Are two
cochlear implants in children better than one?
Dr. Karen
Gordon thinks so, and she has new evidence to back it up.
Dr. Gordon
has and her team at Sick Kids in Toronto have been studying the development of
all the children who have gone through the cochlear implant program for the
past two decades. This has given her an opportunity to study the brain
activity of children from infancy to late teens. The studies have involved
children with unilateral cochlear implantation, those who received a second
implant at an early or later stage and those who received bilateral cochlear
implants.
She says
what they found was "amazing".
The brains of children who have had early bilateral
cochlear intervention develop normally expected patterns of activity in parts
of the brain that determine hearing function.
Early intervention is important to take advantage
of developmental plasticity. “The newborn brain forms many new synapses,” she
explains. “Some of these synapses, or connections between neurons, will become
more efficient and others will be pruned away. These changes depend, to a large
extent, on the input the brain receives. If we don’t provide the child with
access to sound during this important period of brain development, we miss an
important chance to promote connections which are important for hearing. “
Dr. Gordon supports her case using well known
studies comparing the brain activity of children who have been deaf since birth
and those who use sign language to see what happens when the brain develops
without sound.
“Without sound, the auditory parts of the brain can
be taken over by other senses including vision. The auditory brain becomes
responsive to visual stimuli in adults who have been deaf for most of their
lives and use sign language.” Is that because they are deaf or because they
sign? This question was answered by studying CODAs, children of deaf adults.
These are people who use sign language fluently but who have normal hearing.
This group does not show the same reorganization in the auditory brain. That
means that “The observed reorganization of the auditory brain occurred when it
was deprived of hearing and not because sign language was used.”
“What is happening in development that would allow
the auditory brain to process visual input? Let’s imagine a neuron sitting in
the auditory cortex in a very young child who doesn’t hear well. If that neuron
receives input from neurons which should carry auditory information and also
from neurons which carry visual information, it will receive most of its
information from the visual system. Donald Hebb suggested that, in this case,
the target neuron will respond to clear input provided by the visual neurons
and, over time, these synapses will become more mature and more efficient. In
the meantime, connections from the auditory system provide limited input and
can be pruned away.”
“If that happens, and the system matures in this
way, this neuron and others like it in the auditory brain may have impaired
responses to auditory input. That is not a good basis for re-establishing
auditory function with a cochlear implant. We want to avoid this situation by
providing children who are deaf with implants while the auditory system is
still developing.”
Dr. Gordon explained that hundreds of children have
gained hearing by receiving a unilateral cochlear implant at young ages but,
she says, intervention has also meant that doctors have “changed the pathways
because hearing is only provided from one cochlear implant”. “Just remember, we
stimulated from one ear. The other ear was deprived all that time.”
When considering implantation, she says, doctors
and audiologists must consider the importance of binaural hearing during
development. “For children, binaural hearing is critical to localize sounds and
hear in noise. It is not enough to give children audibility, they also need to
be able to use this hearing in their daily lives.”
Audiologists can try to counter noise by using FM
systems in classrooms. This technology is useful if the child needs to hear one
person better than others.
“Children live beyond the classroom and beyond the
sound booth. They need to hear in places where they’re forming social bonds--
on the playground, in school hallways. These are places where they need to
communicate and where noise is often a serious problem.”
“In these
situations, there are a lot of people talking all at once and they’re all
moving, including the child with hearing loss. That child needs to distinguish
multiple speakers in space so they need to hear from all directions. In these
situations, FM systems and directional microphones are not going to help. This
is why I’m focused on binaural hearing in children. Binaural hearing is an
important normal process that we need to bring back to children with hearing
loss.”
Dr. Gordon’s current research
shows that bilateral cochlear implants can restore some binaural hearing when
they are provided to young children simultaneously.
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