Eye position determines audiovestibular integration during whole-body rotation

When a sound is presented in the free field at a location that remains fixed to the head during whole-body rotation in darkness, it is heard displaced in the direction opposing the rotation. This phenomenon is known as the audiogyral illusion. Consequently, the subjective auditory median plane (AMP) (the plane where the binaural difference cues for sound localization are perceived to be zero) shifts in the direction of body rotation. Recent experiments, however, have suggested opposite AMP results when using a fixation light that also moves with the head. Although in this condition the eyes remain stationary in the head, an ocular pursuit signal cancels the vestibulo-ocular reflex, which could induce an additional AMP shift. We tested whether the AMP is influenced by vestibular signals, eye position or eye velocity. We rotated subjects sinusoidally at different velocities, either in darkness or with a head-fixed fixation light, while they judged the laterality (left vs. right with respect to the midsagittal plane of the head) of broadband sounds presented over headphones. Subjects also performed the same task without vestibular stimulation while tracking a sinusoidally moving visual target, which mimicked the average eye-movement patterns of the vestibular experiments in darkness. Results show that whole-body rotation in darkness induces a shift of the AMP in the direction of body rotation. In contrast, we obtained no significant AMP change when a fixation light was used. The pursuit experiments showed a shift of the AMP in the direction of eccentric eye position but not at peak pursuit velocity. We therefore conclude that the vestibular-induced shift in average eye position underlies both the audiogyral illusion and the AMP shift.