Direction of galvanically-induced vestibulo-postural responses during active and passive neck torsion

The direction of a postural response induced by galvanic vestibular stimula tion depends on the head and trunk position. The relative importance of aff erent information (proprioception) and efferent motor command/corollary dis charge is unknown. We studied the direction of body sway evoked by galvanic vestibular stimulation in 9 healthy subjects during active and passive hea d positioning at 0 degrees frontal position, 35 degrees to the left, and 75 degrees to the right, using a custom-built collar. At 0 degrees and 75 deg rees there were no significant differences in sway direction between active and passive head positioning. The galvanic stimulation invoked sway toward the anode, mainly in the inter-aural direction. The sway direction differe d significantly between active and passive positioning at 35 degrees to the side (p < 0.05). When the head was actively kept in this position, the bod y sway was mainly in an inter-aural direction. The sway shifted to a naso-o ccipital direction when the head was passively positioned at 35 degrees. Ou r results indicate that the afferent proprioceptive information has the lar gest influence on the direction of the galvanically-induced postural respon se, although some dependence on efferent motor commands and non-linear cerv ical proprioception cannot be ruled out entirely.