Vection induced re-orientation in a visually evoked postural response
A. E. I. Thurrell and A. M. Bronstein
MRC Human Movement and Balance Unit, Institute of Neurology, National Hospital of Neurology and Neurosurgery, Queen Square, London, WC1E 3BG
For visual control of posture to be successful there must be a compensation for any change in position of the head and eyes relative to the feet. A previous study (Wolsley et al. 1996) showed that visually evoked postural responses (VEPRs) to roll-plane visual-motion are reoriented both by head-on-trunk and eye-in-head rotation. Thurrell et al. (2000) extended these results to show that this reorientation was not dependent on cognitive knowledge of environment geometry. During viewing of large moving scenes subjects may perceive themselves to be moving (vection) or the scene to be moving, perception alternating spontaneously between these two states. The current study investigated the difference in posture between epochs with vection and with object-motion during continuous visual roll motion. In addition, the effect of monocular vs. binocular viewing in the accuracy of the orientation of VEPRs was investigated. Five normal subjects were required to stand, looking (monocularly or binocularly) either straight ahead or ~30o to the right or left using only eye rotation at the centre of a large disk rotating around the line of sight. Posture was measured via the head position (Polhemus Fastrack) and the centre of foot pressure (COP), with alternations between vection and object-motion perception being indicated by a push button. On transition between object-motion and vection, subjects deviated further by an average of 1.4cm (head) and 1.2cm (COP). On average, subjects showed a good match between visual motion direction and sway direction: the mean error between sway direction and disk orientation was 13o during vection compared to 16o during object motion perception. The error was also smaller when subjects viewed the disk binocularly rather than monocularly (mean error 14o vs. 16o at the head and 15o vs. 21o for the COP; data combined across vection and object motion perception). Overall, these results show a more accurate reorientation of VEPR during vection and binocular viewing. Binocular viewing may be more effective because of the additional depth cues and more accurate gaze position signals available in this condition; changes in field size were negligible between monocular and binocular viewing due to the wearing of vision restricting goggles. The extra accuracy observed during vection implies that conscious perception of self-motion, presumably at a cortical level, interacts more effectively with gaze position signals. In turn, this may allow a better internal representation of visually-mediated body motion in space.
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Thurrell AEI, Bertholon P, Bronstein AM. Reorientation of a visually evoked postural response during passive whole body rotation. Exp. Brain Res. 2000; 133; 229-232