BACKGROUND: Exposure to unfamiliar motion patterns commonly results in motion 
sickness and a false perception of motion, termed mal de debarquement, on the 
return to stable conditions.
OBJECTIVE: To investigate whether motion sickness severity is correlated with 
the duration and severity of mal de debarquement; to study the possible 
preventive effect of projecting earth-referenced scenes (an artificial horizon) 
during exposure to motion on the development of mal de debarquement.
METHODS: Thirty subjects were exposed to the recorded motion profile of a boat 
in a 3-degrees-of-freedom ship motion simulator. During the simulated voyage, 
the study participants were repeatedly put through a performance test battery 
and completed a motion sickness susceptibility questionnaire, while 
self-referenced and earth-referenced scenes were projected inside the simulator 
cabin. Six hours post disembarkation, subjects completed a questionnaire on mal 
de debarquement duration and severity.
RESULTS: Mal de debarquement, mostly of mild severity, was reported following 
59% of the exposures to the provocative motion profile, and in 79% of cases 
lasted less than 6 hours. The incidence of mal de debarquement, its duration, 
and the severity of symptoms did not differ between the various artificial 
horizon projection modes. Significant correlations were found between motion 
sickness severity and the duration and severity of the mal de debarquement that 
followed.
CONCLUSIONS: The significant correlations found between motion sickness severity 
and mal de debarquement duration and severity imply that both syndromes might 
stem from a failure to adapt to new motion conditions. There is a disparity 
between the previously reported reduction in motion sickness symptoms by an 
artificial horizon, and its failure to influence the duration and symptoms of 
mal de debarquement. This might be explained by the different response in the 
two syndromes, physical versus cognitive.