Predicted effects of sensorineural hearing loss on across-fiber envelope coding in the auditory nerve

Abstract

Cross-channel envelope correlations are hypothesized to influence speech intelligibility, particularly in adverse conditions. Acoustic analyses suggest speech envelope correlations differ for syllabic and phonemic ranges of modulation frequency. The influence of cochlear filtering was examined here by predicting cross-channel envelope correlations in different speech modulation ranges for normal and impaired auditory-nerve (AN) responses. Neural cross-correlation coefficients quantified across-fiber envelope coding in syllabic (0-5 Hz), phonemic (5-64 Hz), and periodicity (64-300 Hz) modulation ranges. Spike trains were generated from a physiologically based AN model. Correlations were also computed using the model with selective hair-cell damage. Neural predictions revealed that envelope cross-correlation decreased with increased characteristic-frequency separation for all modulation ranges (with greater syllabic-envelope correlation than phonemic or periodicity). Syllabic envelope was highly correlated across many spectral channels, whereas phonemic and periodicity envelopes were correlated mainly between adjacent channels. Outer-hair-cell impairment increased the degree of cross-channel correlation for phonemic and periodicity ranges for speech in quiet and in noise, thereby reducing the number of independent neural information channels for envelope coding. In contrast, outer-hair-cell impairment was predicted to decrease cross-channel correlation for syllabic envelopes in noise, which may partially account for the reduced ability of hearing-impaired listeners to segregate speech in complex backgrounds.

Figure 1

Model threshold elevations for the selective OHC impairment conditions in the present study. Data points represent the CFs used in this study. For CFs above 1 kHz, a fixed 30-dB loss was approximated by choosing appropriate values of C_OHC between 0 and 1 in the model. Threshold shifts less than 30 dB for CFs < 1 kHz represent the maximum amount of OHC loss (i.e., complete OHC loss corresponding to C_OHC = 0) produced by the AN model, which has reduced cochlear gain at lower CFs.

Figure 2

Correlogram analyses of envelope coding within (columns 1 and 2) and across (column 3) spike trains from two chinchilla AN fibers [(A) CF_A=827 Hz; (B) CF_B=618 Hz; 0.4-octave separation] responding to the same broadband noise. (A),(B) Normalized shuffled autocorrelograms [thick line, e.g., SAC(A+)] and cross-polarity correlogram [thin line, e.g., SCC(A+,A−)]. (C) Shuffled cross-fiber correlogram [thick line, e.g., SCC(A+,B+)] and cross-fiber, cross-polarity correlogram [thin line, e.g., SCC(A+,B−)]. The X denotes characteristic delay (CD_SCC = 400 μs), which occurs due to the traveling-wave delay. (D)–(F) Corrected sumcors (see text), which emphasize envelope coding, were based on the average of the cross-polarity and auto- or cross-fiber correlograms. Sumcor peak height [in (D) and (E)] quantifies within-fiber envelope coding. Across-CF envelope coding is quantified with a neural cross-correlation coefficient [ρ_ENV, Eq. (1)] by comparing the peak heights of sumcor(AB) to sumcor(A) and sumcor(B). Spike train data from Heinz and Swaminathan (2009).

Figure 3

Model predictions of the effects of SNHL on across-CF coding of envelope for CFs centered at 500 Hz [panel (A)] and 3500 Hz [panel (B)]. Stimuli: Broadband noise; duration: 1.7 s. ρ_ENV is plotted as a function of CF separation. Each data point represents mean ρ_ENV computed over twenty repetitions. Filled circles: Predictions for normal hearing at best modulation levels (BML = 45 dB SPL for 500 Hz and 30 dB SPL for 3500 Hz). Filled diamonds: Predictions for normal hearing at the same sound level as the predictions for the impaired cases (65 dB SPL for 500 Hz and 60 dB SPL for 3500 Hz). Open triangles: OHC damage (21 dB loss for 500 Hz, see Fig. 1; 30 dB loss for 3500 Hz); open squares: 30-dB IHC damage. BML for the impaired case was 65 dB SPL for 500 Hz and 60 dB SPL for 3500 Hz. The smallest CF separation (ΔCF in octaves) at which ρ_ENV dropped to 0.7 is shown using arrows and the values are inset. The NH value in parenthesis represents the NH prediction for the equal-SPL condition.

Figure 4

Model predictions of the effects of SNHL on across-CF coding of speech envelopes for CFs geometrically centered at 500 Hz. Stimuli: Speech (“A boy fell from the window”), duration: 1.72 s. ρ_ENV is plotted as a function of CF separation for overall envelope [panel (A) 0–300 Hz], syllabic envelope [panel (B) 0–5 Hz], phonemic envelope [panel (C) 5–64 Hz], and periodicity envelope [panel (D) 64–300 Hz]. Each data point represents mean ρ_ENV computed over twenty repetitions. Filled circles: Predictions for NH at BML (40 dB SPL). Filled diamonds: Predictions for NH at the same sound level as the predictions for the impaired cases (65 dB SPL). Open triangles: maximal (21 dB) hearing loss due to OHC damage; Open squares: 30-dB IHC damage. BML for the impaired case was 65 dB SPL. The smallest CF separation (ΔCF in octaves) at which ρ_ENV dropped to 0.7 is shown using arrows and the values are inset. The NH value in parenthesis represents the NH prediction for the equal-SPL condition.

Figure 5

Degree of cross correlation (DCC) is plotted as a function of SNR for speech in quiet and in noise for each modulation frequency range. (A) syllabic (0–5 Hz); (B) phonemic (5–64 Hz); (C) periodicity (64–300 Hz). Filled symbols represent predictions for NH and open symbols represent predictions for selective OHC impairment.

Figure 6

DCC as a function of SNR for envelope vocoded speech in quiet and in noise for each modulation frequency range. Figure layout is the same as for Fig. 5.