By Patricia E. Connelly, PhD, CCCA
Au DK, Hui Y, Wei WI. Superiority of bilateral cochlear implantation over unilateral cochlear implantation in tone discrimination in Chinese patients. American Journal of Otolaryngology 2003;24:19–23.
LEARNING OUTCOMES
As a result of reading this article, you are expected to:
Dr. Au, et al., began with background information that set the stage for the purpose of their project: “to compare the effectiveness between bilateral and unilateral CIs [cochlear implants] in discriminating Cantonese lexical tone in background noise and in quiet” (page 20). They discussed all of the advantages of binaural hearing with and without otoprostheses, reported on speech perception studies in noise for subjects with unilateral CIs with and without amplification on the nonimplanted ear and cited the results from the few studies reported on patients with bilaterally implanted devices. They noted with interest that these studies of speech perception with bilateral CI users were completed mostly in Western countries where English is the official language.
The authors completed their background information with a brief literature review of CI studies of speech perception in Cantonese speakers. Of interest, they noted that Cantonese has six contrastive tones produced by the vocal cords that convey different meanings. Since these meaningful tones are produced solely by the vocal cords, there is no way to lipread these important elements of Cantonese. “The perception of these tones requires good temporal and spectral auditory abilities that theoretically can be provided by speech-coding strategies that use fast stimulation rates” in the implant processor (page 20). This group of researchers wanted to measure the binaural CI advantage over using only one cochlear implant for the discrimination of these Cantonese lexical tones.
This study recruited four subjects with bilateral CIs (two males, two females) with an average age of 33 years. No information was given about the educational attainment of each subject. The average preimplant PTA was 103.3 dB HL, right and left ear included. The sound field PTA was 32.5 dB SPL for the unilateral CI condition and 27.9 dB SPL for the bilateral condition. Each subject acted as his/her own control for the four bilateral and eight unilateral experimental conditions.
The speech stimuli used in this study came from the Tone Discrimination Test which is a part of the Hong Kong Speech Perception Test Manual used to evaluate cochlear implant candidacy in adults. There were 30 items per test, each item being a pair of the Cantonese contrastive tones. The subject simply needed to indicate “same” or “different” after hearing each test item. Test items were presented at seven signal-to-noise ratios (SNR) (+15, +10, +5, 0, –5, –10, –15) and in quiet. Test order and conditions were completely randomized.
The results of this study clearly indicated a significant advantage for improved speech perception in the bilateral CI mode relative to the unilateral listening mode, even under the most extreme SNRs. It was interesting to note that the speech discrimination scores for unilateral and bilateral CI use in quiet were not statistically different.
Obviously, the benefits of binaural hearing are best illustrated when the auditory system is challenged to process a primary signal imbedded in a background of noise at varying and various SNRs. Signal separation in background noise is a central auditory processing task that is enabled by binaural hearing. Without binaural hearing this exquisite ability is disabled, with all of its incumbent challenges and problems manifested as a person’s extreme dissatisfaction with their hearing in the most difficult listening situations.
The issue of bilateral CIs and bimodal hearing has garnered considerable attention lately. The February 8, 2005 edition of The ASHA Leader featured an article authored by Dee Naquin Shafer, entitled, “Bilateral Cochlear Implants on the Rise: Optimal Hearing Could Be Result.” This article presented the background information, rationale in functional terms and concerns about insurance reimbursement for the second CI. The cost-effectiveness of the second implant is still being investigated. Bimodal hearing was explored in this column (THP, July/August 2004) when an article was reviewed on the use of a conventional hearing aid in the nonimplanted ear. The January/February 2005 issue of Hearing Loss, the official publication of Self Help for Hard of Hearing People, contained the article, “Hearing Aid Use in Conjunction with a Cochlear Implant” by Teresa A. Zwolan, PhD. Dr. Zwolan gave excellent advice on when and how to reintroduce the hearing aid to the nonimplanted ear—important considerations to optimize benefit from the implant.
Even though this research was conducted in another language, it nonetheless has implications for our patients who don’t speak Cantonese. First, under the binaural condition subjects were much better able to process speech (Cantonese contrast tones) in noisy environments, subtle acoustic differences in the speech signal that carried different meanings and could not be lipread. We probably have some analogous phenomena in the English acoustic signal, although I can’t think of one right now. Second, this article highlights that speech perception is better with bilateral CIs relative to unilateral cochlear implantation. Efficacy studies are needed to prove to insurance companies and other third party payers that a particular medical or surgical intervention consistently produces a desired result. Although this study is limited in that only four subjects were used, it’s a start and one that crosses cultural and linguistic boundaries. Finally, this study tends to reinforce what most hearing care providers already know; that is, in most applications binaural is better.
This is a very easy article to read and understand. I highly recommend it as one that provides both background and proof that binaural has great advantages over monaural hearing. THP
Dr. Connelly is an assistant professor of surgery at the New Jersey Medical School and director of audiology service at the UMDNJ–University Hospital in Newark, New Jersey.
For a copy of the Au, et al. article, contact
Pat Connelly, PhD, CCCA, at connelpe@umdnj.edu.
