[Interlabyrinthine asymmetry, vestibular dysfunction and space motion sickness]

The data available suggest that a disorder in the labyrinthine paired function can be regarded as a factor responsible for the initiation and development of space motion sickness. This concept is based on the assumption that in the norm the vestibular function is to a certain extent asymmetric which is made for by a compensatory center in the central nervous system. Exposure to an unusual space environment leads to a disorder of this compensation and development of a "new" vestibular asymmetry. This exposure involves: elimination of the difference in the weight of otolith membranes, disorder of the canal-otolith interaction, asymmetric blood-CSF changes, distinct interhemispheric asymmetry, general stress. Vestibular asymmetry that develops in the weightless state may become sufficient for the generalization of afferent impulsation to normal stimuli and development of a strong reaction. Adaptation to weightlessness occurs due to rearrangements of the compensatory center which are responsible for lower vestibular asymmetry. The compensatory mechanisms that have evolved in weightlessness continue to function during a certain time interval after recovery. They facilitate a re-initiation of vestibular asymmetry and motion sickness that is mainly provoked by head movements, as in the weightless state.