Ensemble responses of the auditory nerve to normal and whispered stop consonants

Whispered syllables lack many of the frequency and voicing cues of normally voiced speech, but these two acoustically distinct forms of speech are placed into the same linguistic categories. To examine how whispered and voiced speech are encoded in the auditory system, the responses to speech sounds were recorded from 132 single auditory nerve fibers in 20 ketamine anesthetized chinchillas. Stimuli were the naturally produced syllables /da/ and /ta/ presented in whispered and normal voicing. The results for each syllable presented at a fixed intensity were analyzed by pooling the responses from individual auditory nerve fibers across animals to create a global average peri-stimulus time (GAPST) histogram. For each word-initial consonant, the pattern of peaks in the GAPST was the same for both normal and whispered speech. For the vowel the GAPSTs for the whispered speech sounds did not display the synchronization observed in the responses to the voiced syllables. The temporal pattern of the peaks was constant over a 40 dB intensity range, although peak sizes varied. Grouping fibers within different frequency ranges created local averages (LAPST) that revealed the significant contribution of high frequency fibers in the response to the whispered consonants. Responses of individual fibers varied with both the syllable and the voicing. These findings suggest that the encoding of either a whispered or a normal stop consonant results in the same temporal pattern in the ensemble response.