title: Speech Coding in the Midbrain: Effects of Sensorineural Hearing Loss.
abstract: In response to voiced speech sounds, auditory-nerve (AN) fibres phase-lock to 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 is 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 of F0-synchronized rate across best frequency (BF). This profile of a temporally coded measure is transformed into a mean rate profile in 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. 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. Computational models for AN and IC neurons illustrate how hearing loss would affect the F0-synchronized rate profiles set up in response to voiced speech sounds.
authors.affiliation: Departments of Biomedical Engineering, Neurobiology & Anatomy, Electrical & Computer Engineering, University of Rochester, Rochester, NY, USA. Laurel.Carney@Rochester.edu.
authors.forename: Laurel H
authors.initials: LH
authors.lastname: Carney
authors.affiliation: Department f Neuroscience, University of Connecticut Health Center, Farmington, CT, USA.
authors.forename: Duck O
authors.initials: DO
authors.lastname: Kim
authors.affiliation: Department f Neuroscience, University of Connecticut Health Center, Farmington, CT, USA.
authors.forename: Shigeyuki
authors.initials: S
authors.lastname: Kuwada
mesh_terms: D001309:Auditory Threshold; D003056:Cochlear Nerve / Q000502:physiology; D006319:Hearing Loss, Sensorineural / Q000503:physiopathology*; D006801:Humans; D007245:Inferior Colliculi / Q000502:physiology*; D013067:Speech Perception / Q000502:physiology*
keywords: Modulation transfer function; Neural coding; Vowels