Further simulations of the effect of cochlear-implant pre-processing and head movement on interaural level differences.

Alan W Archer-Boyd,Robert P Carlyon

doi:10.1121/10.0005647

Abstract

We simulated the effect of several automatic gain control (AGC) and AGC-like systems and head movement on the output levels, and resulting interaural level differences (ILDs) produced by bilateral cochlear-implant (CI) processors. The simulated AGC systems included unlinked AGCs with a range of parameter settings, linked AGCs, and two proprietary multi-channel systems used in contemporary CIs. The results show that over the range of values used clinically, the parameters that most strongly affect dynamic ILDs are the release time and compression ratio. Linking AGCs preserves ILDs at the expense of monaural level changes and, possibly, comfortable listening level. Multichannel AGCs can whiten output spectra, and/or distort the dynamic changes in ILD that occur during and after head movement. We propose that an unlinked compressor with a ratio of approximately 3:1 and a release time of 300–500 ms can preserve the shape of dynamic ILDs, without causing large spectral distortions or sacrificing listening comfort.

Highlights

We simulated the effect of several automatic gain control (AGC) and AGC-like systems and head movement on the output levels, and resulting interaural level differences (ILDs) produced by bilateral cochlear-implant (CI) processors
In order to determine the effects of AGC parameter changes on a simple change in level, the simulator was presented with non-spatialized speech-shaped noise (SSN) signals
The columns left to right show the effect of changing the attack time, release time, and compression ratio, respectively, with the different parameters indicated by different colors within each plot

Summary

Introduction

We simulated the effect of several automatic gain control (AGC) and AGC-like systems and head movement on the output levels, and resulting interaural level differences (ILDs) produced by bilateral cochlear-implant (CI) processors. The reduced spectral and temporal resolution of CI processing means that listeners are heavily dependent on temporal envelope cues to understand speech (e.g., Shannon et al, 1995; Shannon et al, 2001; Green et al, 2004) These cues can be distorted by fast-acting compressors (e.g., Stone and Moore, 2007). There is an inevitable trade-off between these factors, and the parameters used in AGC systems are an important part of that trade-off This is especially true in CIs, given listeners’ relatively narrow electrical dynamic range, and their much greater perceptual weighting of interaural level difference (ILD) cues over interaural time difference (ITD) cues for sound localization, relative to NH listeners, who generally weight lowfrequency ITDs more than ILDs for broadband stimuli Even under experimental conditions where ITD cues are preserved by presenting the cues directly to the processor, perception of those cues by CI listeners remains poor (Van Hoesel, 2004; Ihlefeld et al, 2015)

Methods

Results

Discussion

Conclusion