PHYSIOLOGICAL RESEARCH ISSN 0862-8408 © 2006 Institute of Physiology, Academy of Sciences of the Czech Republic, Prague, Czech Republic Fax +420 241 062 164 E-mail: physres@biomed.cas.cz http://www.biomed.cas.cz/physiolres Physiol. Res. 55 (Suppl. 1): S121-S127, 2006 Somatosensory Influence on Postural Response to Galvanic Vestibular Stimulation F. HLAVAČKA, F. B. HORAK 1 Institute of Normal and Pathological Physiology, Slovak Academy of Sciences, Bratislava, Slovakia and 1 Neurological Sciences Institute of Oregon Health Sciences University, Beaverton, OR, USA Received October 27, 2006 Accepted November 17, 2006 On-line available December 22, 2006 Summary We investigated how postural responses to galvanic vestibular stimulation were affected by standing on a translating support surface and by somatosensory loss due to diabetic neuropathy. We tested the hypothesis that an unstable surface and somatosensory loss can result in an increase of vestibulospinal sensitivity. Bipolar galvanic vestibular stimulation was applied to subjects who were standing on a force platform, either on a hard, stationary surface or during a backward platform translation (9 cm, 4.2 cm/s). The intensity of the galvanic stimulus was varied from 0.25 to 1mA. The amplitude of the peak body CoP displacement in response to the galvanic stimulus was plotted as a function of stimulus intensity for each individual. A larger increase in CoP displacement to a given increase in galvanic current was interpreted as an increase of vestibulospinal sensitivity. Subjects with somatosensory loss in the feet due to diabetes showed higher vestibulospinal sensitivity than healthy subjects when tested on a stationary support surface. Control subjects and patients with somatosensory loss standing on translating surface also showed increased galvanic response gains compared to stance on a stationary surface. The severity of the somatosensory loss in the feet correlated with the increased postural sensitivity to galvanic vestibular stimulation. These results showed that postural responses to galvanic vestibular stimulus were modified by somatosensory information from the surface. Somatosensory loss due to diabetic neuropathy and alteration of somatosensory input during stance on translating support surface resulted in increased vestibulospinal sensitivity. Key words Somatosensory • Vestibular • Posture • Sensory integration Introduction The influence of vestibular-somatosensory interaction in human postural control results in different postural responses to galvanic vestibular stimulation depending on the state of the somatosensory system. For example, healthy subjects standing on a translating surface showed increased responses to galvanic vestibular stimulation (Inglis et al. 1995). Postural responses to galvanic current are also larger when subjects simultaneously make voluntary movements (Severac- Cauquiel and Day 1998). Thus, responsiveness to vestibular signals appears to go up whenever somatosensory information from surface contract