Both Interaural and Spectral Cues Impact Sound Localization in Azimuth

Spatial cues for sound azimuth (interaural differences) and elevation (spectral) are processed through distinct central processes. However, it is unclear how they interact to modify perceived sound location. Potentially, such interactions might correct errors in azimuth that are inherent in the geometry of interaural cues as a function of elevation. We studied the influence of target location on sound localization in azimuth in the presence or absence of spectral cues. Human subjects were tested in a dark echo-attenuated room. Their heads were restrained while facing a cylindrical screen at 2 m, behind which a non-visible speaker on a robotic arm randomly presented auditory targets along three vertical slices at 0deg and plusmn45deg azimuth. Stimuli consisted of repeating 150-ms bursts of broadband (BB: 0.1-20 kHz), high-pass (HP: 3-20 kHz) and lowpass (LP: 0.1-1 kHz) noise, which emphasize azimuth (LP), elevation (HP), or both (BB) cues. Targets were localized by manually pointing a visually-guided joystick-mounted laser at perceived target locations. Azimuth accuracy of all three sounds in the median plane remained unaffected by elevation. Away from the median plane, target elevation strongly influenced azimuth accuracy of LP sounds, where accuracy was biased toward the median plane with increasing eccentricity in elevation. Localization of HP and BB sounds showed less spatial distortion. Our results suggest a synergistic interaction between interaural and spectral cues, which corrects errors in perceived sound azimuth as a function of elevation