OBJECTIVES:
In contrast to previous studies, post-flight measures of both otolith-ocular function and orthostatic tolerance were unimpaired in four payload crewmembers exposed to artificial gravity generated by in-flight centrifugation during the Neurolab (STS-90) mission. The aim of the current proposal is to obtain control measures of otolith and orthostatic function following short duration missions, utilizing the centrifugation and autonomic testing techniques developed for the Neurolab mission, from astronauts who have not been exposed to in-flight centrifugation. This will enable a direct comparison with data obtained from the Neurolab crew. Deficits in otolith-ocular reflexes would support the hypothesis that intermittent exposure to in-flight centripetal acceleration is a countermeasure for otolith deconditioning. Furthermore, a correlation between post-flight otolith deconditioning and orthostatic intolerance would establish an otolithic basis for this condition.

1. Measure otolith-mediated eye movements, in particular ocular counter-rolling (OCR), spatial orientation of the vestibulo-ocular reflex (VOR) and optokinetic nystagmus (OKN), in response to tilts of the gravito-inertial acceleration vector (GIA) during pre- and post flight tangential centrifugation.
2. Measure OCR, spatial orientation of the VOR and of OKN during pre- and post-flight static roll tilt. Evaluate pre- and post-flight orthostatic tolerance by monitoring heart rate, blood pressure, and fluid shifts during a standardized tilt test as previously used for the Neurolab mission. Segmental body impedance will be used to estimate fluid shifts and stroke volume, from which vascular resistance can be calculated and used as an estimate of sympathetic activation.

If the hypothesis is correct, post-flight deficits in otolith-ocular function will provide evidence that in-flight centrifugation had prevented otolith deconditioning during the Neurolab mission. In addition, a correlation of post-flight orthostatic intolerance with decrements in otolith-ocular performance will provide evidence for an otolithic basis for deficits in sympathetic outflow related to orthostatic stress. These would be highly significant findings for future long-duration space missions, where providing an artificial gravity countermeasure for otolith and orthostatic deconditioning may prove critical to the well being of the crew. These findings are also relevant to studies of imbalance and orthostatic intolerance on Earth. Many of the postural and locomotor deficits observed in astronauts post-flight are similar to those seen in patients with vestibular disease, and findings from this study may shed light on the otolithic basis for these conditions.

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