Effects of number of maxima and electrical dynamic range on speech-in-noise perception with an “n-of-m” cochlear-implant strategy

Abstract

Cochlear implant (CI) recipients face great challenges in speech-in-noise recognition, partially due to the fact that only temporal envelopes from a limited number of bands are preserved in most CI signal processing strategies. In “n-of-m” strategies (e.g., the Advanced Combinational Encoder, ACE), the number of maxima (nmax) and electrical dynamic range (EDR) are two essential parameters that may affect the envelope representation and further influence speech perception. Speech recognition can be improved by optimizing parameter settings in CI programming. To investigate the effects of nmax and EDR on speech-in-noise perception, Mandarin speech reception thresholds (SRTs) in babble noise were measured in CI recipients using ACE. The nmax was set to 2, 4, 6, 8, and 16. The EDR was set to the base EDR (i.e., participants’ clinical EDR) and 50 % EDR (i.e., 50 %-compressed base EDR). Results showed that: 1) there was no significant interaction effect between nmax and EDR, 2) SRTs with nmax = 2, 4, and 16 were significantly higher (or worse) than those with moderate nmax (6–8), 3) narrower EDRs significantly lead to higher SRTs. Simulation experiments using a Gaussian-Enveloped Tones Vocoder in normal-hearing listeners were also conducted and provided both supportive and additional observations to the CI results. This study suggests that, in CI programming, nmax and EDR are two independent influencing factors. Large nmax (e.g., 16) is not recommended as it may harm speech intelligibility in noisy environments, and inaccurate EDR measurements should be avoided.