Neural Coding of Interaural Time Differences with Bilateral Cochlear Implants in Unanesthetized Rabbits.

Area of Science:

• Neuroscience
• Auditory system research
• Cochlear implant technology

Background:

• Bilateral cochlear implants (CIs) enhance sound localization and speech perception but often show reduced interaural time difference (ITD) sensitivity compared to normal hearing.
• ITD sensitivity in CI users deteriorates with increasing stimulation rates, particularly at high frequencies used for speech processing.

Purpose of the Study:

• To investigate the neural basis of ITD sensitivity degradation in bilateral CI users.
• To characterize ITD tuning in the inferior colliculus (IC) of an unanesthetized rabbit model with bilateral CIs.

Main Methods:

• Single neurons in the inferior colliculus (IC) were studied using pulse train stimuli in an unanesthetized rabbit model with bilateral cochlear implants (CIs).
• ITD sensitivity was assessed based on overall firing rates and synchronized responses using temporal windowing.
• Neural just-noticeable differences (JNDs) in ITD were calculated using signal detection theory to compare with human perceptual data.

Main Results:

• Approximately 73% of IC neurons exhibited significant ITD sensitivity.
• Optimal ITD sensitivity occurred at pulse rates of 80-160 pulses per second (pps), with degradation at lower and higher rates.
• Temporal windowing revealed ITD sensitivity in neurons masked by background activity at low pulse rates.
• With temporal windowing, ITD sensitivity decreased monotonically with increasing pulse rate.
• Neural ITD JNDs, calculated using appropriate methods for different pulse rates, fell within the range of human perceptual JNDs.

Conclusions:

• Neural ITD sensitivity in the IC is influenced by stimulation rate and background neural activity.
• The study demonstrates a correlation between neural ITD processing in the auditory midbrain and human performance with bilateral CIs.
• Findings provide insights into the neural mechanisms underlying ITD perception in bilateral CI users, particularly concerning the impact of stimulation rates.