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Background: During large angles of self-tilt in the roll plane on Earth, measurements of the subjective visual vertical (SVV) in the dark show a bias towards the longitudinal body axis, reflecting a systematic underestimation of self-tilt., Objective: This study tested the hypothesis that self-tilt is underestimated in partial gravity conditions, and more so at lower gravity levels., Methods: The SVV was measured in parabolic flight at three partial gravity levels: 0.25, 0.50, and 0.75 g. Self-tilt was varied amongst 0, 15, 30, and 45 deg, using a tiltable seat. The participants indicated their SVV by setting a linear array of dots projected inside a head mounted display to the perceived vertical. The angles of participants' body and head roll tilt relative to the gravito-inertial vertical were measured by two separate inertial measurement units., Results: Data on six participants were collected. Per G-level, a regression analysis was performed with SVV setting as dependent variable and head tilt as independent variable. The latter was used instead of chair tilt, because not all the participants' heads were aligned with their bodies. The estimated regression slopes significantly decreased with smaller G-levels, reflecting an increased bias of the SVV towards the longitudinal body axis. On average, the regression slopes were 0.95 (±0.38) at 0.75 g; 0.84 (±0.22) at 0.5 g; and 0.63 (±0.33) at 0.25 g., Conclusions: The results of this study show that reduced gravity conditions lead to increased underestimation of roll self-tilt.

Background: The vestibulo-ocular reflex (VOR) is a basic function of the vestibular system that stabilizes gaze during head movement. Investigations on how spaceflight affects VOR gain and phase are few, and the magnitude of observed changes varies considerably and depends on the protocols used., Objective: We investigated whether the gain and phase of the VOR in darkness and the visually assisted VOR were affected during and after spaceflight., Methods: We measured the VOR gain and phase of 4 astronauts during and after a Space Shuttle spaceflight while the subjects voluntary oscillated their head around the yaw axis at 0.33 Hz or 1 Hz and fixed their gaze on a visual target (VVOR) or imagined this target when vision was occluded (DVOR). Eye position was recorded using electrooculography and angular velocity of the head was recorded with angular rate sensors., Results: The VVOR gain at both oscillation frequencies remained near unity for all trials. DVOR gain was more variable inflight and postflight. Early inflight and immediately after the flight, DVOR gain was lower than before the flight. The phase between head and eye position was not altered by spaceflight., Conclusion: The decrease in DVOR gain early in the flight and after the flight reflects adaptive changes in central integration of vestibular and proprioceptive sensory inputs during active head movements.

Background: Ground-based studies have reported shifts of the vestibulo-ocular reflex (VOR) slow phase velocity (SPV) axis toward the resultant gravito-inertial force vector. The VOR was examined during eccentric roll rotation before, during and after an 8-day orbital mission. On orbit this vector is aligned with the head z-axis. Our hypothesis was that eccentric roll rotation on orbit would generate horizontal eye movements., Methods: Two subjects were rotated in a semi-supine position with the head nasal-occipital axis parallel to the axis of rotation and 0.5 m off-center. The chair accelerated at 120 deg/s2 to 120 deg/s, rotated at constant velocity for one minute, and then decelerated to a stop in similar fashion., Results: On Earth, the stimulation primarily generated torsional VOR. During spaceflight, in one subject torsional VOR became horizontal VOR, and then decayed very slowly. In the other subject, torsional VOR was reduced on orbit relative to pre- and post-flight, but the SPV axis did not rotate., Conclusion: We attribute the shift from torsional to horizontal VOR on orbit to a spatial orientation of velocity storage toward alignment with the gravito-inertial force vector, and the inter-individual difference to cognitive factors related to the subjective straight-ahead.

