Noise susceptibility of cochlear implant users: the role of spectral resolution and smearing

Abstract

The latest-generation cochlear implant devices provide many deaf patients with good speech recognition in quiet listening conditions. However, speech recognition deteriorates rapidly as the level of background noise increases. Previous studies have shown that, for cochlear implant users, the absence of fine spectro-temporal cues may contribute to poorer performance in noise, especially when the noise is dynamic (e.g., competing speaker or modulated noise). Here we report on sentence recognition by cochlear implant users and by normal-hearing subjects listening to an acoustic simulation of a cochlear implant, in the presence of steady or square-wave modulated speech-shaped noise. Implant users were tested using their everyday, clinically assigned speech processors. In the acoustic simulation, normal-hearing listeners were tested for different degrees of spectral resolution (16, eight, or four channels) and spectral smearing (carrier filter slopes of -24 or -6 dB/octave). For modulated noise, normal-hearing listeners experienced significant release from masking when the original, unprocessed speech was presented (which preserved the spectro-temporal fine structure), while cochlear implant users experienced no release from masking. As the spectral resolution was reduced, normal-hearing listeners' release from masking gradually diminished. Release from masking was further reduced as the degree of spectral smearing increased. Interestingly, the mean speech recognition thresholds of implant users were very close to those of normal-hearing subjects listening to four-channel spectrally smeared noise-band speech. Also, the best cochlear implant listeners performed like normal-hearing subjects listening to eight- to 16-channel spectrally smeared noise-band speech. These findings suggest that implant users' susceptibility to noise may be caused by the reduced spectral resolution and the high degree of spectral smearing associated with channel interaction. Efforts to improve the effective number of spectral channels as well as reduce channel interactions may improve implant performance in noise, especially for temporally modulated noise.

Fig. 1

Averaged short-term spectra for noise-band vocoder speech processing used in the acoustic cochlear implant simulation for the vowel /i/. The left panel shows processing by 16 channels and the right panel shows processing by four channels. The dotted lines show the short-term spectra without smearing (−24 dB/octave filter slopes) and the solid lines show the short-term spectra with smearing (−6 dB/octave filter slopes).

Fig. 2

Mean speech recognition thresholds as a function of noise conditions in normal-hearing listeners and cochlear implant patients for the original unprocessed speech. The filled circles represent the data from the NH listeners while the open circles represent the data from the CI listeners. The error bars represent one standard deviation.

Fig. 3

Individual and mean SRTs for CI subjects, grouped by device and speech processing strategy. A. Four patients with Nucleus-22 device; B. two patients with Clarion CII/Hi-Res strategy; C. one patient with Med-El device; D. three patients with Clarion/CIS strategy. The error bars represent one standard deviation.

Fig. 4

Speech recognition thresholds as a function of gated noise frequency in NH listeners for 16-, eight-, and four-channel noise-band speech and unprocessed speech. A. Steep carrier filter slope (−24 dB/octave). B. Shallow carrier filter slope (−6 dB/octave). The error bars represent one standard deviation.