Relationships between electrically evoked potentials and loudness growth in bilateral cochlear implant users

Area of Science:

• Audiology
• Neuroscience
• Biomedical Engineering

Background:

• Bilateral cochlear implant (CI) use aims to restore binaural hearing, but achieving matched loudness perception between ears is challenging.
• Understanding the relationship between objective neural responses and subjective loudness is crucial for optimizing CI programming.

Purpose of the Study:

• To investigate the correlation between electrically evoked compound action potential (ECAP) and electrically evoked auditory brainstem response (EABR) amplitude growth functions and loudness perception in bilateral CI users.
• To evaluate whether matching stimulus levels based on ECAP amplitude, EABR amplitude, or current level minimizes loudness discrepancies between the two ears.

Main Methods:

• Ten adult bilateral CI users participated, undergoing loudness judgments, ECAP, and EABR measurements.
• Stimuli involved biphasic current pulses (32 µs/phase) at 23 pps, with loudness assessed via constant stimuli method.
• ECAPs were measured via implant telemetry, and EABRs via surface electrodes; both apical and basal electrodes were tested in each ear.

Main Results:

• Stimuli with equal ECAP amplitudes resulted in average loudness differences of 14-20%, while equal EABR amplitudes yielded 13-50% differences.
• Matching stimulus current levels led to average loudness discrepancies of 14-16%.
• No statistically significant differences in loudness discrepancy were observed between ECAP amplitude matching, EABR amplitude matching, or current level matching.

Conclusions:

• Objective measures like ECAP and EABR amplitude, or matched current levels, do not reliably predict equal loudness perception in bilateral CI users.
• Current matching and neural response amplitude matching strategies showed similar, non-significant differences in approximating equal loudness across different electrode positions.