Phantom limbs and the wiring of the neocortex

A story from NPR this morning, “How Do You Amputate a Phantom Limb?,” got me thinking about the amazing integration of the body and the brain.

Most of us have heard about the phenomenon of phantom limbs experienced by many people who have had limbs amputated. On the NPR segment, Radio Lab interviewers review a story told by Dr. V.S. Ramachandran of the University of California at San Diego, who had a patient plagued by pain in his phantom arm. Ramachandran was able to use a mirror to simulate the missing limb and teach the patient’s brain that the limb was really gone, after which the pain ceased.

For me, this resonates with the brain theory propounded by Jeff Hawkins in his book On Intelligence. Hawkins is best known as the computer architect who founded Palm Computing, but he is also educated as a neuroscientist. His book makes an interesting case that intelligent machines are possible but that the conventional artificial intelligence (AI) model is wrongheaded.

However, in discussing how brains work, he helps the reader to appreciate that the makeup of the brain of any creature is very much determined by that creature’s physical and sensory makeup.

The traditional, rather stereotyped idea of the brain is that brains are mapped out according to various cognitive and sensory functions. And on one level that is true. But Hawkins makes the point that the cortex is an extremely flexible structure:

… the wiring of the neocortex is amazingly “plastic,” meaning it can change and rewire itself depending on the type of inputs flowing into it. For example, newborn ferret brains can be surgically rewired so that the animals’ eyes send their signals to the areas of cortex where hearing normally develops. The surprising result is that the ferrets develop functioning visual pathways in the auditory portions of their brains … they see with brain tissue that normally hears sounds.

Commenting on human brains, Hawkins writes that

Adults who are born deaf process visual information in areas that normally become auditory regions. And congenitally blind adults use the rearmost portion of their cortex, which ordinarily becomes dedicated to vision, to read braille. Since braille involves touch, you might think it would primarily activate touch regions — but apparently no area of cortex is content to represent nothing. The visual cortex, not receving information from the eyes like it is “supposed” to, casts around for other input patterns to sift through — in this case, from other cortical regions.

What this shows, he maintains, is that “brain regions develop specialized functions based largely on the kind of information that flows into them during development.”

Ramachandran’s results with phantom limb pain support the idea that the brain maintains its flexibility in adulthood and its sensory areas can be rewired in dramatic ways.

AB — 18 March 2009


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