Establishment and verification of auditory brainstem implant vocoder model

Objective·To develope an auditory brainstem implant (ABI) vocoder based on cochlear implant (CI) vocoder characteristics and ABI electrode array topology, and to verify its reliability.Methods·An "n-of-m" coding strategy CI/ABI vocoder was constructed based on MATLAB. Within each frame, only the envelopes of the n channels with the highest energy were selected. The interaction coefficient (IC) (range: 1‒3), channel numbers (range: 5‒22), and electrode array topology (CI/ABI) were adjustable parameters, allowing for the synthesis of simulated speech. Psychoacoustic evaluation was employed, recruiting normal hearing subjects to perform closed-set simulated phoneme perception. The phoneme recognition accuracy (20 vowel questions/condition, 11 consonant questions/condition) was compared with the corresponding conditions of CI and ABI from reference literature to determine the IC value of the vocoder and verify its reliability.Results·The vocoder successfully synthesized all test stimuli. In the closed-set CI-simulated speech recognition, the simulated vowel and consonant recognition accuracy for IC2 and IC3 conditions showed no significant difference compared to the accuracy reported in the CI reference literature (P>0.05). The difference in vowel and consonant accuracy between IC2 and the literature was smaller than that between IC3 and the literature (vowel |d|=1.6% vs. 20%, consonant |d|=8.4% vs. 9.9%), thus determining the optimal interaction coefficient of this model as 2. Subsequently, when modifying the electrode array topology to ABI, it was found that the simulated phoneme recognition accuracy for a 16-channel ABI was significantly lower than that for the 16-channel CI group, consistent with the reported literature. The simulated vowel and consonant accuracy within the 5‒8 channel range for ABI showed no significant difference (P>0.05), also aligning with the trend reported in the literature.Conclusion·A CI/ABI vocoder based on "n-of-m" coding strategy is established and the optimal IC is determined. The established ABI encoder has been evaluated for high reliability through psychoacoustic experiments. It provides suitable technical means for validating ABI-specific coding strategies.