Relative sound localisation abilities in human listeners.

Spatial acuity varies with sound-source azimuth, signal-to-noise ratio, and the spectral characteristics of the sound source. Here, the spatial localisation abilities of listeners were assessed using a relative localisation task. This task tested localisation ability at fixed angular separations throughout space using a two-alternative forced-choice design across a variety of listening conditions. Subjects were required to determine whether a target sound originated to the left or right of a preceding reference in the presence of a multi-source noise background. Experiment 1 demonstrated that subjects' ability to determine the relative location of two sources declined with less favourable signal-to-noise ratios and at peripheral locations. Experiment 2 assessed performance with both broadband and spectrally restricted stimuli designed to limit localisation cues to predominantly interaural level differences or interaural timing differences (ITDs). Predictions generated from topographic, modified topographic, and two-channel models of sound localisation suggest that for low-pass stimuli, where ITD cues were dominant, the two-channel model provides an adequate description of the experimental data, whereas for broadband and high frequency bandpass stimuli none of the models was able to fully account for performance. Experiment 3 demonstrated that relative localisation performance was uninfluenced by shifts in gaze direction.