Background: It has been proposed that individual susceptibility to motion sickness is related to the vestibulo-ocular reflex (VOR) through the activation of the velocity storage mechanism., Objective: We investigated whether motion sickness level was related to the gain and phase of the VOR., Methods: VOR gain and phase were measured in 214 subjects while they rotated in yaw at 0.01 Hz, 0.02 Hz, 0.04 Hz, 0.08 Hz, and 0.16 Hz in darkness, and results were compared to the severity of symptoms the subjects experienced during subsequent tests to provoke motion sickness. These tests included cross-coupled angular accelerations, sudden stops in light or in dark, off-vertical axis rotation, and parabolic flight. The subjects were grouped according to the motion sickness level reached during these tests (none, low, medium, or high)., Results: No correlation was found between the horizontal VOR gain and motion sickness level. However, for the subjects with high motion sickness level, the VOR phase lead was significantly lower during rotation at frequencies ranging from 0.04 Hz to 0.16 Hz (i.e. the VOR time constant was longer) than the other motion sickness groups., Conclusion: These results support the theory that the longer the time constant for velocity storage, the more severe the motion sickness.

Background: Artificial gravity using short-radius centrifugation has been proposed as an integrative countermeasure during spaceflight., Objective: To determine the rotation parameters of a short-radius centrifuge so that subjects rotating in the dark would feel as if they were standing upright., Methods: Twelve subjects were lying supine in a nacelle on a 2.8 m-radius centrifuge with their head closer to the axis of rotation and their feet pointing radially outwards. Subjects verbally reported body orientation for 26 combinations of centrifuge rotation rate and nacelle pitch tilt. ECG and respiratory responses were also recorded., Results: Five subjects felt like they were vertical when centrifugation elicited 1 g at their center of mass along their body longitudinal axis, whereas seven subjects felt they were vertical when they experienced about 1 g at ear level, regardless of the nacelle tilt angle. Heart rate variability varied with the subjects' perception of verticality., Conclusions: These results suggest that one group of subject was relying principally on the otolith organs for the perception of verticality, whereas the other group was also relying on extravestibular somatosensory receptors. The crewmember's perception of verticality might be a factor to take into account for the prescription for artificial gravity during spaceflight.

Constant velocity off-vertical axis rotation (OVAR) provides dynamic linear acceleration stimuli that can be used to assess otolith function. Eight astronauts were rotated in darkness about their longitudinal axis 20° off vertical at low (0.125 Hz) and high (0.5 Hz) frequencies and their responses were compared before and after spaceflight. Eye movements were recorded using infrared videography and perceived motion was evaluated using a joystick with four degrees of freedom - pitch and roll tilt, front-back and lateral translation. Low-frequency OVAR generates tilt otolith-induced responses - modulation of ocular counter-roll and counter-pitch with perceived conical motion path - whereas high-frequency OVAR generates translational otolith-induced responses - modulation of horizontal and vergence slow phase velocity with perceived cylindrical motion path. While there were transient changes in the amplitude of the translational ocular responses on landing day, there were no major changes in the tilt ocular reflexes after adaptation to weightlessness. However, there was an increase in sensitivity to motion perception after spaceflight. Direct comparisons of pre- and postflight stimuli suggested that OVAR on landing day was less provocative of motion sickness than before spaceflight. These results confirm that some otolith reflexes elicited during passive motion may not be altered by short-duration spaceflight - or may readapt very quickly - and that the resolution of sensory conflict associated with postflight recovery involves higher-order neural processes.

Ocular counter-rolling (OCR) induced by whole body tilt in roll has been explored after spaceflight as an indicator of the adaptation of the otolith function to microgravity. It has been claimed that the overall pattern of OCR responses during static body tilt after spaceflight is indicative of a decreased role of the otolith function, but the results of these studies have not been consistent, mostly due to large variations in the OCR within and across individuals. By contrast with static head tilt, off-vertical axis rotation (OVAR) presents the advantage of generating a sinusoidal modulation of OCR, allowing averaged measurements over several cycles, thus improving measurement accuracy. Accordingly, OCR and the sense of roll tilt were evaluated in seven astronauts before and after spaceflight during OVAR at 45 degrees/s in darkness at two angles of tilt (10 degrees and 20 degrees). There was no significant difference in OCR during OVAR immediately after landing compared to preflight. However, the amplitude of the perceived roll tilt during OVAR was significantly larger immediately postflight, and then returned to control values in the following days. Since the OCR response is predominantly attributed to the shearing force exerted on the utricular macula, the absence of change in OCR postflight suggests that the peripheral otolith organs function normally after short-term spaceflight. However, the increased sense of roll tilt indicates an adaptation in the central processing of gravitational input, presumably related to a re-weighting of the internal representation of gravitational vertical as a result of adaptation to microgravity.

