No abstract

Ocular counterrolling (OCR) induced by lateroflexion, whole body roll, eccentric rotation or translational acceleration has been explored and promoted as an indicator of the vestibular statolith function. Quantification of eye torsion induced by stimulation of the statolith organ has recently become available for broad clinical application thanks to the development of video nystagmography. The most simple way and maybe clinically the most attractive procedure to induce OCR could have been by changing the orientation of the statolith system relative to the gravitation vector, i.e. by lateroflexion of the head. However, irrespective of the visual condition, irrespective of the type of head roll, static or dynamic, OCR showed considerable inter- and intravariability. Findings in healthy subjects and patients indicate that according to our current insight, measurement of OCR induced by lateroflexion or whole body roll has a limited sensitivity and specificity. The normal range of OCR in healthy subjects is large (4.5–9.0% and could be interpreted as a functional asymmetry in statolith function that does not really exist. At present there are insufficient clinical data available to judge the clinical relevance of this test. OCR induced by sideward translations has the advantage over the previously described techniques that it allows an evaluation of the frequency characteristics of the statolith function. Major problems with this technique are that the equipment required is complex and expensive, that a appropriate head fixation is difficult and that responses are quite small. So far, the scarce clinical data do not yet point to a clear clinical relevance of this approach.

Intermittent short-radius centrifugation is a promising countermeasure against the adverse effects of prolonged weightlessness. To assess the feasibility of this countermeasure, we need to understand the disturbing sensory effects that accompany some movements carried out during rotation. We tested 20 subjects who executed yaw and pitch head movements while rotating at constant angular velocity. They were supine with their main body axis perpendicular to earth gravity. The head was placed at the centrifuge's axis of rotation. Head movements produced a transient elevation of heart-rate. All observers reported head-contingent sensations of body tilt although their bodies remained supine. Mostly, the subjective sensations conform to a model based on semicircular canal responses to angular acceleration. However, some surprising deviations from the model were found. Also, large inter-individual differences in direction, magnitude, and quality of the illusory body tilt were observed. The results have implications for subject screening and prediction of subjective tolerance for centrifugation.