Head Shadow, Summation, and Squelch in Bilateral Cochlear-Implant Users With Linked Automatic Gain Controls

Area of Science:

• Audiology
• Neuroscience
• Biomedical Engineering

Background:

• Bilateral cochlear-implant (BICI) users exhibit reduced speech understanding in noise compared to normal-hearing individuals.
• Independent automatic gain controls (AGCs) in BICI devices may impair binaural hearing by reducing interaural level differences.
• Linked AGCs are hypothesized to enhance binaural benefits by preserving or increasing interaural level differences.

Purpose of the Study:

• To investigate the impact of linked versus independent AGCs on binaural hearing benefits in BICI users.
• To assess how AGCs affect summation, head shadow, and squelch effects in speech-on-speech masking scenarios.
• To explore the interaction between AGC type, interaural asymmetry, and masker location on speech perception.

Main Methods:

• Nine BICI users participated in the study.
• Speech understanding was measured using sentences with a single masker at various azimuths and target-to-masker ratios.
• Research processors manipulated AGC type (independent vs. linked) and tested individual ears or both.
• Sentence recall in quiet assessed interaural functional asymmetry.

Main Results:

• AGC type did not significantly impact overall speech understanding performance or binaural benefits.
• Significant interactions were observed between interaural functional asymmetry and ear/interferer location.
• Greater summation and squelch benefits were linked to larger functional asymmetries, driven by the better ear's performance.

Conclusions:

• Linked AGCs did not significantly alter binaural benefits for speech-on-speech masking in this BICI population.
• No significant detriment was observed with linked AGCs across tested conditions.
• Individual interaural functional asymmetry plays a crucial role in determining binaural benefits, often exceeding the impact of AGC strategy.