Roles of size, position, and speed of stimulus in vection with stimuli projected on a ground surface

INTRODUCTION: Although the induction of vection (perception of illusory self-motion) has been studied for some decades, the eff ect of ground surface properties on vection remains to be assessed quantitatively. This study will be helpful for designing helicopter or airplane fl ight simulation, because pilots often perceive optic fl ow on the ground surface and perceive self-motion from such fl ows. METHOD: Vection stimuli of variable position, size, and optic fl ow speed were presented in a trapezoidal area on a ground surface. Body sway was also measured. RESULTS: Substantial vection was induced by stimuli on a ground surface. Increases in stimulus speed and size were each associated with stronger vection (e.g., the subjective strength increased by 50% as the speed increased from 0.375 m . s 2 1 to 1.5 m . s 2 1 ). When the stimulus occupied a more distant section of the visual fi eld, vection was more effi ciently induced than when the nearer section was occupied (e.g., the subjective strength decreased by 50% when the nearer half section of optical fl ow was removed). These properties of vection were similar to vection induced by upright vertical stimuli. Speed, size, and position of vection stimuli modifi ed both length and direction of body sway signifi cantly. Vection and body sway showed some correlations (e.g., r 5 0.55). CONCLUSION: Stimuli on ground surfaces can induce substantial vection and vection strength can be modifi ed by the stimulus properties of the ground surfaces.