Head movements in non-terrestrial force environments elicit motion sickness: implications for the etiology of space motion sickness.

Space motion sickness has become an operational concern in manned space flight. Considerable evidence exists that head movements in free fall, especially pitch movements, are provocative until adaptation occurs (3,4,8,9,11,17,18,22,26). The question arises whether space motion sickness is an unique nosological entity or is due to body movements in a nonterrestrial force environment, a force environment for which the body's dynamic sensory-motor adaptions to 1 G are no longer appropriate (14,16,18-21). To evaluate this issue, we had subjects make controlled head movements during exposure to high gravitoinertial force levels, 1.8-2.0 G, in parabolic flight maneuvers. Head movements in pitch with eyes open were most evocative of motion sickness, yaw movements with eyes covered were least provocative. This pattern is identical to that which occurs when the same types of head movements are made in the free fall phase of parabolic maneuvers (17,18). It appears that space motion sickness is the consequence of prolonged exposure to a non-terrestrial force background rather than of exposure to free fall per se.