EFFECT OF GALVANIC VESTIBULAR STIMULATION ON HUMAN POSTURAL RESPONSESDURING SUPPORT SURFACE TRANSLATIONS

1. We investigated the role of the vestibular system in postural contr ol by combining galvanic vestibular stimulation (0.2-0.5 mA) with plat form translations in standing subjects. Vestibular stimulation deliver ed 500 ms before and continuously during the platform translation prod uced little change in the earliest center of pressure (COP) and center of mass (COM) movements in response to platform translations. but res ulted in large changes during the execution of the postural movement a nd in the finial equilibrium position. 2. Vestibular stimulation produ ced anterior or posterior shifts in the position of COP and COM, depen ding on the polarity of the galvanic current. These shifts were larger during platform translations than during quiet stance. The peak of th ese shifts in COP and COM occurred at 1.5-2.5 s after the onset of pla tform translation. and increased in magnitude with increasing platform velocity. The final equilibrium positions of COP and COM were also sh ifted. but these shifts were smaller and not dependent on platform vel ocity. 3. These results imply that a tonic step of galvanic current to the vestibular system can change the final equilibrium position for a n automatic postural response. Furthermore, these results indicate tha t the vestibular system may play a larger role in interpreting sensory reafference during postural movements. and especially during fast pos tural movements. than in controlling quiet stance. Finally, these resu lts indicate that the vestibular system does not play a critical role in triggering the earliest postural responses, but it may be critical in establishing an internal reference for verticality.