Responses to cochlear normalized speech stimuli in the auditory nerve of cat

Previous studies of auditory-nerve fiber (ANF) representation of vowels in cats and rodents (chinchillas and guinea pigs) have shown that, at amplitudes typical for conversational speech (60-70 dB), neuronal firing rate as a function of characteristic frequency alone provides a poor representation of spectral prominences (e.g., formants) of speech sounds. However, ANF rate representations may not be as inadequate as they appear. Here, it is investigated whether some of this apparent inadequacy owes to the mismatch between animal and human cochlear characteristics. For all animal models tested in earlier studies, the basilar membrane is shorter and encompasses a broader range of frequencies than that of humans. In this study, a customized speech synthesizer was used to create a rendition of the vowel [E] with formant spacing and bandwidths that fit the cat cochlea in proportion to the human cochlea. In these vowels, the spectral envelope is matched to cochlear distance rather than to frequency. Recordings of responses to this cochlear normalized [E] in auditory-nerve fibers of cats demonstrate that rate-based encoding of vowel sounds is capable of distinguishing spectral prominences even at 70-80-dB SPL. When cochlear dimensions are taken into account, rate encoding in ANF appears more informative than was previously believed.