Relationship between motion sickness susceptibility and vestibulo-ocular reflex gain and phase

BACKGROUND: It has been proposed that individual susceptibility to motion sickness is related to the vestibulo-ocular reflex (VOR) through the activation of the velocity storage mechanism. OBJECTIVE: We investigated whether motion sickness level was related to the gain and phase of the VOR. METHODS: VOR gain and phase were measured in 214 subjects while they rotated in yaw at 0.01 Hz, 0.02 Hz, 0.04 Hz, 0.08 Hz, and 0.16 Hz in darkness, and results were compared to the severity of symptoms the subjects experienced during subsequent tests to provoke motion sickness. These tests included cross-coupled angular accelerations, sudden stops in light or in dark, off-vertical axis rotation, and parabolic flight. The subjects were grouped according to the motion sickness level reached during these tests (none, low, medium, or high). RESULTS: No correlation was found between the horizontal VOR gain and motion sickness level. However, for the subjects with high motion sickness level, the VOR phase lead was significantly lower during rotation at frequencies ranging from 0.04 Hz to 0.16 Hz (i.e. the VOR time constant was longer) than the other motion sickness groups. CONCLUSION: These results support the theory that the longer the time constant for velocity storage, the more severe the motion sickness.