Hearing loss degrades vowel coding primarily through F0-synchrony loss rather than mean firing rates

Evidence

Vowel coding studies have historically focussed on representation of spectral peaks, or formants, in AN rates or synchrony to stimulus components near BF. These representations are vulnerable to additive noise and vary with sound level. The sensitivity of midbrain neurons to low-frequency fluctuations inspired a shift of focus to the contrast in pitch-related fluctuations along the BF axis in vowel coding (Carney et al. 2015 ). In the healthy AN, there is a robust contrast in the profile of the F0-synchronized discharge rate across the BF axis (Fig. 2d ). Midbrain neurons’ sensitivities to these F0-synchronized rates transforms the temporally coded AN profile into a discharge rate profile (Fig. 4a ). The representation of F0-synchronized rates is affected by several mechanisms that are influenced by SNHL: synchrony capture, threshold (and thus rate saturation), and bandwidth of tuning. Bandwidth, in turn, affects the modulation spectrum of peripheral responses. Wider peripheral bandwidths associated with SNHL result in a wider modulation bandwidth and a reduction of the amplitude of low-frequency fluctuations. These factors have little influence on the mean rates of AN fibres; however, they have a large effect on F0-synchrony in the AN, which in turn affects the responses of IC neurons. — Dijk, Pim van, Physiology, Psychoacoustics and…, 5508

Categorization

• XREF: Connects to temporal coding of AM signals and how wideband peripheral filtering smears low-frequency envelope cues — parallels discussions of envelope coding deficits in the earlier auditory nerve chapters.
• QUESTION: The proposed experiment — restoring F0-synchrony profiles and testing speech-in-noise — is a concrete open question worth tracking in the SNHL signal-processing literature.