Signal
Processing, Learning, and the Cochlear Implant
by Cheryl
Heppner
Editor: Here's another of Cheryl's reports from the SHHH convention.
This one addresses how much speech understanding people with different
hearing histories can expect to achieve with a cochlear implant. Very
interesting stuff!
Here's Cheryl!
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Robert V. Shannon, Ph.D., heads the Department of Auditory Implants
and Perception Research at House Ear Institute in Chicago. Here are some
notes from his presentation at the SHHH tenth annual research symposium:
- The hearing collaboration between the ear and brain takes many
years to perfect.
- It takes 10-12 years for the maturity of complex sensory pattern
recognition.
- Once the brain is trained to hear normal patterns of sound, it can
take a long time to learn a different or distorted pattern.
- If the brain is never trained [as a child] to understand sound
patterns, a deaf person may never be able to learn them as an adult.
Cochlear Implants in Children - A study of homes with little
parent-child interaction found results similar to those in homes of
parents with children who have hearing loss. This indicates that the
brain needs information to develop.
- Cochlear implants give that information and children's performance
starts to develop like that of a normal hearing child. The difference is
that it just starts later on the clock, dating to the time of the
implant.
- Studies conclude that modern cochlear implants can provide
sufficient information for children to learn language at a normal rate,
but earlier is better. Vigilance is needed to identify children who are
not getting enough auditory information.
Cochlear Implants in Postlingually Deafened Adults - Adult brains are
trained in speech patterns over a lifetime. To be successful, a cochlear
implant must reproduce those sound patterns to the brain.
- Dr. Shannon looked at the results of shifting a bit (e.g., what
would be one octave) to see what would result if insertion is not done
perfectly. In tests of normal hearing people, a shift of about 4
millimeters rendered sound unintelligible. This shows the importance of
positioning the implant correctly.
- In a group of people who already had cochlear implants, a shift of
3 millimeters on their maps dropped their performance 30%. At the end of
three months, it had risen a bit but still was 20% less than when they
started. This shows the long-term relationship between the ear and
brain, so we have to be concerned about precision in the design,
programming, and surgery of cochlear implants.
- As we age, plasticity and our ability to relearn takes longer.
Cochlear Implants and Prelingually Deafened Individuals
- If you are blind from birth and suddenly regain sight, can you tell
the difference between a ball and a cube? No. The movie "At First
Sight" starring Val Kilmer, and based on a story by Oliver Sacks,
recounts the story of a man who regained 20/40 but whose brain couldn't
make sense of what he saw. He could recognize simple objects but not
faces or depth or complexity.
- A blind man who regained vision at age 52 could recognize some
simple shapes because he was familiar with them by touch. However, he
could tell that two objects were different only if he touched them.
- A similar pattern happens with congenitally deaf people who receive
cochlear implants. They can recognize that speech sounds are different,
but they cannot identify them. Most get no speech recognition even after
years of practice and training. Even if they had only a small amount of
prior hearing, an individual will do much better with a cochlear implant
than someone who had none. - Many congenitally deaf individuals still
like their cochlear implants because of the help in orienting to sound
in their environment and in speechreading.
Conclusions - Children are very adaptable and can make optimal use of
an implant if it is done early.
- Postlingually deaf individuals benefit if the information they
receive from the implant is close enough to their learned patterns.
- Prelingually deaf individuals may be able to make only limited use
of their implants, and complex patterns like speech may take a long
time.
- The brain's flexibility and prior experience are the two most
important factors.
