Integration of acoustic and electric hearing is better in the same ear than across ears

Abstract

Advances in cochlear implant (CI) technology allow for acoustic and electric hearing to be combined within the same ear (electric-acoustic stimulation, or EAS) and/or across ears (bimodal listening). Integration efficiency (IE; the ratio between observed and predicted performance for acoustic-electric hearing) can be used to estimate how well acoustic and electric hearing are combined. The goal of this study was to evaluate factors that affect IE in EAS and bimodal listening. Vowel recognition was measured in normal-hearing subjects listening to simulations of unimodal, EAS, and bimodal listening. The input/output frequency range for acoustic hearing was 0.1-0.6 kHz. For CI simulations, the output frequency range was 1.2-8.0 kHz to simulate a shallow insertion depth and the input frequency range was varied to provide increasing amounts of speech information and tonotopic mismatch. Performance was best when acoustic and electric hearing was combined in the same ear. IE was significantly better for EAS than for bimodal listening; IE was sensitive to tonotopic mismatch for EAS, but not for bimodal listening. These simulation results suggest acoustic and electric hearing may be more effectively and efficiently combined within rather than across ears, and that tonotopic mismatch should be minimized to maximize the benefit of acoustic-electric hearing, especially for EAS.

Figure 1

Illustration of the output and input frequency ranges for simulated residual acoustic hearing (AH; white bars) and electric hearing (CI; black bars). The grey bars represent the regions where the AH and CI input frequency ranges overlap.

Figure 2

Spectral envelopes for the steady portion of the vowels “heed” (left column) and “hod” (right column). The black lines show the original spectral envelope. The green lines show the spectral envelope with the simulated residual acoustic hearing (AH); the input and output frequency range was 0.1–0.6 kHz. The red lines show the spectral envelope with the CI simulations; the output frequency range was 1.2–8.0 kHz and the input frequency range was varied to preserve different amounts of speech information while introducing different amounts of tonotopic mismatch.

Figure 3

Mean percent correct (N = 10) for overall vowel recognition (A), F1 (B), F2 (C), and duration (D). The white bars show performance with simulated residual acoustic hearing (AH), the black bars show performance with the CI simulations alone, the red bars show performance with bimodal listening, and the green bars show performance with EAS. Performance for the CI simulations alone, bimodal, and EAS are shown as a function of the CI input low-cutoff frequency. The error bars show the standard error of the mean.

Figure 4

Mean integration efficiency (N = 10) for bimodal (filled circles) and EAS simulations (open triangles) as a function of the CI input low-cutoff frequency. The error bars show the standard error of the mean.