Artificial vestibular feedback in conditions of a modified body scheme.

Subjects standing in the dark on a rigid immobile support maintained a vertical posture, which was destabilized by vibrostimulation of both Achilles tendons. Feedback was created via the vestibular pathway using transmastoid galvanic stimulation. Changes in the current in the feedback envelope were made using a linear function based on the amplitude and rate of head displacement. Changes in the body scheme were produced using rotation of the head relative to the trunk, rotation of the trunk with the relative head position fixed, and simultaneous rotation of the head and trunk. The result of these manipulations was that the head could be rotated through essentially 90 degrees relative to the feet. In addition, rotation of one foot relative to the other through 90 degrees was used. Artificial feedback damped head oscillations induced by vibration, but only those in the vertical plane passing through the interaural axis of the head. It is suggested that changes in the vectorial characteristics of vestibular responses and the results of applying artificial feedback on the background of modified orientation of the head relative to the feet may be associated with substitution of the ensembles of vestibular hair sets providing the dominant signals in the responses of vestibulospinal neurons.