FEATURE ARTICLE
The Design and Function of Cochlear Implants
Fusing medicine, neural science and engineering, these devices transform human speech into an electrical code that deafened ears can understand
Michael Dorman, Blake Wilson
In the Ears of a Child
Adults who lose their hearing and later receive a cochlear implant
can associate the new stimulation patterns with their memories of
what speech should sound like. Children born deaf do not have this
luxury. Yet a team led by Richard Miyamoto and Mario Svirsky at
Indiana University has found that congenitally deaf children who
receive a cochlear implant during their first or second years can
learn spoken language at a normal or near–normal rate. These
children can enter first grade with age–appropriate language
skills—a testament to the adaptive ability of young neural
systems. This plasticity undoubtedly plays a major role in the
success of implants at an early age.
Scientists can observe the neural changes in young children fitted
with implants using the tools of neurophysiology. Author Dorman and
his colleagues at Arizona State University, in collaboration with
Anu Sharma and her team at the University of Texas at Dallas, found
that the brains of deaf children under the age of four are quickly
reconfigured in response to the signals from an implant. Using
electrodes on the scalp, we were able to record sound–evoked
electrical activity in the cortex. Within a week after the implant
was activated, we saw changes in the latency of neural responses to
sound. Within six months, children who had heard nothing for up to
three and a half years showed age–appropriate timing of
cortical activity in response to sound.
Children who receive the implant after their seventh birthday have
less success than younger patients in developing speech and oral
language. We saw corresponding evidence for this age limit in the
cortical–latency experiments. After an initial change, the
delay of cortical activity in response to sound remained abnormally
long in older children, even after considerable experience with the implant.
Sadly, the same property that helps the implant work so well in
preschoolers limits its effectiveness for older children. During the
extravagant growth of neural connections during the first years of
life, areas of the brain that lack stimulation can be usurped or
recruited to process active signals that usually go to other parts
of the brain. In this case, regions that would normally analyze
auditory inputs might be appropriated by the spread of visual or
other sensory connections as the child gets older. And once an area
is allocated to a different task, returning to the original task is
difficult or impossible, depending on age. This narrow window of
opportunity has also been observed in animal experiments.
The different outcomes of implants in younger and older children
reflect different patterns of neural organization in the children
prior to implantation. Using positron–emission tomography
(PET), Dong Soo Lee and his colleagues at the Seoul National
University found extremely low activity in the auditory cortex and
surrounding brain areas in children who were deaf for a relatively
short period—which is what one would expect given that there
was no auditory input. This group of children adapted well to
cochlear implants.
However, in children deprived of sound for more than 7 years, PET
scans before the implant surgery showed a more normal level of
activity in the auditory cortex and language areas. Because this
cortex was not activated by auditory input, it must have received
input from some other sense—probably vision. It is reasonable
to suppose that the encroachment of other functions into brain areas
normally devoted to auditory processing is one reason that older
children have a much more difficult time acquiring speech and oral
language skills after receiving the cochlear implant. This
biological reality adds an important codicil to the debate over
cochlear implants for the deaf children of deaf parents. By the time
a deaf child reaches the age at which he or she might elect to have
an implant, it will be too late to achieve the best outcome.
» Post Comment