer hope of improved recovery from injury, stroke, and brain disorders through drugs or training regimens.

      During early development, genes prompt the brain's neurons to form trillions of connections. These connections are fine-tuned by the neurons' electrical activity: useful connections are maintained or added, while others often disappear. Early experiments showed that many brain functions have a "critical period" during which most of this fine-tuning takes place -- usually the first few years after birth. Scientists once thought that, except for areas involved with memory, brain functions are usually stable after this time.
      In the 1980s, researchers made a surprising discovery. When nerve impulses in one finger of a monkey were blocked, the part of the brain that previously responded to touch at that finger began over several months to respond to signals from surrounding fingers. The deprived brain regions began responding to different nerves. This helps explain "phantom pain," in which people with part of their body amputated report intense feeling in or near a missing arm or leg -- usually when a nearby region is stimulated.
      Scientists also found that many brain regions' functions were organized differently every time they were examined. This happened even in brain areas unaffected by experiments. Changes in organization also followed limited damage to nerves for vision and hearing. This suggested that all brain areas continually adapt to changing signals.
      Scientists are still uncertain whether adult brain reorganization results from formation of new connections or strengthening of existing, previously unused connections. A loss or increase of neuron activity in a certain area may let normally silent connections gain the upper hand and win more brain territory.
      Understanding the brain's ability to dynamically reorganize itself, even in adulthood, helps scientists understand how patients sometimes recover brain functions damaged by injury or disease. While the brain can't grow new neurons, new neuron connections can emerge with surprising speed. Even learning to read by Braille can increase the brain territory responding to fingertip stimulation.
      Scientists are now looking for ways of making reorganization more likely to occur. Proteins called nerve growth factors are being tested in humans to see if they prompt brain reorganization after stroke and other disorders. Since reorganization seems to be influenced by neural activity, scientists are also testing special "brain exercises" designed to help the brain remodel itself in beneficial ways.
      Brain reorganization may also contribute to the symptoms of some diseases or slow recovery. Since the brain adapts to underlying problems, it must re-adapt once the problems are removed. Understanding how these changes occur may lead to ways of preventing damage and speeding recovery in learning disorders, stroke, and other nervous system diseases.

 

When nerve stimulation changes, as with amputation, the brain reorganizes. In one theory, signals from a finger and thumb of an uninjured person travel independantly to separate regions in the brain's thalamus (left). After amputation, however, neurons that formerly responded to signals from the finger respond to signals from the thumb (right).