Vestibular-visual interactions in balance and orientation

Richard Fitzpatrick, Prince of Wales Medical Research Institute

Abstract
We have argued that galvanic vestibular stimulation (GVS) delivers a pure vestibular perturbation without affecting other sensory channels. Two experiments reveal GVS vestibular-visual interactions important for postural control and orientation. The first study shows that GVS evokes visual as well as vestibular balance responses. When GVS was delivered to standing subjects with the eyes, but not the head, turned to the side, the normal GVS sway response was turned towards the gaze direction, but only with the eyes open. The same was seen for GVS with prism glasses that turned the visual field. As GVS produces ocular torsion though the VOR, it also provides a visual signal that produces secondary postural corrections. Unlike the visual signal, the semicircular canal signal has no absolute reference to external space. The second study shows that the orientation response to GVS is calibrated to prior visual signals. During an initial walk with eyes shut, GVS caused subjects to turn. On opening the eyes, they walked straight ahead despite GVS. On shutting the eyes again, the GVS caused only a slight turn but a large turn when the polarity was reversed, indicating that the vestibular signal was calibrated to straight ahead during the eyes-open period.

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