In space flight, gravitational force is no longer sensed because the space-flight crews are experiencing the effects of microgravity. Also, since there is little locomotion in space, the exposure to centripetal forces is reduced; but the linear accelerations due to side-to-side, up-and-down, and front-to-back motions (translations) persist. Since tilt is meaningless in space (there is no vertical reference from gravity), it has been hypothesized that, during adaptation to weightlessness, the brain would reinterpret all otolith signals to indicate primarily translation, not tilt. It was postulated that this adaptation within the brain underlies the amelioration of space motion sickness symptoms over time. This otolith tilt translation reinterpretation (OTTR) hypothesis has received some support from perceptual studies done after space flight, but it had never been tested during space flight.
In this experiment, astronauts were rotated in a centrifuge. When the centrifuge started, they felt rotation, but this feeling of rotation disappeared after 30-45 seconds of rotation. The net effect of the centrifugation on Earth was that, when seated, the crewmembers felt as if they were tilted either 25 degrees to the side (at 0.5-G acceleration) or 45 degrees to the side (at 1-G acceleration). When lying down, the crewmembers felt as if they were tilted backwards approximately 15 degrees (0.5-G) or 30 degrees (1-G). In space, when weightless, the input to the inner ear from gravity is gone, and the inner ear will only sense the accelerations due to chair rotation. When seated in the chair, this could mean that instead of feeling tilted 30 or 45 degrees, crewmembers would feel as if they were tilted 90 degrees (i.e., as if they were lying on their side). According to the OTTR hypothesis, however, during centrifugation in space crewmembers should not perceive themselves as being tilted 90 degrees relative to their perceived upright, but instead should feel as if they are being translated (moving to one side). The purpose of this study was to determine whether in space the astronauts felt a sense of tilt or translation during constant-velocity centrifugation, as compared to their original position before the centrifuge started.
The subjects were rotated on a short-arm centrifuge, either in darkness or during the presentation of visual stimuli. The results presented in this report concern only those recorded when the subjects were in complete darkness. The centrifuge was accelerated in complete darkness and after 40 seconds; i.e., after the perception of rotation ceased (and the associated eye movements, called nystagmus, had also stopped). The subjects were then prompted by an operator to verbally report whether they had a perceived sensation of tilt or motion during steady-state centrifugation. A typical trial consisted of clockwise (CW) and counterclockwise (CCW) rotation with the left-ear-out (LEO) orientation, CCW and CW rotation with the right-ear-out (REO) orientation, and CW rotation with the lying-on-back (LOB) orientation. The basis for the perceptual measurements on Earth was whether subjects perceived tilt of their body vertical relative to their perception of the spatial vertical. For the LEO/REO orientations, subjects used a scale of 0-90 degrees to represent their roll tilt perception, where zero degree indicated that the subjects felt upright and 90 degrees meant that they felt as if they were "lying-on-side." A similar scale was used in the LOB configuration. If the subject felt horizontal, this would be reported as zero tilt, and a tilt of -90 degrees indicated that the subjects felt "upside-down." Inflight, subjects had no perception of tilt when the centrifuge was stationary. When exposed to the 0.5-G or 1-G centripetal acceleration during rotation, if subjects felt as if they were tilted, they used the same criteria as on Earth to report their tilt relative to a perceived spatial vertical. That is, if the subjects reported a roll tilt of 90 degrees in LEO/REO orientations, this indicated that they felt as if they would have been "lying-on-side" on Earth. A -90-degree pitch tilt when in the LOB position indicated that the subjects would have been "upside-down" on Earth. Subjects were also asked to report any sense of linear motion using a simple estimate of magnitude (in meters/second).
In addition to the centrifugation study, the astronauts' perception during static full-body roll tilt was studied during preflight and postflight testing. Postflight testing started as soon as two hours after landing (R+0). Subjects sat in a tilt chair in darkness and their bodies were passively tilted left-ear-down from the upright position around an axis located under their feet. The chair was tilted in increments of 15 degrees up to 90 degrees. Subjects stayed tilted at each angle for about 40 seconds before reporting their perceived angle of tilt. Three astronauts of the STS-78 Life and Microgravity Spacelab (LMS) mission were also tested during static roll tilt of 30 degrees on L-30, R+0, R+1, R+4 and R+7.
On Earth when sitting upright on a centrifuge and facing into the direction of motion, subjects at first sense that they are in a steep turn and then feel that they are tilted outward. They perceive a 1-G centripetal acceleration as a tilt of about 45 degrees, although they are upright. (The 1-G acceleration of gravity adds to the 1-G centripetal acceleration to cause the 45-degree tilt). Similarly, they perceive a 0.5-G centripetal acceleration as a tilt of about 25 degrees. When they are centrifuged while lying on their back, they perceive a body tilt toward the head-down position. These effects are called somatogravic illusions and are present in every person with an intact vestibular system.
In the test subjects, the perceived body tilt was larger during the first 4-5 runs on the centrifuge, but fairly constant afterward. This result shows that there is a training effect for the somatogravic illusion, which might explain the different values across studies depending on whether the test subjects were naive or not. After several trials, the mean perceived body tilt during 1-G and 0.5-G centrifugation in the subjects was smaller (35 degrees and 20 degrees, respectively) than that of the GIA (45 degrees and 27 degrees) because the subjects were asked to report their angle of body tilt only 40 seconds after constant-velocity rotation. Ground-based studies have shown that it takes about 80 seconds for the sensation of body tilt to reach its full magnitude after the centrifuge has reached its final velocity.
