Binaural weighting of pinna cues in human sound localization

Human sound localization relies on binaural difference cues for sound-source azimuth and pinna-related spectral shape cues for sound elevation. Although the interaural timing and level difference cues are weighted to produce a percept of sound azimuth, much less is known about binaural mechanisms underlying elevation perception. This problem is particularly interesting for the frontal hemifield, where binaural inputs are of comparable strength. In this paper, localization experiments are described in which hearing for each ear was either normal, or spectrally disrupted by a mold fitted to the external ear. Head-fixed saccadic eye movements were used as a rapid and accurate indicator of perceived sound direction in azimuth and elevation. In the control condition (both ears free) azimuth and elevation components of saccadic responses were well described by a linear regression line for the entire measured range. For unilateral mold conditions, the azimuth response components did not differ from controls. The influence of the mold on elevation responses was largest on the ipsilateral side, and declined systematically with azimuth towards the side of the free ear. Near the midsagittal plane the elevation responses were clearly affected by the mold, suggesting a systematic binaural interaction in the neural computation of perceived elevation that straddles the midline. A quantitative comparison of responses from the unilateral mold, the bilateral mold and control condition provided evidence that the fusion process can be described by binaural weighted averaging. Two different conceptual schemes are discussed that could underlie the observed responses.