Thermal stimulation of the vestibular labyrinth during orbital flight.

During the European Spacelab mission (SL1) in 1983, caloric testing was performed for the first time in long-term weightlessness. After 2 days into orbital flight an unequivocal caloric nystagmus was observed in both subjects tested which corresponded in both quality and intensity with that measured in one-g conditions on Earth. The subsequent D1 mission enabled the experiment to be repeated on further subjects and with improved measurement procedures. As with the SL1 findings, the observed caloric nystagmus response proved to be equivalent to that measured during baseline testing on Earth. Renewed consideration of peripheral and central mechanisms, which might be involved in the elicitation of the caloric response--both in one-g and zero-g environments--has led to the reopening of a number of associated issues. One important observation which has been addressed by various research groups concerns the influence of the labyrinth's orientation to the gravity vector on the caloric response. The present authors have examined a group of healthy subjects in various body positions in the sagittal plane. The interindividual variability in the response behavior was found to be high; indeed single cases were observed in which the nystagmus response did not invert from the supine to the prone positions. These findings are discussed together with earlier reports in the literature.