Testing the Precedence Effect in the Median Plane Reveals Backward Spatial Masking of Sound.

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.