The plastic brain

The human brain's 'plasticity' means that it may be learning and adapting even as you read this

Published: Sunday 31 October 1999

Varying shades of grey matter< (Credit: SCIENCE PHOTO LIBRARY)it has intrigued scientists for a long time now. There is no limit to the number of surprises that the human brain can throw up -- the more you discover, the more dynamic it seems. Some call it the world's most complex computer. But recent reports reveal that its 'plasticity' -- the ability to adapt -- makes even the best of computers seem ordinary when compared to this relatively small mass of 'organic circuitry'.

Plasticity implies that the human brain is more flexible and dynamic than previously thought. It is not a computer programme that is finalised by age two or three. Rather, it is growing and developing constantly. "Growth is an intrinsic property of the brain," according to Ira Black, chairperson of neuroscience at the Robert Wood Johnson Medical School in Piscataway, New Jersey, usa . "The notion that the brain is not simply a hard-wired switchboard that can't be changed has led to a revolution in how we approach diseases of the brain or the mind," Black adds.

A report in the Washington Post cites the example of the 18-year-old Maranda Francisco. At the age of four, she was suffering from a rare form of epilepsy that caused as many as 120 seizures a day, throwing the right side of her body into continual paralysis. Surgeons at Johns Hopkins Hospital in Baltimore removed the entire left side of her brain in an operation called "radical hemispherectomy". She made a stunning recovery; there were no more seizures, her speech returned to normal, and she made up lost ground in grade school. Despite initial weakness, she regained nearly full use of her right arm and leg.

The report mentions that today, Maranda leads a remarkably active teenage life as the right side of her brain took over the functions of the missing left side. "Does function transfer? Sure," answers John M Freeman, director of the paediatric epilepsy centre at Johns Hopkins Hospital. "How does it happen? I do not have any idea," he concedes. Black says the idea of the ever-changing, dynamic brain should not have come as a surprise to neuroscientists. The Washington Post report mentions that common tasks like recalling a new friend's face or phone number cannot be performed until something changes in the brain.

"Any time you learn, you've changed the state of your brain," points out Jordan Grafman, chief of the cognitive neurosciences section at the National Institute of Neurological Diseases and Stroke ( ninds ) in Bethesda, usa . "That's plasticity."

The brain is like an incredibly detailed map of a constantly changing topography, the Washington Post report mentions. "I always think of the Balkans -- all those countries that have come and gone and changed their boundaries over the 20th century," says Grafman. The change in the way we view the brain is partly due to advances in neuroimaging techniques that allow scientists to see inside the living brain and "map" its shifting territories. Images from positron emission tomography ( pet ) and magnetic resonance imaging ( mri ) enable scientists to track changes in the brain even as they occur. Activated areas of the brain "light up" on these scans, revealing increased blood flow and electrical energy.

There are up to 100 billion nerve cells (neurones) in the human brain. About 10 billion of these are in the neocortex or the outer layer of "grey matter", which is responsible for all forms of conscious experience. But the brain's real power comes from the connections of these cells, known as the synapses, where messages are exchanged in the form of electrical pulses. There can be thousands of links in each neurone, giving the average brain 100 trillion synapses.

These connections are strengthened with use, leading to the formation of an intricate circuitry of knowledge and memory. Cells that wire together, fire together, say neuroscientists. The connections in an active adult brain increase in number, complexity and sophistication, the Washington Post report points out. Recent research has proved wrong the belief that the brain operates like a circuit board, programmed by thousands of genes shortly before and soon after birth.

The report mentions that in blind people who learn Braille, the role of the visual cortex, which normally helps people see, is "invaded" and taken over by the sense of touch. Brain scans show that the reading finger in Braille readers "recruits" the unused visual cortex for its needs. Another example is of right-handed children who learn to play the violin, cello or guitar, using fingers of the left hand on the instrument's strings, which requires dexterity. The part of the brain dedicated to processing signals from the left hand increases markedly. Brains scans show the expanding "cortical territory" of the left-hand fingers.

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