|Visual Cortex Activity in the Blind|
By Melissa Lee Phillips |
Neuroscience for Kids Consultant
May 20, 2002
Scientists used to believe that each area of the human brain was
specialized only for particular tasks. It was assumed that an area of the
brain that processes a certain type of information can process only
that type of information and can never change its function. Over the past
few years, it has become apparent that the brain actually exhibits more
plasticity -- the ability to change and form new and
different neural connections -- than originally thought. For example, the
occipital cortex is thought to be used mainly
for visual processing. If someone is blind, though, what happens to this
area? Is it simply not used? Can the brain adapt and use these neurons for
something else? A study by Dr. Harold Burton and his colleagues at
Washington University in St. Louis has attempted to answer some of these
Sixteen blind people were studied: nine were blind from birth ("early-blind") and seven became blind later in life ("late-blind"). Using functional magnetic resonance imaging (fMRI), the scientists measured cerebral blood flow while the subjects read Braille words and while they read sequences of nonsense Braille. The major activity detected while the subjects read real words occurred in the visual cortex, even though all the subjects were completely blind. The early-blind subjects had even more activity in the visual cortex than the late-blind subjects. More specifically, the people who were blind from birth had more activation in occipital-temporal cortex areas called V5/MT and V8 and in the occipital cortex on the side of the brain opposite their reading hand.
The researchers think it is possible that the traditionally visual areas might have been recruited for some other function, possibly for processing touch input they receive by reading with their hands. This might explain why the early-blind subjects had more activation in the visual areas: they didn't have the "normal" neural connections to begin with, so these brain areas were free to develop for some other type of input. The late-blind subjects, though, already had many visual connections. It may be more difficult for the visual cortex to adapt to another type of input if these neurons were originally dedicated to visual input.
A second possibility is that these areas are not used solely for processing visual information but are used more generally for encoding information that will later be processed by the brain's language centers. Sighted people often encode visual information for this purpose, but blind people would encode touch information instead. In both cases, the activity is essentially the same: coding information for later use. The scientists stress that more studies will be needed to determine which -- if either -- of these hypotheses is correct.
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