Psychoacoustic and phoneme identification measures in cochlear-implant and normal-hearing listeners.

The purpose of this study is to identify precise and repeatable measures for assessing cochlear-implant (CI) hearing. The study presents psychoacoustic and phoneme identification measures in CI and normal-hearing (NH) listeners, with correlations between measures examined. Psychoacoustic measures included pitch discrimination tasks using pure tones, harmonic complexes, and tone pips; intensity perception tasks included intensity discrimination for tones and modulation detection; spectral-temporal masking tasks included gap detection, forward and backward masking, tone-on-tone masking, synthetic formant-on-formant masking, and tone in noise detection. Phoneme perception measures included vowel and consonant identification in quiet and stationary and temporally gated speech-shaped noise. Results on psychoacoustic measures illustrate the effects of broader filtering in CI hearing contributing to reduced pitch perception and increased spectral masking. Results on consonant and vowel identification measures illustrate a wide range in performance across CI listeners. They also provide further evidence that CI listeners obtain little to no release of masking in temporally gated noise compared to stationary noise. The forward and backward-masking measures had the highest correlation with the phoneme identification measures for CI listeners. No significant correlations between speech reception and psychoacoustic measures were observed for NH listeners. The superior NH performance on measures of phoneme identification, especially in the presence of background noise, is a key difference between groups.

Analysis of speech-based Speech Transmission Index methods with implications for nonlinear operations.

The Speech Transmission Index (STI) is a physical metric that is well correlated with the intelligibility of speech degraded by additive noise and reverberation. The traditional STI uses modulated noise as a probe signal and is valid for assessing degradations that result from linear operations on the speech signal. Researchers have attempted to extend the STI to predict the intelligibility of nonlinearly processed speech by proposing variations that use speech as a probe signal. This work considers four previously proposed speech-based STI methods and four novel methods, studied under conditions of additive noise, reverberation, and two nonlinear operations (envelope thresholding and spectral subtraction). Analyzing intermediate metrics in the STI calculation reveals why some methods fail for nonlinear operations. Results indicate that none of the previously proposed methods is adequate for all of the conditions considered, while four proposed methods produce qualitatively reasonable results and warrant further study. The discussion considers the relevance of this work to predicting the intelligibility of cochlear-implant processed speech.