Guerraz, Michel; (2001) Visual control of posture: The role of motion parallax and cognitive processes. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The thesis investigates visual control of posture in man, in particular the role of visual motion parallax. The moving room paradigm, which simulates self-motion, was used in most experiments. In the first series of experiments the effects of room motion in the presence or absence of a foreground fixation target were investigated. Directionally specific postural responses opposite to background motion were observed when subjects were fixating the foreground target. Neither induced motion, ocular convergence nor binocular vision were sufficient to evoke such responses. It was concluded that such postural response and more generally postural control could be influenced by motion parallax. In experiments carried out in a normal stationary room, it was found that the motion parallax a subject generates during spontaneous body sway is used by the visuo-motor system to control body oscillations. The parallax-based effect was, however, limited to low frequency components of body sway (0.75 Hz). The influence of cognitive processes in visuo-postural control was also investigated. It was found that the predictability of the forthcoming displacement of a moving visual scene helped observers distinguish visual flow due to self-motion from that due to object-motion and therefore inhibit postural readjustments induced by object motion. The visual control of posture in patients with congenital nystagmus, who show involuntary movement of the eyes at a frequency of 3-5 Hz, was also investigated. It was found that, in the moving room paradigm, these patients were able to use motion parallax to control slow reorientation of the body as do control subjects. In contrast, during spontaneous body sway in a 3 dimensional environment they were not able to control rapid oscillations of the body (> 0.5Hz). These findings suggest that in congenital nystagmus, visual control of posture is restricted by low frequency sampling of the visual scene (foveation periods). They also suggest that although motion parallax is a dynamic cue, it can be used to control slow body re-orientation on the basis of solely discrete visual samples.

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