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.