Adaptation to Linear Acceleration in Space ( ATLAS ) Experiments : Equipment and Procedures

During orbital spaceflight, the acceleration of gravity is reduced from one-G on the Earth’s surface to about 10–6-G in orbit. Nevertheless, the head accelerations associated with changing direction and with turning are unaffected. Therefore, the body must selectively adapt to the relative absence of gravity. As a result of this reorganization, astronauts frequently exhibit difficulty with balance upon landing that could pose a serious problem if they had to function efficiently in a gravitational environment immediately after a long-duration spaceflight. They can also become disoriented when in motion. These deficits are as yet incompletely understood. The otoliths, located in the inner ear, sense head accelerations and generate reflexes to the eyes and postural muscles that maintain gaze and posture when moving in a gravitational environment. They are also directly involved in sensing the direction of gravity and contribute importantly to the sense of spatial orientation. It is likely that changes in otolith-mediated reflexes that occur as a result of adaptation to microgravity could be responsible for postflight problems with gaze and balance. The purpose of two Neurolab experiments (see science reports by Moore et al. and Clément et al. in this publication) was to study how spatial orientation and reflexes originating in the otolith organs might be affected by adaptation to microgravity. Flight and ground-based instruments were developed for the Neurolab mission to study how orienting otolith-ocular reflexes and the perception of the spatial vertical are changed by adaptation during spaceflight. These instruments included a flight-rated centrifuge that delivered measured amounts of linear acceleration along different body axes in flight, a functionally equivalent centrifuge to do ground-based testing, and an apparatus that statically tilted the head and body with regard to gravity before and after flight. The static tilt apparatus allowed the teams to obtain data that could be compared with the results of centrifugation. Binocular, infrared video-oculography, a noninvasive technique, was utilized to measure eye movements in three dimensions while the astronauts were being centrifuged. Equipment was also developed that could deliver visual stimuli to calibrate the induced eye movements and to induce optokinetic nystagmus (OKN) and ocular pursuit during centrifugation and static tilt. This report describes the development and use of this equipment. This unique hardware performed flawlessly in flight, provided the first inflight “artificial gravity,” and advanced the technology of eye movement recording.