The effects of sensory inputs and hyperventilation on postural stability in man

The studies involved in this thesis were undertaken to investigate postural stability during auditory and visual stimuli and assess the effect of hyperventilation (HV) on postural mechanisms. Recording techniques included a force platform and an electro-magnetic head motion recorder (for body sway measurements), transcutaneous carbon dioxide tension measurements, surface electromyography and electro-oculography. The research strategy involved establishing relationships between the physical characteristics (frequency, intensity, direction) of the stimuli used (eg visual, proprioceptive) and the physical characteristics of the sway pattern elicited (amplitude, velocity and direction). HYPERVENTILATION EXPERIMENTS: Initially postural instability after voluntary HV was documented. The HV effects were more intense with eyes closed. HV preferentially increased low frequency oscillations, and this effect was present both in healthy and labyrinthine defective subjects. Additional experiments were undertaken in order to clarify by which mechanisms HV can interfere with balance control. Neurophysiological studies showed that the electrically evoked sural nerve action potential increased its amplitude whereas the scalp somatosensory evoked potentials decreased during HV. In contrast, no significant effects of HV on short-latency vestibulo-spinal responses were detected (click-evoked vestibulo-colic EMG response). The hypothesis that the vestibulo-cerebellum might also be affected by HV was also investigated by recording ocular-motor functions mediated by this neural structure (vestibulo-ocular reflex suppression and smooth pursuit eye movements). Ocular performance, however, was unchanged by HV. VISUAL MOTION EXPERIMENTS: The question investigated was if the main direction of a visually evoked postural response can be re-oriented as a function of eye-in-orbit and head-on-trunk position. The visual motion stimulus used consisted of a large disc rotating around the line of sight. Subjects viewed the rotating disc with various combinations of eye and neck deviations. It was found that the plane of motion of the visual stimulus with respect to the body modulated the direction of postural reactions. The findings indicate that signals of the eye-in-orbit and head-on-trunk position are used in the visual control of postural sway. It is argued that these signals are of proprioceptive origin. AUDITORY EXPERIMENTS: Changes in postural stability during sounds of different frequency and intensity were examined with and without visual input. It was shown that increased loudness tended to increase lateral sway; sound frequency influenced anterior- posterior sway. Vision had a significant stabilising effect on most sway parameters. The work undertaken for this thesis has 1) identified a potential role for eye and neck proprioceptors in visuo-postural control, 2) described postural sway under different sound characteristics, 3) shown that HV decreases postural stability and 4) that the effects of HV on balance control are selective, i.e., certain postural mechanisms are affected by HV whereas others not