Functional Interplay Between the Putative Measures of Rostral and Caudal Efferent Regulation of Speech Perception in Noise

Abstract

Efferent modulation has been demonstrated to be very important for speech perception, especially in the presence of noise. We examined the functional relationship between two efferent systems: the rostral and caudal efferent pathways and their individual influences on speech perception in noise. Earlier studies have shown that these two efferent mechanisms were correlated with speech perception in noise. However, previously, these mechanisms were studied in isolation, and their functional relationship with each other was not investigated. We used a correlational design to study the relationship if any, between these two mechanisms in young and old normal hearing individuals. We recorded context-dependent brainstem encoding as an index of rostral efferent function and contralateral suppression of otoacoustic emissions as an index of caudal efferent function in groups with good and poor speech perception in noise. These efferent mechanisms were analysed for their relationship with each other and with speech perception in noise. We found that the two efferent mechanisms did not show any functional relationship. Interestingly, both the efferent mechanisms correlated with speech perception in noise and they even emerged as significant predictors. Based on the data, we posit that the two efferent mechanisms function relatively independently but with a common goal of fine-tuning the afferent input and refining auditory perception in degraded listening conditions.

Keywords: ABR; brainstem; contextual encoding; corticofugal; efferent; olivocochlear bundle; otoacoustic emissions.

FIG. 1

A simplified illustration of the descending pathways in the central auditory pathway based on Terreros and Delano (2015) (With permission). The black paths are the afferent paths and the grey paths are the efferent paths. It can be seen that there are many rich interconnections in the efferent auditory pathway. The efferent innervations originating from the cortex and the upper brainstem are operationally termed rostral efferents, and those originating in the lower brainstem are operationally termed the caudal efferents. AC auditory cortex, MGB medial geniculate body, IC inferior colliculus, SOC superior olivary complex, CN cochlear nucleus.

FIG. 2

The waveforms and spectrograms of the four stimuli used in the study. The salient acoustic characteristics of each syllable are given in the attached table.

FIG. 3

Schematic representation of the two paradigms used in the study. The upper panel shows the variable paradigm and the lower panel shows the repetitive paradigm.

FIG. 4

Mean SNR-50 in the two groups. Error bars represent 95 % confidence interval of mean. The older group had higher mean SNR-50 than the younger group. Lower SNR-50 indicates better speech perception in noise and also greater tolerance to disruptive effects of noise.

FIG. 5

a ABR waveforms in the younger and older groups in the repetitive and the variable paradigms. b A 256 point short-term Fourier transform spectrograms with a 90 % overlapping Tukey window of the difference ABR waveforms (repetitive minus variable) from the repetitive and variable paradigms in the two groups. Warm colours indicate greater contextual effect and cold colours indicate lesser contextual effect. Difference ABR spectrograms show greater contextual effect in younger adults compared to the older adults. c Bar plots of the contextual effect on the spectral magnitudes in the younger and older groups. Error bars represent 95 % confidence interval of mean. It can be seen that the contextual effect on the spectral magnitudes are highest in the younger group at F0 while in the older group, this contextual effect is not present.

FIG. 6

Mean magnitude of contralateral suppression of OAE (CSOAE_mag) in the two groups. Error bars represent 95 % confidence interval of mean. It can be seen that the mean CSOAE_mag is higher in the younger group when compared to the older group.

FIG. 7

a–c The bivariate scatter plots among the different measures of efferent modulation and speech perception in noise. It can be seen that there is no visible trend between the contextual effect of ABR with that of CSOAE. r depicts the Pearson’s correlation coefficient and r _p depicts the partial correlation coefficient, when controlling for the effects of FFR spectral magnitudes (IF0, IH2, IH3) in the variable paradigm. Asterisks show significant pairs with significant correlation at 0.05 level of significance. Removal of the outlier in the elder group changed the CSOAE to SNR-50 correlation coefficient by −0.005, and the CSOAE to contextual effect of ABR by +0.1 (P still not significant); hence, the data was retained.

FIG. 8

Figure depicting the results of within-group multiple linear regression predicting the speech in noise perception based on the efferent modulation measures. IF0 indicates the afferent index of processing, i.e., the spectral magnitude at F0 in the variable paradigm. The thick arrows connect the significant predictors of speech perception in noise and corresponding numbers indicate the standardized regression coefficients. The thin arrows connecting to SNR-50 indicate that they were not significant predictors and the corresponding numbers indicate the standardized regression coefficients. An additional multiple linear regression on the data pooled across groups shows that the model significantly predicted SNR-50 (r ² = 0.622, adjusted r ² = 0.600). CSOAE_mag (β = −0.565, P < 0.05) predicted SNR-50 maximally followed by contextual effect at F0 (β = −0.-0.482, P < 0.05) followed by spectral magnitude at F0 in the variable paradigm (β = −0.272, P < 0.05).