of the hands in relation to the patterns of motor activity controlling the hands and arms and other parts of the body allows identification of properties of the object such as its shape, texture, and weight. In the case of locomotion, similar types of information along with vestibular cues allow—even in the absence of vision—quite accurate judgments of the distance and direction moved and properties of the support surface, such as its inclination and surface properties.

Manipulation of virtual objects and locomotion in VEs will disrupt many of the sensorimotor regularities that are present in the normal environment. This is a significant issue because we have limited experience with violation of these constraints, and most probably only a relatively small set of the relevant ones have yet been identified. Consequently, movements made in VEs that violate terrestrial sensorimotor constraints may initially cause performance decrements and elicit symptoms of motion sickness (Lackner et al., 1991). In fact, reports of motion sickness in simulated environments are becoming commonplace. As with other unusual sensorimotor environments, continued exposure is likely to lead to adaptive compensations that restore accurate performance and alleviate symptoms of motion sickness. However, on return to the normal environment, there may be persistence of adaptation to the virtual situation leading to performance decrements and motion sickness in the normal environment. It may be possible, however, to create states of dual simultaneous adaptation such that accurate performance is possible in the normal as well as in one or more VEs. The possibility of such dual adaptations has been shown for a number of unusual force situations, such as rotating and nonrotating environments (Lackner, 1990), as well as for other types of situations (e.g., Welch et al., 1993).

Head Movements

Head movements made during exposure to increased or decreased gravitoinertial force background levels tend to bring on symptoms of motion sickness because the normal patterning of sensorimotor control of the head and patterns of sensory feedback are disrupted (Lackner and Graybiel, 1984a, 1985, 1987; Lackner et al., 1991). Virtually any alteration in the normal patterning of control can be provocative (Lackner and DiZio, 1989). For example, wearing a neck brace requires head movements to be achieved by motion of the torso; this alteration in motor control can be quite provocative for many individuals. Passive exposure to various types of motion is provocative because of the labyrinthine stimulation involved (e.g., on shipboard), but such exposure is even more provocative if the head is not passively supported but rather actively controlled (Lackner et al., 1991).



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