Sensitivity of bilateral cochlear implant users to fine-structure and envelope interaural time differences

Area of Science:

• Auditory Neuroscience
• Biomedical Engineering
• Speech Processing

Background:

• Bilateral cochlear implant (CI) users exhibit limited sensitivity to interaural time differences (ITDs) in high-rate pulse trains.
• This deficit hinders the use of fine-structure ITD cues crucial for sound localization.
• Prior animal studies suggested amplitude modulation might improve ITD sensitivity in high-rate pulse trains.

Purpose of the Study:

• To investigate if amplitude modulation can restore ITD sensitivity to high-rate pulse trains in humans with bilateral CIs.
• To assess ITD sensitivity when cues are encoded in the carrier, envelope, or whole waveform.
• To evaluate envelope ITD sensitivity across various modulation frequencies relevant to speech.

Main Methods:

• Measured ITD thresholds in six bilateral CI users under three conditions: carrier ITD, envelope ITD, and whole waveform ITD.
• Tested high-rate pulse trains (1000 pulses-per-second) with sinusoidal amplitude modulation.
• Assessed envelope ITD sensitivity as a function of modulation frequency (4-16 Hz and 100 Hz).

Main Results:

• Five out of six subjects showed no significant ITD sensitivity in the 1000-pps carrier, even with modulation.
• One subject demonstrated improved ITD sensitivity with modulation in the carrier.
• Peak envelope ITD sensitivity occurred at 100 Hz, with rapid degradation outside a narrow range; sensitivity was poor for speech-relevant modulation frequencies (4-16 Hz).

Conclusions:

• Current bilateral CI processors are unlikely to effectively encode ITD in the carrier pulse train.
• Human ITD sensitivity to the speech envelope is limited, particularly within crucial frequency ranges for speech perception.
• Further research is needed to optimize ITD encoding strategies for bilateral CI users.