The effect of movement, compression, and cochlear-implant processing on the output of cochlear implants

Abstract

Dynamic-range compression is used in cochlear implants (CIs) to compensate for the greatly reduced dynamic range of CI listeners. It has been found that unlinked bilateral compression in hearing aids (HAs) can affect the spatial location of sounds, reducing interaural level differences (ILDs) and causing sound sources to appear closer to 0° azimuth and/or more diffuse (Wiggins and Seeber, 2011). The compressors in CI processors are often much stronger than hearing aids (e.g., CI compression ratios of 12:1 vs. ≤3:1 in HAs). We investigated the effect of compression on monaural and binaural level cues during simulated rotational movements in the horizontal plane. Moving signals were created by combining static behind-the-ear hearing aid microphone impulse responses at successive angles. For sound levels above the compressor threshold, fast movements over wide angles (120°/s) altered the size and extent of monaural level changes and ILDs considerably, relative to those from those that would occur naturally and without compression. The relatively slow attack and release times of the compressor also had the effect of producing a “lag” in the level changes after the movement had stopped. Microphone directionality was also found to have an effect on the output of the compressor during movement.