Abstract
Spatial hearing, which largely relies on binaural time/level cues, is a challenge for patients with asymmetric hearing. The degree of the deficit is largely variable, and better sound localization performance is frequently reported. Studies on the compensatory mechanism revealed that monaural level cues and monoaural spectral cues contribute to variable behavior in those patients who lack binaural spatial cues. However, changes in the monaural level cues have not yet been separately investigated. In this study, the use of the level cue in sound localization was measured using stimuli of 1 kHz at a fixed level in patients with single-sided deafness (SSD), the most severe form of asymmetric hearing. The mean absolute error (MAE) was calculated and related to the duration/age onset of SSD. To elucidate the biological correlate of this variable behavior, sound localization ability was compared with the cortical volume of the parcellated auditory cortex. In both SSD patients (n = 26) and normal controls with one ear acutely plugged (n = 23), localization performance was best on the intact ear side; otherwise, there was wide interindividual variability. In the SSD group, the MAE on the intact ear side was worse than that of the acutely plugged controls, and it deteriorated with longer duration/younger age at SSD onset. On the impaired ear side, MAE improved with longer duration/younger age at SSD onset. Performance asymmetry across lateral hemifields decreased in the SSD group, and the maximum decrease was observed with the most extended duration/youngest age at SSD onset. The decreased functional asymmetry in patients with right SSD was related to greater cortical volumes in the right posterior superior temporal gyrus and the left planum temporale, which are typically involved in auditory spatial processing. The study results suggest that structural plasticity in the auditory cortex is related to behavioral changes in sound localization when utilizing monaural level cues in patients with SSD.
Highlights
Unilateral loss of hearing deteriorates sound localization ability, which relies largely on binaural time and level differences in the sound source that reaches the two ears
When analyzing sound localization performance, the two single-sided deafness (SSD) groups were pooled and subdivided into three subgroups according to their duration of SSD (SSD < 7 years: 0.17–7 years of SSD, SSD10−29 years: 10–29 years of SSD, and SSD43−46 years: 43–46 years of SSD)
Between the two SSD groups (SSD-R vs. SSD-L), there were no significant differences in pure-tone average (PTA) in the poor ear (Wilcoxon rank sum test, p = 0.936), duration of SSD (p = 0.716) or age at onset of SSD (p = 0.755)
Summary
Unilateral loss of hearing deteriorates sound localization ability, which relies largely on binaural time and level differences in the sound source that reaches the two ears. The variable impact of hearing loss on localization ability, even in individuals with unilateral sensorineural single-sided deafness (SSD), indicates that hearing loss in the poorer ear alone does not eradicate the potential to localize sound. Instead, this variability suggests that malleable processes in higher-level structures occur naturally to improve localization accuracy. The degree of attenuation varies according to the azimuthal location relative to the intact ear This level cue by head shadowing would be unreliable if the source level varies frequently, but it serves as an essential localization cue in a familiar environment. Reflecting the high variability of localization performance and the diversity of the audiologic profile, clinical factors that contribute to improved auditory spatial ability have only been reported in a few studies; longer duration of SSD (Liu et al, 2018; Nelson et al, 2019) and younger age at SSD onset (Firszt et al, 2017; Nelson et al, 2019) have been associated with better localization performance
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