The influence of visual stimuli on posture

Abstract

Humans move through the environment without loosing balance or bumping into other objects or organisms. In order to achieve this ability, the brain uses incoming information from several senses. If stationary observers receive information from the visual system signaling self-motion through the environment, while information from the vestibular and proprioceptive systems suggest a stationary observer, visual information overrides the information from both other sensory systems. As a consequence, observers start to sway. The main focus of this thesis was on whether only direct visual stimulation (i.e. visual motion presented to the retina; a stimulus-response mechanism) or also indirect visual stimulation (i.e. motion not presented to retina but generated internally) induces postural sway. Specific stimulus characteristics were manipulated and illusory visual motion signals in the absence of external visual stimulation were used to investigate this question. As a first question, we have investigated whether the strength of perceived self-motion and the induced postural sway magnitude are directly related. Radial optic flow patterns simulating self-motion through the environment were presented to stationary observers. Rigid optic flow patterns generated a stronger sensation of self-motion but generated less postural sway than non-rigid optic flow patterns. Apparently, visuo-vestibular interactions are tailored to compensate for rigid optic flow stimuli. Additionally, we showed that an often-observed directional anisotropy in postural sway between expanding and contracting optic flow did not have a biomechanical origin. To avoid this directional anisotropy, we used horizontal translating dot patterns to examine whether the perceptual strength (manipulated by varying the speed and dot contrast) of a stimulus influences the postural sway magnitude. Results showed that the induced sway magnitude and sway direction depend on the speed and contrast of a stimulus moving in a single direction. This result suggests that visuo-vestibular interactions are influenced by an internal representation of visual motion, rather than being a direct consequence of the stimulus. Further evidence for such an internal representation of visual motion influencing postural sway, was provided by the finding that the illusory motion generated by the motion aftereffect induced postural sway as well. We also examined two perceptual predictions derived from the sway results. First, we expected an asymmetry in perceptual strength between radial optic flow directions to mirror the directional anisotropy apparent in postural sway. However, the results showed the opposite asymmetry, suggesting an inverse relationship between the perceptual strength of radial optic flow directions and the induced postural sway magnitude. This perceptual asymmetry was also observed at the level of perceptual grouping during binocular rivalry. Grouped expanding optic flow was perceived for longer durations than grouped contracting optic flow, but only when the structure of the optic flow pattern was fully visible. Another interesting observation of this study was that grouping of optic flow patterns is, like for static images, primarily influenced by its eye-of-origin. All in all, the findings of the current thesis show that postural sway is not a direct stimulus-response (postural sway) process, since an internal representation of visual motion (e.g. feedback, adaptation, integration) appears to influence postural sway.