Pitch head-and-trunk movements during constant velocity rotation are a provocative vestibular stimulus that produces vertigo and nausea. When exposed to this stimulus repeatedly, motion sickness symptoms diminish as the subjects habituate. Acetylleucine is a drug that is used to treat acute vestibular vertigo. In this study, we wanted to ascertain whether this drug (a) lessened motion sickness or delayed habituation; (b) accelerated the recovery following habituation; and (c) whether changes in the subjective vertical accompanied habituation. Twenty subjects were administered acetylleucine or placebo in a double-blind study during a five-day vestibular training. Horizontal vestibulo-ocular reflex, optokinetic nystagmus, smooth pursuit, and subjective visual vertical were evaluated before, during, and up to two months after the vestibular training. Based on Graybiel's diagnostic criteria, motion sickness decreased steadily in each vestibular training session, and there was no difference between the scores in the acetylleucine and placebo groups. Post-rotatory nystagmus peak velocity and time constant also declined in both groups at the same rate. Thus, acetylleucine neither reduced the nausea associated with this provocative stimulus, nor hastened the acquisition or retention of vestibular habituation of motion sickness and nystagmus. There was no difference in optokinetic nystagmus and smooth pursuit between the acetylleucine and placebo groups. However, subjects showed larger error in the subjective visual vertical after habituation, which indicates that spatial orientation is also affected by vestibular training.

Prolonged microgravity during orbital flight is a unique way to modify the otolith inputs and to determine the extent of their contribution to the vertical vestibulo-ocular reflex (VOR) and optokinetic nystagmus (OKN). This paper reviews the data collected on 10 astronauts during several space missions and focuses on the changes in the up-down asymmetry. Both the OKN elicited by vertical visual stimulation and the active VOR elicited by voluntary pitch head movements showed an asymmetry before flight, with upward slow phase velocity higher than downward slow phase velocity. Early in-flight, this asymmetry was inverted, and a symmetry of both responses was later observed. An upward shift in the vertical mean eye position in both OKN and VOR suggests that these effects may be related to otolith-dependent changes in eye position which, in themselves, affect slow phase eye velocity.

In this paper we review space flight experiments performed by our laboratory. Rhesus monkeys were tested before and after 12 days in orbit on COSMOS flights 2044 (1989) and 2229 (1992-1993). There was a long-lasting decrease in post-flight ocular counter-rolling (70%) and vergence (50%) during off-vertical axis rotation. In one animal, the orientation of optokinetic after-nystagmus shifted by 28 degrees from the spatial vertical towards the body vertical early post-flight. Otolith-ocular and perceptual responses were also studied in four astronauts on the 17-day Neurolab shuttle mission (STS-90) in 1998. Ocular counter-rolling was unchanged in response to 1-g and 0.5-g Gy centrifugation during and after flight and to post-flight static roll tilts relative to pre-flight values. Orientation of the optokinetic nystagmus eye velocity axis to gravito-inertial acceleration (GIA) during centrifugation was also unaltered by exposure to microgravity. Perceptual orientation to the GIA was maintained in-flight, and subjects did not report sensation of translation during constant velocity centrifugation. These studies suggest that percepts and ocular responses to tilt are determined by sensing the body vertical relative to the GIA. The findings also raise the possibility that 'artificial gravity' during the Neurolab flight counteracted adaptation of these otolith-ocular responses.