Auditory nerve representation of naturally-produced vowels with variable acoustics

This investigation compared the encoding of naturally-produced, whispered and normally-voiced vowels by auditory nerve fibers. Speech syllables containing the vowels /o/ and /æ/ were produced by two female speakers and presented at three intensities to ketamine-anesthetized chinchillas. Six different representations of the spectral components in the vowels in the responses of the auditory nerve fibers were evaluated. For both normal and whispered vowels over a 30 dB range, the formant peaks in the vowel were best displayed using rate-place representations. The spectral detail in the vowel was revealed by average localized synchronized rates (ALSR) and autocorrelations of individual peristimulus time histograms. The average localized interval rates (ALIR), autocorrelations of ensemble responses, and autocorrelations of individual spike trains demonstrated poor representations of vowel spectra, although the frequency components of normally-voiced vowels had better representations than those of whispered vowels. These analyses suggest that rate-based and synchronization-based measures yields two very different pieces of information, but only a normalized rate-based measure consistently identified the formants of both the whispered and normally-voiced vowels.