Eye-position effects in directional hearing.

The influence of gaze direction on azimuthal sound localization was investigated by presenting free-field acoustical stimuli in combination with a visual fixation task. In Experiment 1, a two-alternative forced-choice method was employed. While fixating visual targets, subjects judged whether noise bursts, presented from various directions, were perceived as being on the left or right of either a visual reference indicating straight ahead or the subjective straight-ahead direction. The psychometric functions measured with the first task shifted consistently opposite to the direction of eccentric gaze, i.e., the location of the auditory stimulus was perceived as shifted toward the direction of gaze. The mean magnitude of the shift was 4.7 degrees over a range of fixation angles up to 45 degrees on either side. Without an external reference indicating straight ahead, shifts of sound localization were inconsistent, either opposite or toward the direction of fixation in individual subjects. In Experiment 2, subjects orientated their head toward sound stimuli while fixating visual targets in various directions. As in Experiment 1, head position as a measure of sound localization shifted significantly toward the direction of eccentric gaze when a visual reference of the head median plane was present, and the results were inconsistent across subjects when it was absent. The results indicate a significant effect of gaze direction on the spatial agreement of auditory and visual perception which may be based on the superposition of distinct auditory and visual eye-position effects. The effect is in agreement with previous neurophysiological results that have suggested an incomplete neural transformation of auditory spatial coordinates from a craniocentric into an oculocentric frame of reference.