Combined Acoustic and Electric Stimulation The average level of performance with a single cochlear implant for sentence understanding has been very good (80%–90% correct) for a long time. The average level of performance for consonant-nucleus-consonant (CNC) word recognition has been poorer, in the range of 50%–60% correct for the same amount of time (Helms et ... Feature sidebar
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Combined Acoustic and Electric Stimulation
Author Notes
  • Michael F. Dorman, PhD, is professor and director of the Cochlear Implant Laboratory at Arizona State University in Tempe, Ariz. Contact him at mdorman@asu.edu.
    Michael F. Dorman, PhD, is professor and director of the Cochlear Implant Laboratory at Arizona State University in Tempe, Ariz. Contact him at mdorman@asu.edu.×
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Hearing & Speech Perception / Acoustics / Swallowing, Dysphagia & Feeding Disorders / Feature sidebar
Feature sidebar   |   March 01, 2011
Combined Acoustic and Electric Stimulation
The ASHA Leader, March 2011, Vol. 16, 17-19. doi:10.1044/leader.FTR3sb2.16032011.17
The ASHA Leader, March 2011, Vol. 16, 17-19. doi:10.1044/leader.FTR3sb2.16032011.17
The average level of performance with a single cochlear implant for sentence understanding has been very good (80%–90% correct) for a long time. The average level of performance for consonant-nucleus-consonant (CNC) word recognition has been poorer, in the range of 50%–60% correct for the same amount of time (Helms et al., 1997). The best chance to improve sentence and word recognition performance significantly may be to provide patients with a combination of electric (E) stimulation and low-frequency acoustic stimulation (A) or EAS.
A chart review of recent unilateral CI patients indicates that a majority (60%) have potentially usable hearing at 250 Hz in the contralateral ear (Dorman & Gifford, in press). If this is true today, then it will certainly be the case in the future with expanded candidacy criteria in terms of auditory thresholds and speech understanding.
Ching and colleagues (2007) provide a review of the benefit to speech understanding with combined electrical and acoustic stimulation. Dorman and colleagues (2008) report an improvement in CNC scores from 53% to 73% correct. An even greater performance improvement can be seen for speech in noise. Zhang et al. (2010) report a gain of 45 percentage points in the acoustic-plus-electric condition for the AzBio sentences presented at +10 dB signal-to-noise ratio.
Zhang et al. (2010) presented a full-bandwidth signal to the implanted ear of EAS patients and a filtered signal to their not-implanted ear. When the full-bandwith acoustic signal was added to the electric signal, performance on the AzBio sentences in noise improved by 45 percentage points. When a 125-Hz low-pass signal was added to the electric signal, performance improved by 30 percentage points. Thus, the majority of the benefit from the low-frequency signals came from the 125-Hz low-pass band (see also Brown & Bacon, 2009).
Li and Loizou (2008) proposed that speech recognition in noise is facilitated when listeners have accessto robust, low-frequency acoustic landmarks (Stevens, 2002), such as the onset of voicing, that mark syllable structure and word boundaries. In a recent study, Spitzer and colleagues (2009) analyzed lexical (word) boundary errors of EAS patients. Fewer lexical boundary errors were found in the EAS conditions than in the electric-only conditions. Thus, it appears that the acoustic signal aids in the recognition of word onsets when CI signals are corrupted by noise.
In one of the newest CI applications, an electrode array of 10, 16, or 20 mm is implanted into one cochlea with the intention of preserving hearing in the frequency region apical to the array (e.g., von Ilberg et al.; 1999, Kiefer et al., 2005; Gantz & Turner, 2004). When this surgery is successful and hearing is preserved in the implanted ear, patients receive electrical stimulation from one cochlea and low-frequency acoustic stimulation from both the implanted and the non-implanted cochleas. These patients should show the benefit of added acoustic information to the electrical signal and should benefit from access to low-frequency, binaural cues processed by two ears with relatively good spectral selectivity.
Gifford, Dorman, and Brown (2010) have tested unilateral CI patients, bilateral CI patients, bimodal patients, and patients with preserved hearing after CI surgery for speech understanding in noise in an eight-loudspeaker, “surround sound” environment—an environment that should maximize the benefit of having two CIs or two partially hearing ears. In this environment, unilateral CI patients showed the poorest performance, bilateral CI and bimodal patients showed better performance, and the patients whose hearing was preserved showed the best performance. These data suggest that an attempt should be made to preserve hearing in CI candidates.
The near future holds the promise of drug-delivery coverings for electrodes that release, for example, a compound that mitigates the damage caused by electrode array insertion into the scala tympani. If apoptosis, or programmed cell death, can be arrested, then hearing-preservation CI surgery could become the standard in the future. Patients with bilateral CIs who have bilateral low-frequency acoustic hearing are possible candidates for this type of surgery. These patients may be more successful in real-world noise environments than current CI patients.
Recent trials in Europe (Van de Heyning et al., 2008; Vermiere et al., 2008; Vermiere & Van de Heyning, 2009) have shown dramatic improvements in speech understanding and sound localization from a CI in patients with single-sided deafness (SSD). More than 80 patients with SSD have received a CI in the deaf ear and show a 2–3 dB improvement in the speech recognition threshold in noisy listening situations, a significant reduction in tinnitus in the deaf ear, and a notable improvement in the ability to localize sounds. In addition, most of these patients report a high level of satisfaction with the CI, saying that “the world sounds whole again” and “I now forget that I have a deaf ear.” Although the quality of sound provided by the CI is poorer than the normal hearing ear, it appears that subjectively the sound quality from the normally hearing ear captures or overrides the poorer quality from the CI ear. CIs are not yet available for patients with SSD in the United States.
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March 2011
Volume 16, Issue 3