[Nfbf-l] Super Powers for the Blind and Deaf, Long article from Scientific American

[Alan Dicey]

Super Powers for the Blind and Deaf
It's an oft-repeated idea that blind people can compensate for their lack of 
sight with enhanced hearing or other abilities. The musical talents of 
Stevie Wonder and Ray Charles, both blinded at an early age, are cited as 
examples of blindness conferring an advantage in other areas. Then there's 
the superhero Daredevil, who is blind but uses his heightened remaining 
senses to fight crime.

It is commonly assumed that the improvement in the remaining senses is a 
result of learned behavior; in the absence of vision, blind people pay 
attention to auditory cues and learn how to use them more efficiently. But 
there is mounting evidence that people missing one sense don't just learn to 
use the others better. The brain adapts to the loss by giving itself a 
makeover. If one sense is lost, the areas of the brain normally devoted to 
handling that sensory information do not go unused - they get rewired and 
put to work processing other senses.

A new study provides evidence of this rewiring in the brains of deaf people. 
The study, published in The Journal of Neuroscience, shows people who are 
born deaf use areas of the brain typically devoted to processing sound to 
instead process touch and vision. Perhaps more interestingly, the 
researchers found this neural reorganization affects how deaf individuals 
perceive sensory stimuli, making them susceptible to a perceptual illusion 
that hearing people do not experience.

These new findings are part of the growing research on neuroplasticity, the 
ability of our brains to change with experience. A large body of evidence 
shows when the brain is deprived of input in one sensory modality, it is 
capable of reorganizing itself to support and augment other senses, a 
phenomenon known as cross-modal neuroplasticity.

Understanding how the brain rewires itself when a sense is lost has 
implications for the rehabilitation of deaf and blind individuals, but also 
for understanding when and how the brain is able to transform itself.
Researchers look to the brains of the deaf and blind for clues about the
limits of brain plasticity and the mechanisms underlying it. So far, it
appears that some brain systems are not very plastic and cannot be changed
with experience. Other systems can be modified by experience but only during
particular sensitive periods (as is the case with language acquisition).
Finally, some neural systems remain plastic and can be changed by experience
throughout life. Discovering factors that promote brain plasticity will
impact several areas: how we educate normally developing as well as blind
and deaf children; rehabilitation after brain injury; and the treatment (and
possible reversal) of neurodegenerative diseases and age-related decline.

Most of the research on cross-modal neuroplasticity has focused on blind
individuals, who often have enhanced auditory abilities. Brain imaging
studies show the visual cortex in the blind is taken over by other senses,
such as hearing or touch, and contributes to language processing. However,
researchers do not know as much about how deafness changes the auditory
cortex. Helen Neville, one of the authors of the new study, previously
showed people born deaf are better at processing peripheral vision and
motion. Some animal studies indicate both vision and touch play a role in
altered cross-modal organization of auditory cortex, but until now, the
evidence in humans has been limited.

In the new study, Christina Karns, Neville, and Mark Dow of the University
of Oregon asked how the early loss of hearing affected neuroplasticity in
the deaf brain, and if that reorganization translated to altered perceptual
abilities in deaf people. They were especially interested to see if deafness
would affect how the brain processes touch and vision together, a question
that had not been answered before now because it required precise tactile
stimuli. Karns and her co-authors designed a unique device worn like
headphones by the study's participants while inside an fMRI scanner.
Flexible tubing, connected to a compressor in an adjacent room, delivered
puffs of air to precise locations on participants' faces. These served as
the tactile stimuli. Visual stimuli - brief pulses of light - were delivered
through fiber optic cables mounted directly below the air-puff nozzle.

The researchers used the fMRI scanner to monitor changes in blood flow to
different areas of the brain, with increased blood flow indicating an
increase in brain activity. They paid special attention to an area known as
Heschl's gyrus, which is the site of the primary auditory cortex in the
human brain.

The participants included 13 congenitally deaf adults and 12 hearing adults
for comparison. The deaf participants had more activation in the auditory
cortex in response to touch and visual stimuli than did the hearing
participants. Karns and her colleagues were surprised to find the primary
auditory cortex in deaf people responded even more to touch than to vision.

The researchers took advantage of a known perceptual illusion in hearing
people to look at how deaf brains processed both touch and vision together.
The auditory-induced double-flash is a phenomenon in which a single flash of
light, paired with two or more brief sounds, is perceived as multiple
flashes of light. The researchers modified the illusion for their experiment
by using a double puff of air as a tactile stimulus to replace the auditory
stimulus. Deaf people were susceptible to the illusion of a double flash of
light when a single flash was paired with double air puffs; hearing people
were not.

The deaf participants all showed greater activity in Heschl's gyrus in
response to the air puffs and light, but their responses differed in degree.
Those with the highest level of auditory cortex activity in response to
touch also had the strongest response to the illusion. This supports the
interpretation that the double-flash illusion is a functional consequence of
the altered cross-modal organization in the deaf brain.

Cross-modal plasticity can cause problems. If the brain has reorganized
itself to compensate for the loss of hearing, what happens when hearing is
restored? Stephen Lomber, a psychologist who studies cross-modal plasticity
at the University of Western Ontario, compares it to a cottage you aren't
using, so you let a friend stay there. The friend gets comfortable,
rearranges the furniture, and settles in. If you come back, they may not
want to leave. This could be why older people who are partially deaf find
hearing aids confusing or unhelpful.

Cross-modal reorganization may also interfere with the success of cochlear
implants. Incomplete reversal of deafness-induced brain reorganization might
limit the benefit from cochlear implants, especially in children born deaf
who receive implants after the age of four. These children, who have lacked
auditory input since birth, may struggle with language comprehension and
speech because the auditory areas of their brain have taken on the
processing of other senses. Karns and other researchers studying cross-modal
plasticity hope a better understanding of how the brain reorganizes will
ultimately help deaf people. The ability to measure how much the auditory
cortex has been repurposed for other sensory processing could help in
designing intervention programs to retrain the auditory cortex to process
sounds again. This research will not produce a real-life Daredevil, but it
is a reminder that our brains have some hidden superpowers.