The long-term goal of the proposal is to elucidate how somatosensory and vestibular input interact to provide the sensory information which determines postural control. Using Mergner's model of this interaction as the hypothetical construct the proposed studies manipulate vestibular input with anodal, galvanic vestibular stimulation or use patients with vestibular lesions to vary this sensation. In the presence of altered surface somatosensory input the investigators propose that they will demonstrate postural control deficits in these subjects. These defects will be demonstrated by quantified muscle activation patterns (surface EMG), surface reactive forces under each foot (torque, CoP movement), and kinematics of body movement (e.g., CoM movement). For the first series of experiments, the investigators hypothesize that alterations in somatosensory input by surface movement and sway referencing of the surface will reveal vestibulospinal asymmetries and deficits in subjects with vestibular loss. In experiment 1, they will determine if patients with unilateral vestibular loss show postural asymmetries when standing on a slowly tilting or translating platform. In experiment 2, they seek to determine if binaural, anodal galvanic vestibular stimulation produces a temporary, acute loss of vestibular input required for postural control. In the second series of experiments the somatosensory input is limited by selecting subjects with sensory neuropathy to determine if patients compensate for somatosensory loss by altering vestibular sensitivity for posture. They hypothesize that loss of somatosensation will result in an increased sensitivity to vestibular disruption of postural control. In the 3rd experiment they will """"""""total"""""""" body somatosensory loss in a patient with a severe loss of large sensory fibers affects responses to galvanic stimulation and surface perturbation. The 4th experiment will characterize how subjects with somatosensory loss in the lower legs caused by diabetic neuropathy respond to galvanic vestibular stimulation during surface perturbations and on a sway referenced surface. The third series of experiments determines the extent to which haptic sensory information can compensate for vestibular loss in supporting postural control. The investigators propose that very light contact of one index finger stabilizes posture by improving control of the trunk when subjects stand on unstable surfaces and that subjects with bilateral loss of vestibular function will show more improvement than normal subjects. Experiment 5 will determine how light touch improves postural stability in narrow stance, on an unstable surface and during surface perturbations. The investigators propose that the studies will lead to a better understanding of the interactive mechanisms underlying compensation for vestibular and somatosensory deficits and which will facilitate development of new approaches to postural rehabilitation in patients with sensory deficits.
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