Abstract
Two synchronous sounds at different locations in the midsagittal plane induce a fused percept at a weighted-average position, with weights depending on relative sound intensities. In the horizontal plane, sound fusion (stereophony) disappears with a small onset asynchrony of 1–4 ms. The leading sound then fully determines the spatial percept (the precedence effect). Given that accurate localisation in the median plane requires an analysis of pinna-related spectral-shape cues, which takes ~25–30 ms of sound input to complete, we wondered at what time scale a precedence effect for elevation would manifest. Listeners localised the first of two sounds, with spatial disparities between 10–80 deg, and inter-stimulus delays between 0–320 ms. We demonstrate full fusion (averaging), and largest response variability, for onset asynchronies up to at least 40 ms for all spatial disparities. Weighted averaging persisted, and gradually decayed, for delays >160 ms, suggesting considerable backward masking. Moreover, response variability decreased with increasing delays. These results demonstrate that localisation undergoes substantial spatial blurring in the median plane by lagging sounds. Thus, the human auditory system, despite its high temporal resolution, is unable to spatially dissociate sounds in the midsagittal plane that co-occur within a time window of at least 160 ms.
Highlights
Asynchrony effects have been studied extensively in the horizontal plane (e.g.,10–12, in humans;[13], in cats), little is known about their effects in the median plane
Dizon and Litovsky[23] included a larger range of locations, and sound durations up to 50 ms. They identified a weak precedence effect in the median plane, which, may be described as weighted averaging, as there was no clear dominance of the leading sound
The single-sound trials (BZZ), and those in which the BZZ and Gaussian white noise (GWN) emanated from the same speaker, were all pooled, as we obtained no significant differences in response behavior for these trial types
Summary
Asynchrony effects have been studied extensively in the horizontal plane (e.g.,10–12, in humans;[13], in cats), little is known about their effects in the median plane Extension to the latter is of interest, as the neural mechanisms underlying the extraction of the azimuth and elevation coordinates are fundamentally different, and initially processed by three independent brainstem pathways[14,15,16]. Dizon and Litovsky[23] included a larger range of locations, and sound durations up to 50 ms They identified a weak precedence effect in the median plane, which, may be described as weighted averaging, as there was no clear dominance of the leading sound. To assess the influence of the latter on the localisation response, we employed a large range of inter-stimulus delays: (i) from synchronous presentation, up to 30 ms delay, (ii) delays with acoustic overlap of both sounds, but >30 ms, and (iii) full temporal segregation of the sounds (delays >100 ms)
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