A neural basis of speech-in-noise perception in older adults

Abstract

Objective: We investigated a neural basis of speech-in-noise perception in older adults. Hearing loss, the third most common chronic condition in older adults, is most often manifested by difficulty understanding speech in background noise. This trouble with understanding speech in noise, which occurs even in individuals who have normal-hearing thresholds, may arise, in part, from age-related declines in central auditory processing of the temporal and spectral components of speech. We hypothesized that older adults with poorer speech-in-noise (SIN) perception demonstrate impairments in the subcortical representation of speech.

Design: In all participants (28 adults, age 60-73 yr), average hearing thresholds calculated from 500 to 4000 Hz were ≤ 25 dB HL. The participants were evaluated behaviorally with the Hearing in Noise Test (HINT) and neurophysiologically using speech-evoked auditory brainstem responses recorded in quiet and in background noise. The participants were divided based on their HINT scores into top and bottom performing groups that were matched for audiometric thresholds and intelligent quotient. We compared brainstem responses in the two groups, specifically, the average spectral magnitudes of the neural response and the degree to which background noise affected response morphology.

Results: In the quiet condition, the bottom SIN group had reduced neural representation of the fundamental frequency of the speech stimulus and an overall reduction in response magnitude. In the noise condition, the bottom SIN group demonstrated greater disruption in noise, reflecting reduction in neural synchrony. The role of brainstem timing is particularly evident in the strong relationship between SIN perception and quiet-to-noise response correlations. All physiologic measures correlated with SIN perception.

Conclusion: Adults in the bottom SIN group differed from the audiometrically matched top SIN group in how speech was neurally encoded. The strength of subcortical encoding of the fundamental frequency appears to be a factor in successful speech-in-noise perception in older adults. Given the limitations of amplification, our results suggest the need for inclusion of auditory training to strengthen central auditory processing in older adults with SIN perception difficulties.

Figures

Figure 1

Mean pure tone thresholds (average of right and left) for top and bottom SIN groups from 125 to 12.5 kHz. No significant group differences were noted at any frequency. Error bars equal 1 standard error of measurement (SEM).

Figure 2

Top: The stimulus waveform of the speech syllable [da]. Middle: Grand average response waveforms to the speech syllable [da] (N=28) in quiet (gray) and noise (black). Bottom: Grand average frequency spectra of brainstem responses to the speech syllable [da] in quiet (gray) and noise (black) contain energy at the fundamental frequency (F0 = 100 Hz) and integer multiples up to 1000 Hz.

Figure 3

Top left: Grand average responses for top (red) and bottom (black) SIN groups to the speech syllable [da] in quiet. A significant RMS difference was noted (*p = 0.039). Top right: Grand average frequency spectra with significant difference for F0 (*p = 0.010) calculated over the entire response range (5 to 190 ms). Bottom: HINT scores are significantly related to RMS (left) and F0 (right) amplitudes.

Figure 4

Top left: Grand average responses for top (red) and bottom (black) SIN groups to the speech syllable [da] in six-talker babble. A significant RMS difference was noted (*p = 0.047). Top right: Grand average frequency spectra with the difference for F0 approaching significance (~ p = 0.062) when calculated over the entire range (5 to 190 ms). Bottom: HINT scores are significantly related to RMS (left) and F0 (right) amplitudes.

Figure 5

Responses in the bottom SIN group changed significantly in noise compared to those in the top SIN group. Response waveforms in quiet (gray) and noise (black) are shown for an individual in the top SIN group (top panel) and the bottom SIN group (middle panel). Bottom panel, left: HINT scores are highly correlated with quiet-to-noise correlation r-values (***p < .001) over the entire response (5 to 190 ms) Bottom panel, right: Bar graphs illustrating significant group differences in quiet-to-noise correlation r-values (p = 0.035). Error bars equal 1 SEM.