Combined effects of vestibular stimulation and gaze direction on orientation of sound lateralization

Dichotic sound discrimination is influenced by either visual or vestibular stimulation. This study investigated the effect of simultaneous gaze and vestibular inputs on dichotic sound discrimination. The subjects (n = 12) closed their eyes or gazed at a red target light placed at a distance of 50 cm from their eyes, and the ITD discrimination test was simultaneously performed in either the supine or in the right lateral decubitus position, in which gravitational linear acceleration causes utricular stimulation in the lower ear. In the ITD discrimination tests, the amplitudes of saw-tooth waves in the supine position with straight gaze were significantly different from those in the lateral decubitus position with downward or upward gaze. The saw-tooth waves in the lateral decubitus position with eye closed significantly shifted toward the upper ear compared to that in the supine position with eye closed. The saw-tooth waves in the lateral decubitus position with upward and with downward gaze shifted significantly toward the upper and lower ears, respectively, as compared to that in the supine position with straight gaze. We concluded that a sound image resulting from dichotic stimulation may be more dominantly influenced by the direction of eccentric gaze than by utricular stimulation that occurs due to gravitational linear acceleration.