Velocity storage activity is affected after sustained centrifugation: a relationship with spatial disorientation

Prolonged exposure to hypergravity in a human centrifuge can lead to post-rotary spatial disorientation and motion sickness. These symptoms are mainly provoked by tilting head movements and resemble the Space Adaptation Syndrome. We hypothesized that the occurrence of these post-rotary effects might be related to changes in the velocity storage (VS) mechanism, which is suggested to play an important role in spatial orientation. In particular, we investigated whether the re-orientation of the eye velocity vector (EVV) towards gravity during off-vertical optokinetic stimulation was affected by centrifugation. Twelve human subjects were exposed to a hypergravity load of 3G (G-load directed along the naso-occipetal axis) for a duration of 90 min. Before and after centrifugation we recorded optokinetic nystagmus (OKN) elicited by a stimulus pattern moving about the subject's yaw axis, with the head erect and tilted 45 degrees to both sides. During OKN with the head erect, we observed a pitch-down component, reorienting the EVV on average 4.5 degrees (SD 3.6, pretest values) away from the stimulus axis. Head tilt induced an additional shift of the EVV towards the spatial vertical of 6.4 degrees on average (SD 3.2). This head-tilt induced reorientation was significantly decreased after centrifugation to 4.7 degrees (SD 2.9), suggesting a reduction of VS-activity. By means of a vector model we estimated the reduction in VS-activity at 31%. Such a decrease in VS-activity might reflect a deterioration of the ability to integrate sensory signals to obtain an estimate of gravity during tilting head movements, resulting in motion sickness in susceptible subjects.