http://web.mit.edu/newsoffice/2011/brain-language-0301.html


Parts of brain can switch functions
 
In people born blind, brain regions that usually process vision can tackle
language.
 
Anne Trafton, MIT News Office



 

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In these fMRI brain scans, regions that are active during language
processing tasks appear as red or blue (for two different types of tasks.)
Parts of the left visual cortex are active in blind participants, but not
sighted participants. 
Image courtesy of the Rebecca Saxe laboratory



March 1, 2011



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When your brain encounters sensory stimuli, such as the scent of your
morning coffee or the sound of a honking car, that input gets shuttled to
the appropriate brain region for analysis. The coffee aroma goes to the
olfactory cortex, while sounds are processed in the auditory cortex.

That division of labor suggests that the brain's structure follows a
predetermined, genetic blueprint. However, evidence is mounting that brain
regions can take over functions they were not genetically destined to
perform. In a landmark 1996 study of people blinded early in life,
neuroscientists showed that the visual cortex could participate in a
nonvisual function - reading Braille.

Now, a study from MIT neuroscientists shows that in individuals born blind,
parts of the visual cortex are recruited for language processing. The
finding suggests that the visual cortex can dramatically change its function
- from visual processing to language - and it also appears to overturn the
idea that language processing can only occur in highly specialized brain
regions that are genetically programmed for language tasks.

"Your brain is not a prepackaged kind of thing. It doesn't develop along a
fixed trajectory, rather, it's a self-building toolkit. The building process
is profoundly influenced by the experiences you have during your
development," says Marina Bedny, an MIT postdoctoral associate in the
Department of Brain and Cognitive Sciences and lead author of the study,
which appears in the Proceedings of the National Academy of Sciences the
week of Feb. 28.

Flexible connections

For more than a century, neuroscientists have known that two specialized
brain regions - called Broca's area and Wernicke's area - are necessary to
produce and understand language, respectively. Those areas are thought to
have intrinsic properties, such as specific internal arrangement of cells
and connectivity with other brain regions, which make them uniquely suited
to process language.

Other functions - including vision and hearing - also have distinct
processing centers in the sensory cortices. However, there appears to be
some flexibility in assigning brain functions. Previous studies in animals
(in the laboratory of Mriganka Sur, MIT professor of brain and cognitive
sciences) have shown that sensory brain regions can process information from
a different sense if input is rewired to them surgically early in life. For
example, connecting the eyes to the auditory cortex can provoke that brain
region to process images instead of sounds.

Until now, no such evidence existed for flexibility in language processing.
Previous studies of congenitally blind people had shown some activity in the
left visual cortex of blind subjects during some verbal tasks, such as
reading Braille, but no one had shown that this might indicate full-fledged
language processing.

Bedny and her colleagues, including senior author Rebecca Saxe, assistant
professor of brain and cognitive sciences, and Alvaro Pascual-Leone,
professor of neurology at Harvard Medical School, set out to investigate
whether visual brain regions in blind people might be involved in more
complex language tasks, such as processing sentence structure and analyzing
word meanings.

To do that, the researchers scanned blind subjects (using functional
magnetic resonance imaging) as they performed a sentence comprehension task.
The researchers hypothesized that if the visual cortex was involved in
language processing, those brain areas should show the same sensitivity to
linguistic information as classic language areas such as Broca's and
Wernicke's areas. 

They found that was indeed the case - visual brain regions were sensitive to
sentence structure and word meanings in the same way as classic language
regions, Bedny says. "The idea that these brain regions could go from vision
to language is just crazy," she says. "It suggests that the intrinsic
function of a brain area is constrained only loosely, and that experience
can have really a big impact on the function of a piece of brain tissue."

Amir Amedi, a neurophysiologist at the Hebrew University of Jerusalem, says
the paper convincingly shows that the left occipital cortex is processing
language. "I think it suggests that in principle, and if the changes are
forced early in development and early in life, any brain area can change its
skin and do any task or function," he says. "This is pretty dramatic."

Bedny notes that the research does not refute the idea that the human brain
needs Broca's and Wernicke's areas for language. "We haven't shown that
every possible part of language can be supported by this part of the brain
[the visual cortex]. It just suggests that a part of the brain can
participate in language processing without having evolved to do so," she
says.

Redistribution

One unanswered question is why the visual cortex would be recruited for
language processing, when the language processing areas of blind people
already function normally. According to Bedny, it may be the result of a
natural redistribution of tasks during brain development.

"As these brain functions are getting parceled out, the visual cortex isn't
getting its typical function, which is to do vision. And so it enters this
competitive game of who's going to do what. The whole developmental dynamic
has changed," she says.

This study, combined with other studies of blind people, suggest that
different parts of the visual cortex get divvied up for different functions
during development, Bedny says. A subset of (left-brain) visual areas
appears to be involved in language, including the left primary visual
cortex. 

It's possible that this redistribution gives blind people an advantage in
language processing. The researchers are planning follow-up work in which
they will study whether blind people perform better than sighted people in
complex language tasks such as parsing complicated sentences or performing
language tests while being distracted.

The researchers are also working to pinpoint more precisely the visual
cortex's role in language processing, and they are studying blind children
to figure out when during development the visual cortex starts processing
language.

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