Formant-frequency discrimination in listeners with hearing loss: Behavioral results and neural models

Abstract

In responses of the healthy ear to harmonic sounds, the amplitude of f0-related fluctuations of auditory-nerve (AN) responses varies along the tonotopic axis. Neural fluctuation 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 IHCs tuned between spectral peaks. This code is robust across a wide range of sound levels and in background noise, and is converted into a rate-place representation in the auditory midbrain, where neurons are sensitive to low-frequency fluctuations. This neural-fluctuation code is vulnerable to sensorineural hearing loss (SNHL), because it depends upon saturation of IHCs and the interaction of cochlear amplification with IHC transduction. We tested formant-frequency discrimination in listeners with normal hearing or mild to moderate SNHL. The difficulty of the task was modulated by varying formant bandwidth. The relationship between spectral peaks and harmonic frequencies was also varied. Formant frequencies were 600 and 2000 Hz, in the range of first and second formants of many vowels. Results were compared to predictions made using model AN and midbrain neurons, with audiograms for the individual listeners included in the peripheral model. [Work supported by NIDCD-R01-001641]