ms or gain changes between the body movement and the stimulus update, performance problems can be anticipated. Similarly, if the optical verticals in a VE do not correspond with the gravitational vertical of the real environment, orientation and movement difficulties may be experienced by the user.

Whole-body movements and locomotion raise a large set of issues concerning the forms of compensation that take place during self-movement, the perception of forces on the surface of the body during movement, the perception of self-displacement through space, and of one's voluntary actions. The way in which adaptive compensations for unusual environments and for maintenance of accurate sensorimotor calibration are achieved is also crucial. All of these issues are critical to understanding how people will adapt to VEs involving locomotory movements, passive transport, and head, arm, and torso movements. However, it is important to recognize that our knowledge of these areas is incomplete. Motion sickness is a factor that is certainly going to affect performance in VEs involving locomotion and experienced self-motion. It is necessary to be aware of the wide range of factors that contribute to motion sickness, the variety of the symptoms, and its sometimes subtle characteristics.

STATUS OF THE RELEVANT HUMAN RESEARCH

Sensorimotor Stability During Self-Motion

Under normal circumstances, an individual accurately perceives his or her voluntary movements and perceives the surroundings to be stable when they are actually stable. This stability that we take for granted is the result of complex sensorimotor adaptations to the 1G force field of earth. The existence of these adaptations becomes apparent during exposure to non-1G force levels or conditions of sensory rearrangement (Lackner and Graybiel, 1981). One aspect of these adaptations includes not perceiving veridically the forces acting on the body surface when they are due to voluntary activity or passive support of the body against the force of gravity. For example, the forces on the bottom of the feet feel roughly comparable when one is standing on one or both feet even though in the former case the force on the stance foot is twice as large (Lackner and Graybiel, 1984c). Similarly, in running, the forces on the feet can vary from 0 to 3G, yet these huge changes are not perceptually registered. When these same force levels are passively applied, the sensation is very strong.

Body movements are accompanied by various sensorimotor consequences. In the case of object manipulation, the pattern of sensory stimulation



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