Vestibular influences on postural instability induced by movements of the visual environment and support.

Subjects were maintained in a vertical posture standing on a hard support with a limited degree of freedom in the frontal plane. The stability of the vertical posture was assessed on the basis of the standard deviations (sigma) from the mean amplitude of head oscillations (in the frontal and sagittal planes) relative to the origin of the coordinate system. Sinusoidal rotations of the optokinetic cylinder in which subjects stood, sinusoidal rotations of the support, and combination of these rotations, with phase discordance between movements of the cylinder and the support, led to increases in sigma in all subjects. Feedback via the vestibular input was created using transmastoid galvanic vestibular stimulation. Changes in the feedback current showed a linear function relating to the amplitude and speed of head movement. Introduction of variations in the feedback function could be used to decrease sigma for lateral oscillations; increases (compared with values on calm standing in the dark) resulted from the use of any of the destabilizing treatments. Changes in sigma for oscillations in the sagittal plane were not systematic.