At no point during or after the mission did the subjects perceive translation during constant-velocity centrifugation. Instead during space flight, the perceived body tilt increased from about 45 degrees on FD1 to nearly 90 degrees on FD16 during centrifugation at 1-G. Inflight tilt perception during 0.5-G centrifugation on FD7 and FD12 was approximately half that reported during 1-G centrifugation. For comparison, the GIA was actually tilted by 45 degrees and 27 degrees on Earth and by 90 degrees in space relative to the spatial vertical during 1-G and 0.5-G centrifugation respectively. The error in perceived tilt was very large early inflight and early postflight. A similar error was seen postflight during static full-body tilt. Prior to flight, the perceived tilt angle was close to the actual tilt angle, and subjects underestimated or overestimated the extent of body tilt by only 3-5 degrees for tilts larger than 60 degrees, similar to the precision of setting a visual line to the vertical. On R+0 and R+1, the extent of body tilt was overestimated by about 15 degrees. Four days after landing, the astronauts' perception of tilt during static body tilt had returned to preflight values. Thus, tilt was underestimated at the beginning of space flight during centrifugation and overestimated on return to Earth during both centrifugation and actual body tilt. The time course of return of subjects' estimates to preflight values was similar in both conditions.
Before the Neurolab mission, astronauts had experienced sustained centripetal acceleration in space only on rare occasions. During the Gemini XI flight, in 1966, the manned spacecraft was tethered to an Agena target vehicle by a long Dacron line. This caused the two vehicles to spin slowly around each other for several minutes. According to the Gemini commander, a television camera fell "down" in the direction of the centrifugal force, but the crew on board Gemini did not sense the centripetal acceleration. Subjects sitting on a linear sled flown during the Spacelab-D1 mission in 1985 perceived linear acceleration but not tilt. Similarly, during off-axis rotation on the Spacelab IML-1 mission, subjects perceived only rotation, not tilt. In both experiments, however, linear accelerations were below 0.22-G, which ground-based studies have shown to be insufficiently strong to yield a perception of tilt. Humans had never been exposed to steadystate linear acceleration of 0.5-G and 1-G in space before Neurolab. The results showed that the astronauts perceived a body tilt relative to a perceived spatial vertical when exposed to 0.5-G and 1-G, and that the magnitude of this perception adapted throughout the mission.
After two weeks in space, the subjects perceived an almost 90-degree tilt when they received a 1-G sideways linear acceleration in space, and about half of this when they received a 0.5-G acceleration. Although they had never encountered this stimulus before, their perception was essentially veridical in that it represented the actual levels of linear acceleration experienced by the graviceptors. It suggests that the otoliths are operating normally in space when exposed to 0.5-G and 1-G steady-state linear acceleration, after the initial period of adaptation. The reduced response to the 0.5-G stimulus, whether it was directed along the interaural axis or the longitudinal axis, shows that not only the direction of GIA but also its magnitude is taken into account by the brain. This result could not have been obtained on the Earth's surface in a 1-G environment.
The finding that none of the astronauts felt translation instead of tilt in response to the 0.5-G or 1-G constant linear accelerations in space indicates that the OTTR hypothesis is incorrect. Tilt is perceived as tilt, regardless of whether the subjects are in microgravity or the 1-G environment of Earth, and is not sensed as translation. A model, which references perceptions of tilt with regard to a weighted sum of all linear acceleration and body vertical (idiotropic vector) as the perceived spatial vertical, could explain these results. The underestimation of tilt at the beginning of the flight suggests that the subjects continued to weight their internal estimate of body vertical to compute the direction of the GIA. However, as the flight progressed, the weight of this internal estimate of body vertical gradually decreased and the subjects finally adopted the centripetal acceleration as the new spatial vertical. On return to Earth, perceived body tilt was larger than preflight. This overestimation of body tilt can be interpreted as the result of the continued small weighting of the internal representation of body vertical in estimating the spatial vertical, after adaptation to the weightless environment. Eye movements during both centrifugation in darkness and horizontal optokinetic stimulation shifted toward the GIA in space, consistent with the perceptual data. Thus, the underestimation of tilt on entry into microgravity, and the exaggerated sense of tilt on return, could both be due to the lag in readjusting the weight of the sense of body vertical in determining the perceived spatial vertical reference. Eye movement recordings during these studies also showed that the vector of eye velocity in darkness and of horizontal optokinetic nystagmus during centrifugation continued to shift toward the GIA in space as on Earth. Therefore, both the eye movement data and perceptual findings are consistent and do not support the OTTR hypothesis.
Information from this research could be used to develop countermeasures to overcome lags in adaptation or changes in gaze and balance that occur after return from space. Such information and countermeasures are critical in the long-duration space flights planned for planetary exploration. When astronauts go to Mars, for example, they may have to fend for themselves immediately after landing on a planet with a significant gravitational force (0.38-G), although they will have been in a microgravity environment for months. Anything that could hasten their re-adaptation to a gravitational environment would be valuable and important to them in overcoming difficulties with gaze, posture, walking, and running. One consequence of the findings is that if low-frequency linear acceleration is always perceived as tilt-whether subjects are in weightlessness or on Earth-long-duration missions can proceed with the expectation that the astronauts will respond normally to artificial gravity or to the gravitational fields of other planets.
There are also substantial clinical implications from these experiments. There is little understanding of why there is imbalance when the vestibular system is damaged. Investigators also do not understand why older people are so prone to falling. Alignment of the body axis to the GIA during walking or turning is likely to be an important source of this imbalance. The evaluation of the perceived tilt during centrifugation might prove to be a useful test of the capability for the brain to evaluate the direction of the GIA in a dynamic situation.
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