Active high-frequency vestibulo-ocular reflex and seasickness susceptibility.

Objectives/hypothesis: The vestibular autorotation test (VAT) examines responses to active head oscillations at frequencies between 2 and 6 Hz in the horizontal and vertical planes while the subject is fixating a visible target. At these frequencies, the vestibulo-ocular reflex (VOR) is the main source of eye movement for ocular stabilization, although other visual and somatosensory information interacts with the response. Because the neural mismatch theory places emphasis on multimodal sensory interactions as the cause of motion sickness, using the VAT, which measures eye movements resulting from vestibular, visual, and, to a certain extent, proprioceptive information and depends on the conscious participation and cooperation of the subject, could be of advantage in evaluating individuals with differing susceptibility to motion sickness. The purpose of the present study was to evaluate high-frequency VOR parameters in seamen at the two extremes of the seasickness susceptibility scale.

Study Design: Cross-sectional, parallel-group design.

Methods: Participants in the study were 35 healthy male volunteers aged 18 to 23 years, of whom 20 were highly susceptible to seasickness and 15 were nonsusceptible. The vestibulo-ocular reflex was evaluated by the VAT at frequencies ranging from 2.0 to 5.9 Hz.

Results: The lag of the vertical phase was significantly higher in the susceptible group. A significant interaction was also found between group and frequency, the vertical phase being significantly higher in the 3.9- to 5.9-Hz range. Although no group effect was detected for the lag of the horizontal phase, there was a significant interaction between group and frequency, the horizontal phase being higher in the susceptible group at 5.5 and 5.9 Hz. No significant group differences were found for horizontal or vertical gain.

Conclusions: The present findings support the contention that the VAT, which measures eye movements resulting from multimodal vestibular, visual, and, to a certain extent, proprioceptive information and depends on the conscious participation and cooperation of the subject, may produce different results in subjects at the two extremes of the seasickness susceptibility scale. Despite the statistical differences that were found, VAT measurements could not be used for practical purposes to categorize individual motion sickness susceptibility.