Effects of hearing loss on the subcortical representation of speech cues

Abstract

Individuals with sensorineural hearing loss often report frustration with speech being loud but not clear, especially in background noise. Despite advanced digital technology, hearing aid users may resort to removing their hearing aids in noisy environments due to the perception of excessive loudness. In an animal model, sensorineural hearing loss results in greater auditory nerve coding of the stimulus envelope, leading to a relative deficit of stimulus fine structure. Based on the hypothesis that brainstem encoding of the temporal envelope is greater in humans with sensorineural hearing loss, speech-evoked brainstem responses were recorded in normal hearing and hearing impaired age-matched groups of older adults. In the hearing impaired group, there was a disruption in the balance of envelope-to-fine structure representation compared to that of the normal hearing group. This imbalance may underlie the difficulty experienced by individuals with sensorineural hearing loss when trying to understand speech in background noise. This finding advances the understanding of the effects of sensorineural hearing loss on central auditory processing of speech in humans. Moreover, this finding has clinical potential for developing new amplification or implantation technologies, and in developing new training regimens to address this relative deficit of fine structure representation.

Figure 1

Top: Average pure-tone thresholds for the normal hearing group (gray) and the hearing impaired group (black). Bottom. Response waveforms to the click stimulus from individuals in the normal hearing group and the hearing impaired group. An apparent delay at wave I in the hearing impaired participant is no longer present at wave V.

Figure 2

(A) The spectrogram of the 40-ms syllable [da]. Fast Fourier transforms were calculated from 20–42 ms for the stimulus (B) and in responses to the envelope (C) and the TFS (D) in quiet and in noise. The average responses for the NH group are displayed.

Figure 3

Comparison of response spectra to the 40-ms stimulus over the frequency following response (20–42 ms) for the NH (red) versus HI groups (unamplified stimulus: gray; amplified stimulus: black) in quiet and in noise. (A), (B) Response spectra to the envelope. The HI group (in response to the unamplified stimulus) has higher amplitudes in the envelope-dominated low frequencies (F₀-H₃) in noise. (C), (D) There are no group differences in the response to the TFS-dominated high frequencies (H₄–H₆). (E), (F) In response to the envelope, the HI group (amplified stimulus) has higher amplitudes in the low frequencies (overall) in quiet and noise. (G), (H) There are no group differences in response to the TFS between the HI (amplified stimulus) and NH groups. (I), (J) Larger delta H_x differences over the range of F₀–H₆ are noted in the quiet condition for the HI group (amplified stimulus) compared to the NH group and in the noise condition for the HI group in response to both unamplified and amplified stimuli. Errors bars in (I) and (J): 1 S.E. *p < 0.05, **p < 0.01.