Visually Induced Involuntary Movements
Looking at a virtual 3D environment with structural features rotating at 60{degrees}/s in a head-mounted display soon elicits an illusion of self-rotation and displacement in the opposite direction. We explored in 75 s long trials the effects of visually induced self-rotation on the head, torso, and horizontally extended right arm of standing subjects. The degree of body and limb movement was contingent on whether the arm was extended out freely or pointing at a briefly proprioceptively specified target position, but did not depend on whether the hand held a rod or not. Most subjects in the Free condition showed significant unintentional arm deviations, which averaged approximately 55{degrees} in the direction opposite the induced illusory self-motion, and were more than 150{degrees} in some cases. In contrast, on average, the deviations in the Pointing condition were a quarter of those in the Free condition. Deviations of head and torso positions also occurred in all conditions. Total arm and head deviations were the sum of deviations of the arm and head with respect to the torso plus deviations of the torso with respect to space. When given a pointing target, subjects were largely able to detect and correct for arm and head deviations with respect to the torso but not for the parts of arm and head deviation that were due to deviations of the torso with respect to space. In all conditions, the arm, head, and torso deviations occurred before subjects began to experience compelling self-rotation and displacement. This is contrasted with the compensations for expected but absent Coriolis forces that are made when stationary subjects make reaching movements to targets during exposure to structured moving visual scenes. These compensations do not occur until subjects experience self-rotation and spatial displacement. These results have implications for vehicle control and maneuvering in environments that induce illusory motion and displacement, and in situations where there is motion in a large area of the visual field. The impact of these effects on joystick control is described and discussed. We also describe the subjective sense of ownership attributed to hand-held objects when experiencing illusory self-motion and displacement.