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In the framework of further space exploration, countermeasures to combat the drawbacks of human space flights are essential. The present study focuses on the influence of microgravity on the otolith-ocular reflex and aims to test the hypothesis of artificial gravity being an adequate countermeasure for the deconditioning of the aforementioned reflex. The so-called SPIN study, commissioned by the European Space Agency, can be considered as a control experiment in the broad sense for the Neurolab mission (STS-90) during which 4 crewmembers of the space shuttle were subjected to in-flight centrifugation on the visual and vestibular investigation system (VVIS). After their nearly 16-day mission, they did not suffer from orthostatic intolerance and spatial disorientation. In addition, the relevant parameters of the otolith-ocular interaction remained unaffected. For this study cosmonauts from a long duration stay in the International Space Station that were not centrifuged in-flight were tested on the VVIS (1 g centripetal interaural acceleration) on 6 different days. Three measurements were taken about 1.5-2 months prior to launch and 3 were taken at 1, 4 and 9 days after return from space. Ocular counter-rolling was measured before, during and after rotation on the VVIS using infrared video goggles and compared pair wise using Friedman tests. The perception of verticality was monitored using an ultrasound system for perceptual evaluation. The preliminary results of 4 cosmonauts showed a surprisingly large inter-individual variability of the measurements. Although OCR and perception of verticality appeared to be influenced overall by the exposure to microgravity, the wide variability among the cosmonauts obscured any statistical significance, in particular due to one cosmonauts being inconsistent with the other 3. Despite the specificity of the tests under normal conditions, the diverse response to spaceflight of our subjects exposes the complexity of the peripheral and central neural adaptive processes. [ABSTRACT FROM AUTHOR]

Abstract: We recently conducted experiments where 24 seated participants were subjected (with eyes closed) to small amplitude, high-jerk impulses of linear acceleration. Responses were distributed as a continuum between two extremes. The “stiff” participants showed little movement of the head relative to the trunk, whereas the “floppy” participants showed a large head rotation in the direction opposite the sled movement. We hypothesized that the stiff behavior resulted from the spontaneous use of an imagined visual frame of reference and undertook this larger-scale study to test that idea. The distribution along the “stiff–floppy” continuum was compared with the scores on psychophysiological tests measuring vividness of imagery, visual field-dependence and motion sickness susceptibility. Multivariate regression analysis revealed that the “stiffness” of individuals was loosely, but significantly related to the vividness of their imagery. However, “stiffness” was not linked to visual field-dependence or motion sickness susceptibility. Even if it explains only 20% of the variance of the data, the increase of “stiffness” with vividness of imagery fits our hypothesis. With eyes closed, stiff people may use imagined external visual cues to stabilize their head and trunk. Floppy people, who are poorer imagers, may rely more on “egocentric”, proprioceptive and vestibular inputs. [Copyright &y& Elsevier]