Across-site variation in detection thresholds and maximum comfortable loudness levels for cochlear implants

Area of Science:

• Audiology
• Biomedical Engineering
• Neuroscience

Background:

• Cochlear implants (CIs) exhibit significant variation in loudness perception across stimulation sites, particularly with narrow-bipolar (BP) configuration.
• Monopolar (MP) stimulation generally shows less across-site variation than BP stimulation.
• Two models explain this variation: differences in electrode-to-neuron distance or neural population activation spread.

Purpose of the Study:

• To investigate the factors influencing across-site variation in detection thresholds (T levels) and maximum comfortable loudness levels (C levels) in cochlear implants.
• To test two competing models explaining how electrode configuration affects loudness perception variability.
• To examine the impact of electrode configuration, stimulus level, and array design on across-site variation.

Main Methods:

• Measured T and C levels in subjects with Nucleus CI24M and CI24R(CS) cochlear implants.
• Utilized both bipolar (BP) and monopolar (MP) electrode configurations for stimulation.
• Analyzed site-to-site variation in psychophysical levels across different conditions.

Main Results:

• BP stimulation showed 2.1-3.3 times greater site-to-site variation in T and C levels compared to MP stimulation.
• No significant difference in variation between T and C levels for the BP configuration, challenging the neural integration hypothesis.
• CI24M implants had lower mean C level variation than CI24R(CS) implants.

Conclusions:

• Monopolar stimulation significantly reduces loudness perception variability compared to bipolar stimulation in cochlear implants.
• Findings support models where electrode-to-neuron distance and current spread influence loudness variation.
• Electrode-array design also plays a role in modulating loudness perception variability.