Galvanic Vestibular Stimulation Produces Sensations of Rotation Consistent with Activation of Semicircular Canal Afferents

axis of real rotation (i.e., with the head pitched fully up or down). With the head close to the neutral position, such that Reid’s plane was tilted 18.8° above horizontal (i.e., slight nose-up tilt), the effect of GVS upon rotation sensation was zero. This suggests that GVS evokes a sensation of head roll around a naso-occipital axis. Using a modeling approach, the authors elegantly demonstrated that this axis is a direct consequence of the anatomical orientation of the canals (Blanks et al., 1975). Based on the assumption that GVS modulates all vestibular afferents equally (Goldberg et al., 1984), they calculated the theoretical axis of head rotation when equal signals from all six canals are combined. It transpires that the resulting axis is naso-occipital, and elevated 16.4° relative to Reid’s plane. This tallies remarkably well with the data gained from the chair rotation experiment. Evoked eye movements corroborate these data. Many studies have described a torsional eye movement response to GVS (Schneider et al., 2000, 2002; Jahn et al., 2003; MacDougall et al., 2005). This consists not only of a fixed offset of eye position as one might expect from pure otolith activation, but contains alternating fast and slow phases, consistent with a canal-evoked nystagmus caused by head roll. Schneider et al. (2002) compared the ocular response to GVS with that caused by head roll. They found that GVS produced essentially the same eye movement as pure head rotation; i.e., torsional offset accompanied by nystagmus. This raises the possibility that both characteristics of the GVS-evoked eye movement can be explained entirely on the basis of rotation. Galvanic Vestibular Stimulation-evoked body movements agree with the perception and eye movement data. With the head Galvanic vestibular stimulation produces sensations of rotation consistent with activation of semicircular canal afferents