Chapter 22 On the role of vestibular, visual and somatosensory information for dynamic postural control in humans

Postural stabilization in altered visual, vestibular, and somatosensory conditions was investigated in humans subjected to either a fast unidirectional displacement or to a continuous sinusoidal movement of the standing support. Visual inputs were varied in four ways: (1)stroboscopic illumination, (2)stabilizing the visual surround with respect to head movements, (3)inducing apparent body movement in pitch using stripe patterns which moved continuously up or down in front of the subject, (4)eye closure. Static vestibular (and neck) input was modified by bending the head forwards or backwards, or to the right or left shoulder with the eyes closed. Somatosensory input from both feet was reduced by an ischaemic block at a level just above the ankle joints. With fast, transient, toe-up platform displacements (high-frequency test) neither the biomechanical parameters as measured by the displacement of the centre of foot pressure nor the early EMG responses of the anterior tibial and triceps surae muscles were modified by a manipulation of visual, vestibular or somatosensory feedback conditions. Sudden disturbances are obviously compensated by an early set of reflex-like muscle responses that, depending on the starting position, are stereotypically released without feedback control to save time at the expence of accuracy (emergency reaction). Continuous regulation of upright stance during sinusoidal displacement at 1 or 0.3 Hz (low-frequency test), however, clearly depends on visual, vestibular, and somatosensory feedback. Studies in patients should contain both tests, since each examines different functions of the very complex posture stabilizing network. Manipulations of sensory feedback, however, are only recommended in the low-frequency test. The experimental suppression or disturbance by disease of two of the three feedback loops invariably causes a conspicuous postural instability.