Modification of human postural responses to soleus muscle vibration by rotation of visual scene.

Sensory interaction in posture control in 20 healthy subjects was investigated by postural responses to differently timed proprioceptive and visual stimulation. As proprioceptive stimulation was used both soleus muscles vibration. Visual stimulation was rotating disc moving in forward and backward direction. Centre of foot pressure (CoP) and trunk tilts in antero-posterior (AP) direction were measured during stance by two accelerometers on the upper and the lower trunk level. Subjects performed four series of eight trials lasted 20s: vibration with eyes open and closed, visual scene motion forward or backward, vibration together with scene motion forward or backward, scene motion forward or backward 3s before vibration. The results showed that early velocities and final angles of body tilt induced by soleus muscle vibration were modified by motion of visual scene. Early part of postural responses was changed slightly if sensory stimulation starts together. When visual stimulation started 3s before muscle vibration, the induced early CoP and trunk tilt records of postural response to vibration occurred with the faster slope similar for both directions of scene motion and for condition of vibration alone with eyes closed. Significantly different final CoP shifts and trunk angles were measured between condition with visual scene motion forward and backward. The effect of visual input on posture control was two times enhanced in paired sensory stimulation. The results indicated an important influence of unstable visual field on posture response to somatosensory stimulation where the early part of postural reaction indicated absence of visual influence.