Understanding speech in modulated interference: Cochlear implant users and normal-hearing listeners

Abstract

Many competing noises in real environments are modulated or fluctuating in level. Listeners with normal hearing are able to take advantage of temporal gaps in fluctuating maskers. Listeners with sensorineural hearing loss show less benefit from modulated maskers. Cochlear implant users may be more adversely affected by modulated maskers because of their limited spectral resolution and by their reliance on envelope-based signal-processing strategies of implant processors. The current study evaluated cochlear implant users' ability to understand sentences in the presence of modulated speech-shaped noise. Normal-hearing listeners served as a comparison group. Listeners repeated IEEE sentences in quiet, steady noise, and modulated noise maskers. Maskers were presented at varying signal-to-noise ratios (SNRs) at six modulation rates varying from 1 to 32 Hz. Results suggested that normal-hearing listeners obtain significant release from masking from modulated maskers, especially at 8-Hz masker modulation frequency. In contrast, cochlear implant users experience very little release from masking from modulated maskers. The data suggest, in fact, that they may show negative effects of modulated maskers at syllabic modulation rates (2-4 Hz). Similar patterns of results were obtained from implant listeners using three different devices with different speech-processor strategies. The lack of release from masking occurs in implant listeners independent of their device characteristics, and may be attributable to the nature of implant processing strategies and/or the lack of spectral detail in processed stimuli.

Concurrent Sound Segregation in Electric and Acoustic Hearing

“…Nelson et al (2003) measured speech perception in the presence of speech-shaped noise that was either steady or modulated at rates of between 1 and 32 Hz. NH listeners performed substantially better at all modulation rates compared to steady noise.…”

“…However, this may involve aspects of the NH neural response that would be difficult to replicate in a CI. These may include a close match between place and rate of stimulation (Oxenham et al 2004;Moore and Carlyon 2005), a reliable phase transition around the peak of the traveling wave (Kim et al 1980;Shamma 1985;Loeb 2005;Moore and Carlyon 2005), and the ability to convey periodicities of up to a few thousand Hertz, despite evidence that most CI users do not exploit temporal cues to pitch at rates above about 300 Hz (Shannon 1983;Townshend et al 1987;McKay et al 2000).…”

“…Some insight into the reasons for this poor performance has come from speech studies that manipulated the nature of the interfering sounds. For example, when the masker is a noise, then, unlike normal-hearing (NH) listeners, CI users do not perform better when the masker is amplitude-modulated at rates between 2 and 8 Hz, compared to when it is steady (Nelson et al 2003). This suggests that, over a range of modulation rates, they are unable to exploit portions of the stimulus where the signal-to-masker ratio is momentarily high.…”

“…In contrast, most CIs encode F0 by the rate of envelope modulation applied to a fixed-rate pulse train on each channel. Such modulations do not convey place-of-excitation cues, and modulations of a few hundred Hertz may be conveyed by AN fibers tuned to frequencies of several thousand Hertz (Ketten et al 1998;Oxenham et al 2004). To investigate the use of this Bpurely temporal^code to F0, we studied simplified stimuli in which each stimulated channel of a CI conveyed a periodic pulse train consisting of a single pulse per period.…”

Discrimination of Schroeder-Phase Harmonic Complexes by Normal-Hearing and Cochlear-Implant Listeners

“…Speech recognition with no background noise is often quite good in cochlear implant (CI) users; however, speech recognition deteriorates significantly in the presence of background noise (Zeng 2004) and especially in fluctuating background noise (Nelson et al 2003;Zeng et al 2005). Additionally, music perception and localization abilities are much worse in CI users than in normal-hearing listeners (Gfeller et al 1997(Gfeller et al , 2002(Gfeller et al , 2005Senn et al 2005;Nimmons et al 2007), highlighting the need for improvement in CI technology.…”