This article was contributed by Michael Pelc at 16 Mar 1996.

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THE USE OF 22-CHANNEL COCHLEAR IMPLANTS BY DEAF CHILDREN

(a summary of data taken from the published research, and an audiological "frame-of-reference" to help understand what the data mean in practical, functional terms)

A growing number of studies are being done to determine how well profoundly deaf children with a 22-channel cochlear implant can understand spoken words. However, it is frequently difficult for parents, professionals and other interested persons to make sense out of these studies for a variety of reasons.

The types of stimuli that are used are not always the same from one study to the next. Most commonly, stimuli include "open-set word discrimination," "closed-set word recognition," lipreading, sentence materials and "auditory tracking." Additionally, the results are often reported in different ways. Some studies report the average score for the group, others report the percent of the group reaching a pre-determined criterion level, and still others report improvement from pre-implant performance. Finally, the reader may not have the audiological background or "frame of reference" that would be needed to understand what particular scores mean. In order to understand how well children perform with cochlear implants, the average group score needs to be compared to that typically obtained by persons with normal hearing and to groups of children with particular, known degrees of hearing loss.

This brief paper attempts to address some of these issues by reviewing several published studies of children who have received a 22-channel implant. Not all studies which have used this device with this population are included for a variety of reasons. First, the author works at a school, not a university or major medical center, and so many studies that have been published in various medical journals and other professional documents are not available within the environment of a school library. Additionally, studies had to meet three distinct criteria in order to be included in the following analysis:

1. The subjects must all be children;
2. They must use a 22-channel cochlear implant;
3. They must report the average score on an "open-set word identification task" done by listening only.

All the studies known to the author that met each of these three criteria are included; no attempt was made to exclude studies for any other reason. The following table summarizes the average open-set word discrimination scores for groups of children who use a 22-channel cochlear implant The reader will note that some studies are listed more than once. Most often this was because the researcher sampled and reported performance at different time intervals of implant use (e.g. after one year, after two years, etc.). Additionally, a few studies reported separate group scores according to the hearing history of the children (the performance of those born deaf was analyzed separately from the performance of those born hearing who later became deaf).

average             months   percent
score in      N     of c-i    born    reference
% correct            use      deaf

    6        10       ??      100        1
   46         3       ??        0        1
    2        38       12       81        2
    9        29       24       81        2
   17        19       36       81        2
   15        11       48       81        2
   16         5       60       81        2
   10         6       36       33        3
   10         8       40      100        4
    6        11       31        0        4
    6        24       20       ??        5
    9        19       ??       50        6
   12         9       24       50        6
   12        25       18       43        7
   12         8       36       ??        8
   34         5       12.5     20        9
   62         3       24       ??       10
   48         4       36       ??       10
   53         2       48       ??       10
  ----      ----     ----     ----
   11.6%    239 kids  22 mo.   58%  (avg. or total)

The average score, the average length of use and the percent of subjects born deaf are all "weighted averages." They were calculated by taking into account the number of subjects in each study.

The "word score" column in the table above refers to the average percent of items correctly identified by the group on a standard "open-set" word identification test done by hearing only (no lipreading) in a sound booth under optimal listening conditions (no interfering noise). An open-set word identification test typically consists of directing the subject to "say the word ____." The subject/listener has no idea what word will be used to complete the phrase. Open-set word identification testing within the controlled environment of a sound booth does not represent performance under typical everyday listening conditions, which would be expected to be poorer. It is assumed that the children in these studies would have scored 0% correct on open-set word identification tasks prior to receiving their cochlear implants. As a frame of reference, open-set word identification scores for children with normal hearing and for children with varying degrees of hearing loss are summarized below.

hearing status       avg score       % born deaf

normal hearing          >95%              0%
50-70dB loss             68%             93%
71-80dB loss             44%             93%
81-90dB loss             21%             84%
91-100dB loss            10%             90%
101-110dB loss            2%             90%
>110dB loss               0%             80%

It appears that an implanted child who used to function like other children with a hearing loss in excess of 110dB before the implant can now function more like children who were born with hearing losses in the 91-100dB range. To understand what this means in practical terms, children with hearing losses from 91-100dB had a range of open-set word identification scores, from 0% to 65% correct. However the distribution of scores within this range was not "normal," but rather it was "skewed" with most scores occurring near one end of the range. 66% of these children scored 0% correct; they could not identify any words on open-set tasks (in fact, it is not until the 71-80dB range that we find a majority of children who are able to identify at least one word on an open-set word identification task). The distribution of scores for implanted children is not known; it may be either normal or skewed. A study which did not report average scores, and therefore is not included in the first table of results, did report a skewed distribution, with 70% of their subjects unable to identify any words under open-set test conditions (11). If the distribution of scores for the other studies summarized here is similarly skewed, with most scores being 0% correct, in practical terms that would mean that the average score for the group may be misleading because the most likely outcome would still be no open-set word identification instead of a score around 10% correct. While the improved performance of a group of implanted children is substantial when compared to their pre-implant abilities, it would also be important to maintain the frame of reference that children with hearing losses in excess of 90dB are considered audiologically deaf.

