Comparing Patient-Specific Variations in Intra-Cochlear Neural Health Estimated Using Psychophysical Thresholds and Panoramic Electrically Evoked Compound Action Potentials (PECAPs)

Purpose: Variations in neural survival along the cochlear implant electrode array leads to off-place listening, resulting in poorer speech understanding outcomes for recipients. Therefore, it is important to develop and compare clinically viable tests to identify these patient-specific intra-cochlear neural differences.

Methods: Nineteen experienced cochlear implant recipients (9 males and 10 females) were recruited for this study. We estimated the neural health along the electrode array for a group of experienced adult implant recipients using two methods: the difference between psychophysical detection thresholds in bipolar vs. monopolar mode and the panoramic electrically evoked compound action potential method (PECAP). We hypothesised that: neural health estimated using both methods at single electrodes will be correlated at the participant level and the group level; and participants with larger variations in neural health along the electrode array will have poorer speech outcomes.

Results: At the individual level, the two neural measures correlated significantly across electrodes (p < 0.05) for 5 out of 15 participants. At the group level, we observed a weak but significant across-electrode correlation (R² = 0.111, p < 0.001). While a larger variation in neural measures estimated from psychophysical thresholds was associated with lower phoneme speech scores (R² = 0.499, p < 0.01), no significant association was found between variations in PECAP's neural health estimates and phoneme speech scores (R² = 0.082, p = 0.366).

Conclusion: Our evidence suggests that both methods likely quantify a shared underlying neural basis, hypothesised to be the neural health along the cochlear implant array. The differences between the two measures may be attributed to differences in stimulus rate or loudness used to elicit responses and/or the influence of factors arising more centrally than the auditory nerve.