Speech Perception with Noise Vocoding and Background Noise: An EEG and Behavioral Study.

This study explored the physiological response of the human brain to degraded speech syllables. The degradation was introduced using noise vocoding and/or background noise. The goal was to identify physiological features of auditory-evoked potentials (AEPs) that may explain speech intelligibility. Ten human subjects with normal hearing participated in syllable-detection tasks, while their AEPs were recorded with 32-channel electroencephalography. Subjects were presented with six syllables in the form of consonant-vowel-consonant or vowel-consonant-vowel. Noise vocoding with 22 or 4 frequency channels was applied to the syllables. When examining the peak heights in the AEPs (P1, N1, and P2), vocoding alone showed no consistent effect. P1 was not consistently reduced by background noise, N1 was sometimes reduced by noise, and P2 was almost always highly reduced. Two other physiological metrics were examined: (1) classification accuracy of the syllables based on AEPs, which indicated whether AEPs were distinguishable for different syllables, and (2) cross-condition correlation of AEPs (r) between the clean and degraded speech, which indicated the brain's ability to extract speech-related features and suppress response to noise. Both metrics decreased with degraded speech quality. We further tested if the two metrics can explain cross-subject variations in their behavioral performance. A significant correlation existed for r, as well as classification based on early AEPs, in the fronto-central areas. Because r indicates similarities between clean and degraded speech, our finding suggests that high speech intelligibility may be a result of the brain's ability to ignore noise in the sound carrier and/or background.