The representation of noise vocoded speech in the auditory nerve of the chinchilla: Physiological correlates of the perception of spectrally reduced speech

This study investigated the neural representation of naturally produced and noise vocoded speech signals in the auditory nerve of the chinchilla. The syllables /bɔ/,/də̢/,/tu/and/pə̢/ produced by male speakers were used to synthesize noise vocoded speech stimuli containing one, two, three and four bands of envelope modulated noise. The ensemble response of the auditory nerve, computed by pooling the PST histograms across many auditory nerve fibers, revealed temporal patterns in the responses to the natural tokens that uniquely identified the stop consonants. The responses to the 3- and 4-band noise vocoded tokens contained temporal patterns that were nearly identical to those observed for the natural tokens, while the responses to the 1- and 2-band tokens were significantly different (p < 0.0001). The ALSR, ALIR and autocorrelation of the pooled PST histograms represented the detail of the frequency spectrum for a naturally produced vowel, while the driven rate was unreliable. Each of these spectral analyses failed to reveal significant information about the noise vocoded vowels. These results suggest that temporal patterns in the responses of the auditory nerve can provide the cues necessary for the recognition of noise vocoded stop consonants.