Effects of artificial feedback to the vestibular input on postural instability induced by asymmetric proprioceptive stimulation.

The subjects in this study maintained a vertical posture standing on a rigid support. Postural stability was assessed in terms of the standard deviations (sigma) from the mean amplitude of movements of the head relative to the null coordinate. Feedback at the vestibular input was created by transmastoid bipolar galvanic stimulation. Changes in the current in the feedback envelope were governed by a linear function based on the amplitude and rate of head movement. Variation in the coefficients of the feedback function could decrease the magnitude of sigma for lateral movements which were increased (compared with values in calm standing in the dark) by unilateral vibrational stimulation of the gluteus medialis muscle. These results provide evidence that "rate" and "position" information have different values for maintaining the vertical posture in different subjects. They also demonstrate the ability of the central nervous system (CNS) to reevaluate the weightings of the different types of information arriving via a single channel. These results support the hypothesis that galvanic vestibular input can provide the CNS with sufficient information relating to the current orientation of the body. This information can be used for postural stabilization.