The timing of galvanic vestibular stimulation affects responses to platform translation

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 postural respo nses. The effect of the galvanic stimulation was largest on the final equil ibrium position of the center of pressure (CoP). The largest effects occurr ed 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 coinci ded. The shift in the final equilibrium position was also larger than the s um of the shifts for the galvanic stimulus and the platform translation alo ne for the 0.5 and 0-s pre-periods. The initial rate of change of the CoP r esponse to the platform translation was not significantly affected in any c ondition. 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 sug gest that the contribution of vestibulospinal information is greatest durin g the dynamic phase of the postural response, and that the vestibular syste m contributes most to the later components of the postural response, partic ularly to the final equilibrium position. These findings suggest that a non linear interaction between the vestibular signal induced by the galvanic cu rrent and the sensory stimuli produced by the platform translation occurs w hen the two stimuli are presented within 1 s, during the dynamic phase of t he postural response to the galvanic stimulus. When presented at greater se parations in time, the stimuli appear to be treated as independent events, such that no interaction occurs. (C) 1999 Elsevier Science B.V. All rights reserved.