Sooner is Better with CIs
Editor: The fact that kids who receive a cochlear implant (CI) early
in life are better able to understand spoken language has been known for
a long time. Now scientists at Stanford and the University of Maryland
have demonstrated one of the reasons. Their studies indicate that
children who receive their implant after the age of 30 months do not
develop the ability to integrate lipreading information with auditory
information. Here's the press release.
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STANFORD, Calif. - Cochlear implants allow the deaf to hear. Their
brains learn to understand the artificial electrical stimulation that
the implants provide to the cochlea as sound. A Stanford neurobiologist
teamed with child-development specialists at the University of Maryland
to see if children with cochlear implants were able to meld their newly
acquired hearing capability with their ability to read lips. In other
words, did their brains process speech the same way as people who are
born with the ability to hear?
In the Dec. 5 online edition of the Proceedings of the National
Academy of Sciences, the researchers discuss how they used a simple
auditory test and found that some children with cochlear implants fuse
the visual and auditory aspects of speech-just like people with normal
hearing. But the effect was only seen in children who received their
implants before the age of 30 months, adding to the body of evidence
suggesting that the earlier a hearing-impaired child receives a cochlear
implant, the better.
"I see this as a fascinating experiment that reveals the
tremendous capacity for plasticity in the developing brain," said
Eric Knudsen, PhD, the paper's senior author and the Edward C. and Amy
H. Sewall Professor at the Stanford University School of Medicine.
Most people don't appreciate that speech is a product of both hearing
and vision, explained Knudsen, who is also chair of Stanford's
Department of Neurobiology. The brain has learned that in conversation,
the lips and face make certain gestures that always occur together with
certain sounds. "The brain is always combining what it sees with
what it hears and making the best guess at what was said," he said.
In most situations, hearing speech alone is just fine-that's why we can
talk on telephones. But in noisy situations, when hearing is poor, our
brains rely heavily on lip and facial movements to figure out what
someone is saying.
Knudsen has a long history of studying the ability of the brain to
adapt to new circumstances and learn new skills. But most of his work is
done with owls. The inspiration for the study in children came from a
casual talk between Knudsen and Nathan Fox, PhD, director of the
University of Maryland' s Child Development Lab. In their conversation,
they discussed a phenomenon called the McGurk effect, which is an
illusion first reported in 1976. If a listener is presented with an
audio recording of a single syllable while watching a synchronized video
of a speaker's face mouthing a different syllable, in some cases a third
syllable is heard, due to the brain's fusion of non-matching audio and
visual information. It's an unnatural situation that a person wouldn't
normally encounter, but it simply and powerfully reveals that speech
perception is a product of both sound and sight.
"The McGurk effect is so amazing because it demonstrates clearly
that speech is not just what you hear," said Knudsen. "If I
thought I heard 'ba' but the lips look like they are mouthing 'ga' then
it must have been 'da.'" Another classic example of the McGurk
effect is that the sound "pa" combined with the lip movements
for "ka" ends up being heard as "ta." It is
impossible to hear the proper sound while viewing the speaker's lips,
noted Knudsen.
Knudsen and Fox wondered if children born deaf would experience the
McGurk effect after receiving cochlear implants. Funded by the National
Institute on Deafness and Other Communication Disorders and the American
Hearing Research Foundation, their team looked at a total of 36
children, ranging in age from 5 to 14, born profoundly deaf and each
having had a minimum of one year with a cochlear implant. At the time of
the study, they could perceive spoken language and communicate verbally.
All of the children could read lips, but until they received hearing
input from their implants they could not make the connection between lip
movement and sound.
The only kids who fully merged the visual and auditory effects of
speech were those who had received implants before they were 30 months
old, which indicates that the earlier the implant is done, the better
the chances for fully integrated speech perception in the brain. Kids
who received implants later showed little evidence of the McGurk effect.
Instead, they relied solely on what the lips were doing when they were
presented with the conflicting auditory and visual information. As a
result, they reported that the sound was the syllable being mouthed.
The paper's first author, Efrat Schorr, who was earning her PhD from
the University of Maryland when the study was done, emphasized that
their findings demonstrate just one of the influences that early
cochlear implantation can have on children with hearing loss and it
reinforces the importance of early detection and intervention for
children with hearing loss. Children who need an implant would benefit
from having it sooner rather than later, the authors conclude in their
paper.
As a developmental psychologist, Fox said that he has always heard
that earlier is better for interventions. "But there is not much
empirical data in humans demonstrating the importance of early
experience on brain development and behavior," he said. "This
study shows just that."
For a demonstration of the McGurk effect,visit http://www.media.uio.no/personer/arntm/McGurk_english.html
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Stanford University Medical Center integrates research, medical
education and patient care at its three institutions - Stanford
University School of Medicine, Stanford Hospital & Clinics and
Lucile Packard Children's Hospital. For more information, please visit
the Office of Communication & Public Affairs site at http://mednews.stanford.edu.
