Ž . Brain Research 821 1999 8–16 Research report The timing of galvanic vestibular stimulation affects responses to platform translation Frantisek Hlavacka a , Charlotte L. Shupert b , Fay B. Horak b,c, ) a Institute of Normal and Pathological Physiology, SloÕak Academy of Sciences, BratislaÕa, SloÕak Republic b Neurological Sciences Institute, Oregon Health Sciences UniÕersity, 1120 NW 20th AÕenue, Portland, OR 97209, USA c Departments of Neurology and Physiology and Pharmacology, Oregon Health Sciences UniÕersity, 3181 SW Sam Jackson Park Rd., Portland, OR 97201, USA Accepted 24 November 1998 Abstract We compared the effects of galvanic vestibular stimulation applied at 0, 0.5, 1.5 and 2.5 s prior to a backward platform translation on Ž . postural responses. The effect of the galvanic stimulation was largest on the final equilibrium position of the center of pressure CoP . The largest effects occurred for the 0.5 and 0-s pre-period, when the dynamic CoP pressure changes in response to both the galvanic stimulus and the platform translation coincided. The shift in the final equilibrium position was also larger than the sum of the shifts for the galvanic stimulus and the platform translation alone for the 0.5 and 0-s pre-periods. The initial rate of change of the CoP response to the platform translation was not significantly affected in any condition. Changes in the peak CoP position could be accounted for by local interaction of CoP velocity changes induced by the galvanic and translation responses alone, but the changes in final equilibrium position could only be accounted for by a change in global body orientation. These findings suggest that the contribution of vestibulospinal information is greatest during the dynamic phase of the postural response, and that the vestibular system contributes most to the later components of the postural response, particularly to the final equilibrium position. These findings suggest that a nonlinear interaction between the vestibular signal induced by the galvanic current and the sensory stimuli produced by the platform translation occurs when the two stimuli are presented within 1 s, during the dynamic phase of the postural response to the galvanic stimulus. When presented at greater separations in time, the stimuli appear to be treated as independent events, such that no interaction occurs. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Human; Posture; Sensory interaction; Vestibular; Somatosensory; Galvanic stimulation 1. Introduction The role of vestibular information in the control of equilibrium responses to external displacements is debated w x 1,12–14,16,17,21,23 . However, studies of responses to galvanic stimulation show that the vestibular system plays an important role in the determination of the tonic equilib- w x rium position of the body 2–4,6,9,15,24,25 . Bipolar gal- vanic current passed between the mastoid processes results in tilt of body position due to an asymmetrical change in vestibular nerve firing, with anodal currents decreasing tonic firing in the eighth nerve and the vestibular nuclei w x 7,18 . In response to bipolar galvanic vestibular stimula- ) Corresponding author. Fax: q1-503-413-7229; E-mail: fay@otolith.lhs.org tion, subjects shift their equilibrium position toward the ear Ž . with suppressed vestibular nerve input anodal current w x regardless of the position of the head 2–4,6,15,19,22,24 . This is similar to patients with vestibular disorders, who also lean or sway to the side of an acute unilateral vestibu- wx lar loss 5 . When galvanic vestibular stimulation is paired with body movement, either in response to a platform transla- tion or to a voluntary movement, the effect of galvanic w x stimulation can be increased 15,22,24 . We previously reported that a step of galvanic vestibular stimulation initiated 0.5 s prior to the onset of a platform translation shifted subjects’ final postural equilibrium position; that is, instead of realigning their bodies to a normal upright position after being translated, the subjects aligned them- Ž . selves to a leaning position toward the anode . The effect 0006-8993r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0006-8993 98 01356-0