Representation of whispered vowels in discharge patterns of auditory-nerve fibers

We have previously shown that the spectra of speech sounds can be represented in the temporal patterns of discharge of populations of auditory-nerve fibers. These results were obtained using perfectly periodic stimuli, for which a temporal representation is straightforward. In order to see if our results could be generalized to nonperiodic stimuli, we have studied responses to a whispered vowel with formant frequencies typical of /epsilon/. The whispered vowel was generated by exciting a vocal tract model with noise; this signal was therefore aperiodic. Temporal patterns of responses to the vowel in populations of auditory-nerve fibers were analyzed using interval histograms. Fourier transforms of these histograms show large components at frequencies near the formant frequencies of the vowel. With these Fourier transform components as a measure of the temporal response, a temporal place representation of the response of populations of fibers preserves the spectral features of the aperiodic vowel stimulus. Profiles of average rate versus characteristic frequency for fibers with spontaneous rates greater than 1/s show little if any formant-related structure. On the other hand, such profiles for fibers with spontaneous rates less than 1/s may show peaks in the region of the formants.