Auditory efferents facilitate sound localization in noise in humans

Abstract

The mammalian auditory system contains descending neural pathways, some of which project onto the cochlea via the medial olivocochlear (MOC) system. The function of this efferent auditory system is not entirely clear. Behavioral studies in animals with olivocochlear (OC) lesions suggest that the MOC serves to facilitate sound localization in noise. In the current work, noise-induced OC activity (the OC reflex) and sound-localization performance in noise were measured in normal-hearing humans. Consistent with earlier studies, both measures were found to vary substantially across individuals. Importantly, significant correlations were observed between OC-reflex strength and the effect of noise on sound-localization performance; the stronger the OC reflex, the less marked the effect of noise. These results suggest that MOC activation by noise helps to counteract the detrimental effects of background noise on neural representations of direction-dependent spectral features, which are especially important for accurate localization in the up/down and front/back dimensions.

Figure 1.

a, Apparatus used to measure sound-source localization. b, Illustration of the contralateral OAE suppression effect used to measure the MOC reflex in humans. The blue and red curves show examples of OAE traces recorded in the ear of one subject without (blue) and with (red) noise in the contralateral ear. Note that in the presence of contralateral noise, OAEs are reduced; this reflects the suppressive influence of noise-induced efferent MOC activity on OHC motility. c, Individual psychometric functions showing percentage correct as a function of SNR in the localization task. d, Confusion matrix showing the proportion of times that a speaker was identified by listeners as having emitted the signal out of the total number of times that this speaker actually emitted the signal, for the −2.5 dB SNR (group-mean data). Speakers are identified by numbers from 1 to 8. These numbers corresponded to those indicated in a. The speakers were ordered based on, first, whether they were below or above the horizontal plane (down/up); second, whether they were in the front of or behind the listener (front/back); and finally, whether they were to the right (R) or the left (L) of the listener. Red arrows indicate entries corresponding to substantial front/back and up/down confusions. e, Scatter plot and regression line relating the differences in percentage correct between the quiet and noisiest conditions in the localization task to the contralateral OAE suppression effects (circles, right ear; triangles, left ear; squares, mean) across the 18 listeners. The regression line is drawn from the mean contralateral OAE suppression (squares). f, Spearman correlation coefficients between contralateral OAE suppression effects and decreases in percentage correct sound-localization because of noise for each SNR tested. The error bars show ± one standard error (SE) of the mean. Significant correlations at the 0.05 level are indicated by an asterisk (*); one correlation, which was significant at the 0.001 level, is indicated by two asterisks (**).