Speech Coding in the Midbrain: Effects of Sensorineural Hearing Loss

Laurel H. Carney, Duck O. Kim, Shigeyuki Kuwada

In response to voiced speech sounds, auditory-nerve (AN) fibres phase-lock to both harmonics near best frequency (BF) and to the fundamental frequency (F0) of voiced sounds. Due to nonlinearities in the healthy ear, phase-locking in each frequency channel tends to be dominated either by a single harmonic, for channels tuned near formants, or by F0, for channels between formants. The alternating dominance of these factors sets up a robust pattern in the F0-synchronized rate across best frequency (BF). This profile of a temporally coded measure is transformed into a mean rate profile at the level of the midbrain (inferior colliculus, IC), where neurons are sensitive to low-frequency fluctuations. In the impaired ear, the F0-synchronized rate profile is affected by several factors: Reduced synchrony capture decreases the dominance of a single harmonic near BF on the response, resulting in more widespread F0-synchrony. Elevated thresholds also reduce the effect of rate saturation, resulting in increased F0-synchrony. Wider peripheral tuning results in a wider-band envelope with reduced F0 amplitude. In general, sensorineural hearing loss reduces the contrast in AN F0-synchronized rates across BF. Here, we use computational models for AN and IC neurons to illustrate the F0-synchronized rate profiles set up in response to voiced speech sounds. The IC models include those for neurons with band-enhanced, band-suppressed, and hybrid modulation transfer functions. These models provide an illustration of the effects of peripheral hearing loss on central representations of voiced speech sounds.