As could be seen in the first table summarizing research results on children with cochlear implants, the average open-set word identification score exceeds 30% correct in 5 of the 19 research summaries (1, 9, 10). However, the number of children in these studies only accounts for 7% of the total population. Additionally, where data on age of onset of deafness are available (1, 9), the majority of children in studies showing greater than 30% open set word identification scores were born with normal hearing and then became deaf. Individuals with a memory of sound (either children or adults) tend to do better with the device than individuals who were born profoundly deaf. The best average open-set word identification score from those studies in which a majority of implanted children were born profoundly deaf is only 17% correct, obtained after 36 months of using the device (2).

The cochlear implant research also reports typical "implant thresholds" in the range of 30-40dB. This represents a miraculous improvement when compared to the child's pre-implant ability to hear sounds. It is now possible for the child to hear various sounds in their environment that were previously inaudible (phones ringing, pots banging, doors closing, etc.). And it is now also possible for the child to hear his name being called and even to hear many of the different speech sounds (phonemes) that people produce when they talk. However, there is a big difference between being able to hear spoken words and being able to process and understand them. For example, if I do not know French but I hear two Frenchmen engaged in animated conversation, being able to hear them does not mean I can understand them. I can hear the different sounds they are making, and I can even hear the phonemes of French, but that alone does not help me understand what they are saying. The difference between "hearing" and "understanding" is particularly important to keep in mind in terms of the functional abilities of implanted children. Although they may be able to detect as many sounds, implanted children do not use what the implant allows them to hear as well as children with thresholds in the 30-40dB range (in fact the data suggest they do not function as well as children with hearing losses in the 80-90dB range, children who would be classified as having a severe hearing loss characterized by significant difficulty understanding spoken language).

In summary, a cochlear implant enables a profoundly deaf child to hear many more sounds, including speech, that the child could not hear before being implanted. The device does not enable a deaf child to hear normally. In fact, the data suggest that an implanted child will function like children with hearing losses in the profound hearing loss range (91-100dB). Children who were born deaf do not get as much benefit from the device as children who were born hearing and then became deaf. When all the data are pooled together, on average implanted children can identify approximately 10% of the words in an open-set listening task performed under "ideal" conditions. However, the average performance for the group may be somewhat misleading. There are indications that the distribution of scores may be skewed, which means that most implanted children will not be able to understand any words presented in this manner.

Without doubt, as a group, children with multi-channel cochlear implants can hear more and do more with what they hear than they could before they were implanted. Although the improvement is probably significant for the group as a whole, it also seems limited because implanted children appear as though they still function like other audiologically deaf children, albeit those with considerably more residual hearing than the implant group has. Perhaps most importantly, the improvement that can be attributed to the implant does not apply to all the children in the group (in fact, it actually may only apply to a minority of them).

REFERENCES

1. Fryauf-Bertschy, Tyler, Kelsay & Gantz, Performance Over Time of Congenitally Deaf and Postlingually Deafened Children Using a Multichannel Cochlear Implant, Journal of Speech & Hearing Research, vol 35, 913-920, 1992
2. Gantz, B., Tyler, R., Woodworth, G., Tye-Murray, N., and Fryauf-Bertschy, H., Results of Multi-Channel Cochlear Implants in Congenital and Acquired Prelingual Deafness in Children: Five-Year Follow-Up, American Journal of Otology Supplement 2, 1-7, 1994
3. Geers, A., and Moog, J., The CID Cochlear Implant Study: A Progress Report, 1992
4. Miyamoto, R., Osberger, M., Robbins, A., Myres, W., and Kessler, K, Prelingually deafened children's performance with the nucleus multichannel cochlear implant, Amer. J. of Otology, Vol. 14, 437-445, September 1993
5. Osberger, Miyamoto, Zimmerman-Phillips, Kemink, Stroer, Firszt & Novak, Independent Evaluation of the Speech Perception Abilities of Children with the Nucleus 22-Channel Cochlear Implant System, Ear & Hearing supplement, 66S-80S, 1991
6. Staller, Beiter and Brimacombe, Children and Multichannel Cochlear Implants, Practical Aspects of Cochlear Implants (book), Taylor and Francis, 1990?
7. Staller, Dowell, Beiter and Brimacombe, Perceptual Abilities of Children with the Nucleus 22-Channel Cochlear Implant, Ear & Hearing supplement, vol 12, no. 4, 34S-47S, 1991
8. Tye-Murray, N. and Spencer, L., Cochlear Implant Users Who Use Simultaneous Communication: Speech/Language Performance, ASHA Convention, 1994
9. Tyler, Speech Perception with the Nucleus Cochlear Implant in Children Trained with the Auditory Verbal Approach, Amer Journal of Otology, Vol 11, 99-107, 1990
10. Waltzman, S., Cohen, N., Gomolin, R., Shapiro, W., Ozdamar, S., and Hoffman, R., Long-Term Results of Early Cochlear Implantation in Congenitally and Prelingually Deafened Children, American Journal of Otology Supplement 2, 9-13, 1994
11. Geers, A., and Moog, J., presentation of data from the final year of the CID cochlear implant study, Annual Convention of the American Speech-Language-Hearing Association, Nov., 1994