Abstract
This study simulated the effect of unlinked automatic gain control (AGC) and head movement on the output levels and resulting inter-aural level differences (ILDs) produced by bilateral cochlear implant (CI) processors. The angular extent and velocity of the head movements were varied in order to observe the interaction between unlinked AGC and head movement. Static, broadband input ILDs were greatly reduced by the high-ratio, slow-time-constant AGC used. The size of head-movement-induced dynamic ILDs depended more on the velocity and angular extent of the head movement than on the angular position of the source. The profiles of the dynamic, broadband output ILDs were very different from the dynamic, broadband input ILD profiles. Short-duration, high-velocity head movements resulted in dynamic output ILDs that continued to change after head movement had stopped. Analysis of narrowband, single-channel ILDs showed that static output ILDs were reduced across all frequencies, producing low-frequency ILDs of the opposite sign to the high-frequency ILDs. During head movements, low- and high-frequency ILDs also changed with opposite sign. The results showed that the ILDs presented to bilateral CI listeners during head turns were highly distorted by the interaction of the bilateral, unlinked AGC and the level changes induced by head movement.
Highlights
Automatic gain control (AGC) is used both in hearing aids and cochlear implants (CIs) to compensate for the reduced dynamic range of hearing compared to that of normal hearing (NH) listeners
A simulation of bilateral CI processors on a rotating head was used to investigate the effect of unlinked AGC on output inter-aural level differences (ILDs) during different speeds and extents of head movement
It was found that the interaction between AGC and head movement induces ILDs that were very different from the input ILDs
Summary
Automatic gain control (AGC) is used both in hearing aids and cochlear implants (CIs) to compensate for the reduced dynamic range of hearing compared to that of normal hearing (NH) listeners. The AGCs used in CIs generally include a slow-acting system with a low threshold and high compression ratio, combined with a fast-acting system having a higher threshold (known as a “dual-loop compressor”). The present study investigates the effects of the slow-acting compressor on the inter-aural level differences (ILDs) presented to CI listeners, with particular reference to the combined effects of AGCs and head movements. The slow AGC used in CIs has a longer time constant and much higher compression ratio [e.g., 12:1 in devices made by Advanced Bionics Ltd., (AB), Valencia, CA] than that implemented in hearing aids. This is likely to increase the distortions of ILDs induced by the combination of the AGC and head movement
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