head movements may be extremely provocative. In addition, the time delays in HMDs associated with updating the visual scene to compensate for head movements disrupt the normal patterning of the vestibulo-ocular reflex. This is especially nauseogenic in wide-field HMDs in which large-gaze shifts occur as individuals turn their head and eyes to acquire targets in the peripheral visual field (Lackner and DiZio, unpublished observations). The point to be taken is that there is a spectrum of factors that can be expected to evoke sickness.

Arm Movements

Arm movements made to objects or to control devices are accompanied by patterns of sensory feedback related to the control of the arm and the object manipulated. Object characteristics include inertia, mass, weight, texture, compliance, etc. Expectancies about the properties of objects are exceedingly important. For example, when visually similar objects of different physical sizes but identical masses are hefted in succession, the larger sized one will be perceived as being considerably lighter. The motor plans for hefting the objects probably include compensation for a greater expected mass of the larger object. This effect, known as the size-weight (or Charpentier's) illusion, persists, however, even when the objects are known to be of comparable weight. Insofar as object manipulation in VEs violates cognitive expectancies about object behavior, performance decrements will occur, adaptation will be required, and after-effects can be anticipated on return to the normal environment.

During passive transport of the body in vehicles as well as during voluntary locomotion and turning of the body, arm movements are usually quite accurate. In a familiar vehicle, even one not being self-controlled, compensation can be made for the ongoing and expected motion of the vehicle (e.g., on shipboard, the rolling and scending motions are periodic and can be anticipated). During exposure to linear and angular acceleration, adjustments of the entire body as well as arm movement control may be required. For example, the driver of a vehicle will physically lean into a turn (Fukuda, 1975).

Arm movements made during exposure to passive rotation of the body generate Coriolis forces that deviate the arm from its intended trajectory and target. With repeated reaches, even without visual feedback about movement accuracy, accuracy will be rapidly regained if the hand makes physical contact at the end of the reaching movement. However, in the absence of vision, if terminal contact of the hand is not present at the end of the movement, adaptation does not take place or is greatly slowed (Lackner and DiZio, 1994). On cessation of body rotation, pointing movements made to targets show error patterns that are mirror images



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