Synchronized Automatic Gain Control in Bilateral Cochlear Implant Recipients Yields Significant Benefit in Static and Dynamic Listening Conditions

Abstract

Individuals with bilateral cochlear implants (BiCIs) rely mostly on interaural level difference (ILD) cues to localize stationary sounds in the horizontal plane. Independent automatic gain control (AGC) in each device can distort this cue, resulting in poorer localization of stationary sound sources. However, little is known about how BiCI listeners perceive sound in motion. In this study, 12 BiCI listeners’ spatial hearing abilities were assessed for both static and dynamic listening conditions when the sound processors were synchronized by applying the same compression gain to both devices as a means to better preserve the original ILD cues. Stimuli consisted of band-pass filtered (100–8000 Hz) Gaussian noise presented at various locations or panned over an array of loudspeakers. In the static listening condition, the distance between two sequentially presented stimuli was adaptively varied to arrive at the minimum audible angle, the smallest spatial separation at which the listener can correctly determine whether the second sound was to the left or right of the first. In the dynamic listening condition, participants identified if a single stimulus moved to the left or to the right. Velocity was held constant and the distance the stimulus traveled was adjusted using an adaptive procedure to determine the minimum audible movement angle. Median minimum audible angle decreased from 17.1° to 15.3° with the AGC synchronized. Median minimum audible movement angle decreased from 100° to 25.5°. These findings were statistically significant and support the hypothesis that synchronizing the AGC better preserves ILD cues and results in improved spatial hearing abilities. However, restoration of the ILD cue alone was not enough to bridge the large performance gap between BiCI listeners and normal-hearing listeners on these static and dynamic spatial hearing measures.