twoimplants
Second cochlear implant can restore two important facets of binaural
hearing
by Dave Tenenbaum, University of Wisconsin, Madison News
Cochlear implants are electronic devices that stimulate auditory nerves
directly, bypassing damage in the inner ear, and thus restoring some
hearing. Although cochlear implants have revolutionized the treatment of
deafness, many users have trouble understanding speech, particularly in
crowds.
That prevents implants from fulfilling their primary task: breaking the
isolation that can accompany deafness.
Scientists who are studying implants have found that two implants are
better than one, and a key to that improvement comes from the ability to
detect the source of the sound.
In a study published this week in the Journal of the Acoustical Society of
America, Ruth Litovsky, an associate professor of communicative disorders
and director of the binaural hearing and speech lab at the Waisman Center
at the University of Wisconsin-Madison, showed that deaf adults with two
cochlear implants regained some ability to localize sound.
Some of the 11 adults she studied had been deaf since early childhood.
Hearing people use subtle differences in the intensity and timing of the
vibrations reaching their ears to identify where a sound is coming from.
This “binaural” hearing has obvious benefits for personal safety. But
because it also helps to focus our attention on a speaker in a noisy room,
binaural hearing is critical to social relationships.
In the study, Litovsky found that adults with two implants, even those who
had been deaf since early childhood, could detect differences in sound
intensity. Adults who had lost their hearing as older children or adults
could also detect subtle differences in the timing of sound arrival at the
two ears. “This paper shows that people who have been deaf for decades can
retain sensitivity to important binaural cues,” Litovsky says, “especially
if they had normal hearing before becoming deaf.”
Even though the study is more proof that cochlear implants can help after
decades of deafness, adults who had a longer experience with hearing were
substantially better at locating the source of a sound. Like many neural
systems, “use it or lose it” applies to auditory systems, Litovsky says.
Auditory nerves degenerate if they are not stimulated.
When cochlear implants were first introduced about a decade ago, adults or
older children often received a single implant, Litovsky says. Now, the
trend is to implant two devices at a much earlier age. Both measures are
consistently tied to better outcomes.
Although both timing and intensity help us find the source of a sound,
timing seems more important. Yet Litovsky finds commercial implants a bit
sloppy in their timing. “Differences in timing between the two ears are
calculated in the brain stem, where the neurons expect the information to
be highly synchronized, but current implant processors are not designed to
be synched. You might think a few millionths of a second would not matter,
but the brain doesn’t deal well with ‘wrong’ time differences.”
The study used research processors, made by Litovsky’s collaborator,
Richard van Hoesel of the Bionic Ear Institute in Melbourne, Australia, to
deliver electrical impulses to the auditory nerve with more accurate
timing.
Because some of the adults in the study could locate the source of sound
even after decades of deafness, commercial implants with better timing
could improve the ability to localize sound. “These people have retained
the ability to perceive the microsecond difference in timing between the
two ears, but current implants are not giving them that information,” says
Litovsky.
And because localizing sound is critical to the ability to focus on one
speaker among many, future implants may reduce social isolation even more
effectively, Litovsky says. “Using these research processors, our study
participants could distinguish location; the circuits in the brain have
retained that ability. That gives me hope that if we can build this type
of processor into commercial implants, implant users can function more
like hearing people.”
Litovsky, who has spent years studying the effectiveness of cochlear
implants, recently was awarded a $2.9 million grant from the National
Institutes of Health to study how the brain changes when hearing is
restored through two implants.
Among brain researchers, it’s axiomatic that treating at a younger age or
sooner after the loss of hearing will produce better results, but the
current study shows that the brain can learn to interpret sensations after
decades of silence. “Even people who are deaf from birth and implanted as
adults can have access to some binaural hearing,” says Litovsky.
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