  This study examined the effect of passengers' active head-tilt and
eyes-open/closed conditions on the severity of motion sickness in the lateral
acceleration environment of cars. In the centrifugal head-tilt condition,
participants intentionally tilted their heads towards the centrifugal force,
whereas in the centripetal head-tilt condition, the participants tilted their
heads against the centrifugal acceleration. The eyes-open and eyes-closed cases
were investigated for each head-tilt condition. In the experimental runs, the
sickness rating in the centripetal head-tilt condition was significantly lower
than that in the centrifugal head-tilt condition. Moreover, the sickness rating
in the eyes-open condition was significantly lower than that in the eyes-closed
condition. The results suggest that an active head-tilt motion against the
centrifugal acceleration reduces the severity of motion sickness both in the
eyes-open and eyes-closed conditions. They also demonstrate that the eyes-open
condition significantly reduces the motion sickness even when the head-tilt
strategy is used.
