The effect of body posture on visual orientation

Visual orientation is precise and accurate, when the observer is upright, in the absence of any visual surround cues. Lateral tilt of the body relative to gravity results in a close approximation of the visual and gravitational reference axes. There are, however, relatively small, systematic differences between them. For body tilts less than 70° the visual vertical is located on the opposite side of the gravitational vertical to tilt, whereas the reverse occurs with greater degrees of tilt. These are referred to as the E- and A-effects, respectively. The precision of visual orientation with body upright was confirmed by experiments reported in this thesis, and it was not changed by variations in the stimulus or procedural conditions. However, the visual vertical was influenced by the eye used for observation. The hypothesis that visual orientation is a function of retinal orientation was examined. This hypothesis was rejected because stimulation of various postural systems, while maintaining the head upright, resulted in systematic variations in visual orientation. It was considered that the accuracy of visual orientation with body upright is due to the precise postural information for this orientation. [...]