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The aim of this study was to evaluate the role of the vestibular system in cardiovascular control in a varying gravito-inertial field induced by parabolic flight. We measured variations in arterial pressure and heart rate in eight awake rats, four of which had undergone bilateral labyrinthectomy 3 months previously. While the control rats showed heart rate and mean arterial pressure modulations depending on gravity level, no such variation was observed in the lesioned rats. This study confirms the role of the vestibular system in cardiovascular control and opens up new prospects for interpreting cardiovascular variations observed during space flights.

Head posture and neck muscle activity (EMGs) were examined in unilateral (UL) and bilateral (BL) vestibularly lesioned rats in hypergravity (1.7 g) and hypogravity (0 g) during parabolic flights. Compared with BL rats taken as control, the head and the body of UL deviated toward the lesion side at 0 g and toward the intact side at 1.7 g. Recorded in head fixed condition, left and right EMGs remained symmetrical in BL while UL rats displayed an asymmetry between left and right muscles at 1.7 g, but not at 0 g. These results demonstrate that an experimental otolithic asymmetry, compensated on Earth, can become unbalanced in altered gravity. Paradoxically, the utricular system appears to play a major role in that process.

The aim of this study was to investigate whether figure skaters, as individuals who experience intense vestibular stimulation, presented modification of the otolith-ocular reflex. The reflexes of 12 figure skaters were assessed using off vertical axis rotation (OVAR). Horizontal otolith-ocular reflex during OVAR is characterized by two parameters: the eye velocity horizontal modulation, assumed to compensate for perceived lateral linear translation, and the bias, assumed to compensate for the perceived rotation. We observed that skaters presented smaller amplitude of modulation and truly compensatory bias compared with control participants. Thus, the otolithic signal during OVAR seems to be interpreted more as rotation and less as translation or inclination in figure skaters.

Since motion sickness (MS) never occurs in individuals who lack functional vestibular apparatus, it has been suggested that MS susceptible individuals have more sensitive vestibular systems than non-susceptible people. However, previous investigations involving only stimulation of the semi-circular canals have been inconclusive. We measured gain and time constant (TC) of horizontal canal-ocular reflex (COR) and magnitude of otolith-ocular reflex (OOR). We found that MS susceptibility was not correlated to COR gain but was negatively correlated to OOR magnitude. Thus, MS susceptible individuals do not have more sensitive vestibular systems. We also found a positive correlation between MS susceptibility and TC. We hypothesize that central vestibular integration (velocity storage mechanism), by increasing low frequency vestibular inputs, would favour MS.