Effects of sensorineural hearing loss on formant-frequency discrimination: Measurements and models

In the response of the healthy ear to a harmonic sound, auditory-nerve rate functions fluctuate at the fundamental frequency, f0. The amplitude of these fluctuations varies along the tonotopic axis. Neural fluctuation (NF) amplitudes encode spectral peaks, including formant frequencies of vowels, because responses of inner-hair-cells (IHCs) tuned near spectral peaks are captured (or dominated) by a single harmonic, resulting in lower fluctuation amplitudes than responses of IHCs tuned between spectral peaks. This NF code is robust across a wide range of sound levels and in background noise. The NF profile is converted into a rate-place representation in the auditory midbrain, wherein neurons are sensitive to low-frequency fluctuations. The NF code is vulnerable to sensorineural hearing loss (SNHL) because capture depends upon saturation of IHCs, and thus the interaction of cochlear gain with IHC transduction. In this study, formant-frequency discrimination limens (DLFFs) were estimated for listeners with normal hearing or mild to moderate SNHL. The difficulty of the task was modulated by varying formant bandwidth to modulate the contrast in the NF profile. The formants were either aligned with harmonic frequencies or placed between harmonics. Formant frequencies were 600 and 2000 Hz, in the range of first and second formants of several vowels. Results were compared to predictions from model auditory-nerve (AN) and inferior colliculus (IC) neurons, with listeners audiograms used to individualize the AN model. Correlations between DLFFs, audiometric thresholds near the formant frequencies, age, and scores on the Quick speech-in-noise test are reported. SNHL had a strong effect on DLFF for the second formant frequency (F2), but relatively small effect on DLFF for the first formant. The IC model provided good predictions of elevated listener thresholds to F2 as a function of SNHL.