Abstract
Direction-specific interactions of sound waves with the head, torso, and pinna provide unique spectral-shape cues that are used for the localization of sounds in the vertical plane, whereas horizontal sound localization is based primarily on the processing of binaural acoustic differences in arrival time (interaural time differences, or ITDs) and sound level (interaural level differences, or ILDs). Because the binaural sound-localization cues are absent in listeners with total single-sided deafness (SSD), their ability to localize sound is heavily impaired. However, some studies have reported that SSD listeners are able, to some extent, to localize sound sources in azimuth, although the underlying mechanisms used for localization are unclear. To investigate whether SSD listeners rely on monaural pinna-induced spectral-shape cues of their hearing ear for directional hearing, we investigated localization performance for low-pass filtered (LP, <1.5 kHz), high-pass filtered (HP, >3kHz), and broadband (BB, 0.5–20 kHz) noises in the two-dimensional frontal hemifield. We tested whether localization performance of SSD listeners further deteriorated when the pinna cavities of their hearing ear were filled with a mold that disrupted their spectral-shape cues. To remove the potential use of perceived sound level as an invalid azimuth cue, we randomly varied stimulus presentation levels over a broad range (45–65 dB SPL). Several listeners with SSD could localize HP and BB sound sources in the horizontal plane, but inter-subject variability was considerable. Localization performance of these listeners strongly reduced after diminishing of their spectral pinna-cues. We further show that inter-subject variability of SSD can be explained to a large extent by the severity of high-frequency hearing loss in their hearing ear.
Highlights
Listeners with total single-sided deafness (SSD) lack the ability to localize sounds on the basis of interaural differences in time (ITD) and sound level (ILD)
When stimuli are presented at a single sound level, SSD listeners could rely on the perceived sound level at the hearing ear because of the azimuth-dependent attenuation produced by the head-shadow effect (HSE)
Others, demonstrated that the ability to use spectral localization cues diminished as soon as normal hearing was experienced (Keating et al, 2013). As it is unclear whether a critical period for this auditory plasticity might be present in humans, and it is postulated that the etiology of subjects with SSD may be unrelated to their localization abilities (Colburn, 1982), we investigated whether the onset of unilateral deafness affects sound-localization performance in azimuth and elevation when tested at a later age
Summary
Listeners with total single-sided deafness (SSD) lack the ability to localize sounds on the basis of interaural differences in time (ITD) and sound level (ILD). The possibility that these listeners have learned to use monaural pinna-induced spectral-shape cues of their hearing ear for localization in azimuth, has been postulated (Batteau, 1967; Colburn, 1982; Häusler et al, 1983; Slattery and Middlebrooks, 1994; Wightman and Kistler, 1997; Van Wanrooij and Van Opstal, 2004; Shub et al, 2008; Kumpik et al, 2010; Rothpletz et al, 2012).
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