Postural responses to combined vestibular and hip proprioceptive stimulation in man.

Vestibular-proprioceptive interaction in human postural control in the frontal plane was studied by analysing the lateral body sway evoked in a standing subject by a weak, near-threshold galvanic vestibular stimulation combined with a balanced, bilateral vibration of the medial gluteus muscles. The intensities of the stimuli were adjusted so that none of them produced a consistent postural response when delivered alone. The pattern of the lateral body sway evoked by the combined stimulation was compared with postural responses to suprathreshold vestibular stimulation and asymmetric (unilateral) vibration of the hip abductors. During the vestibular stimulation alone the head movement started earlier and was larger than movement of the hip. During unilateral vibration the head movement was delayed with respect to the hip movement and the amplitude of head deviation was less than that of the hip. The pattern of postural response to combined vestibular stimulation and balanced vibration resembled that observed under unbalanced, unilateral vibration in terms of both the latencies and amplitudes of deviation of the body segments from their respective baseline positions. It is suggested that the asymmetric vestibular signal provided by galvanic stimulation of the labyrinth introduces a bias into the reference frame for central interpretation of proprioceptive signals so that a symmetric proprioceptive input gives rise to a lateral body sway when referenced to an asymmetric vestibular input.