Synchronization of automatic gain control restores sound-localization cues in bimodal hearing
Authors: Heidjt Jasper B. van der , Sharma Snandan , Mens Lucas H.M. , Chalupper Josef , Opstal A. John van , Wanrooij Marc M. van 
Published: August 2023

Objectives
This study investigated the effect of synchronized activation of automatic gain control (AGC) on sound-localization and speech-recognition performance in cochlear implant users who use a contralateral hearing aid (bimodal listeners). Bimodal listeners often have poor spatial-hearing abilities, which could be partly due to perturbed interaural spatial acoustic cues from the two devices, as these act independently and provide incongruent or mismatched information. Synchronized AGC activation may remove this perturbation and improve spatial hearing.
Design
Ten bimodal listeners were fitted with a bimodal prototype allowing real-time communication between hearing aid and CI. They were tested for their horizontal localization ability of brief (150 ms) noise bursts presented in the frontal hemisphere. To test for frequency- and level-specific effects, sounds were band-passed (broadband 0.25-6 kHz, high-pass 1.5-6 kHz, and low-pass 0.25-0.85 kHz) and presented at levels of 50 and 80 dBA (i.e., below and above AGC activation threshold).
Speech-in-noise recognition was assessed for short sentences presented at 75 dBA with a noise-adaptive algorithm. Sentences were presented from the front, with the noise presented either from the front or at a horizontal angle of ±70 deg.
The sound-localization and speech-recognition tests were performed both with independently acting and synchronized AGC settings.
Results
Localization bias (mean error [95% confidence interval lower, upper range]) varied with sound type: -46 [-53, -38], -31 [-37, -23] and -2 [-16, 10] deg for high-pass, broadband and low-pass sounds, respectively. AGC synchronization did not systematically affect bias (paired difference in mean error with independent and synchronized AGCs: 6 [-3, 20] deg). However, AGC synchronization did reduce sound-localization error variability for 80-dB broadband sounds by 15 [6, 26] deg from an error standard deviation of 67 [63, 77] to 52 [45, 59] deg for independent and synchronized AGCs, respectively (no systematic difference was found for 50-dB and the high-pass and low-pass sounds).
Speech-in-noise thresholds differed for noise presented from the front (0.3 [-1.3, 1.1] dB), implanted side (0.6 [-1.3, 2.3] dB), and non-implanted side (-2.4 [-4.6, 0.1] dB), but synchronization did not have an effect in these conditions. However, spatial release from masking thresholds did improve with synchronization when the noise was presented at the hearing-aid side: -2.7 [-4.5, -0.6] dB.
Conclusions
AGC synchronization improved the localization of broadband sounds presented above the AGC activation threshold in bimodal listeners. This is in line with the hypothesis that bimodal interaural level differences can be preserved for loud broadband sounds through AGC synchronization.
Synchronization did not compromise the listener’s speech perception abilities, despite the reduction of the level of speech when AGC kicks in. Moreover, spatial release from masking was enhanced, suggesting that improved spatial hearing benefits speech recognition in noise