Pitch body orientation influences the perception of self-motion direction induced by optic flow

We studied the effect of static pitch body tilts on the perception of self-motion direction induced by a visual stimulus. Subjects were seated in front of a screen on which was projected a 3D cluster of moving dots visually simulating a forward motion of the observer with upward or downward directional biases (relative to a true earth horizontal direction). The subjects were tilted at various angles relative to gravity and were asked to estimate the direction of the perceived motion (nose-up, as during take-off or nose-down, as during landing). The data showed that body orientation proportionally affected the amount of error in the reported perceived direction (by 40% of body tilt magnitude in a range of ±20°) and these errors were systematically recorded in the direction of body tilt. As a consequence, a same visual stimulus was differently interpreted depending on body orientation. While the subjects were required to perform the task in a geocentric reference frame (i.e., relative to a gravity-related direction), they were obviously influenced by egocentric references. These results suggest that the perception of self-motion is not elaborated within an exclusive reference frame (either egocentric or geocentric) but rather results from the combined influence of both.     

Perception of longitudinal body axis in microgravity during parabolic flight

It has been proposed that an internal representation of body vertical has a prominent role in spatial orientation. This investigation investigated the ability of human subjects to accurately locate their longitudinal body axis (an imaginary straight body midline running from head to toes) while free-floating in microgravity. Subjects were tested in-flight, as well as on ground in normal gravity in both the upright and supine orientations to provide baseline measurements. The subjects wore a goggle device and were in total darkness. They used knobs to rotate two luminous lines until they were parallel to the subjective direction of their longitudinal body axis, in the roll (right/left) and the pitch (forward/backward) planes. Results showed that the error between the perceived and the objective direction of the longitudinal body axis was significantly larger in microgravity than in normal gravity. This error in this egocentric frame of reference is presumably due to the absence of somatosensory cues when free-floating. Mechanical pressure on the chest using an airbag reduced the error in perception of the longitudinal body axis in microgravity, thus improving spatial orientation.     

Grip forces exerted against stationary held objects during gravity changes

 In the present study, grip forces exerted against a stationary held object were recorded during parabolic flights. Such flight maneuvers induce changes of gravity with two periods of hypergravity, associated with a doubling of normal terrestrial gravity, and a 20 s period of microgravity. Accordingly, the object’s weight changed from being twice as heavy as normally experienced and weightless. Grip-force recordings demonstrated that force control was seriously disturbed only during the first experience of hyper- and microgravity, with the grip forces being exceedingly high and yielding irregular fluctuations. Thereafter, however, grip force traces were smooth, the force level was scaled to the object’s weight under normal and high-G conditions, and the grip force changed in parallel with the weight during the transitions between hyper- and microgravity. In addition, during weightlessness, when virtually no force was necessary to stabilize the object, a low force was established, which obviously represented a reasonable safety margin for preventing possible perturbations. Thus, all relevant aspects of grip-force control observed under normal gravity conditions were preserved during gravity changes induced by parabolic flights. Hence, grip-force control mechanisms were able to cope with hyper- and microgravity, either by incorporating relevant receptor signals, such as those originating from cutaneous mechanoreceptors, or by adequately including perceived gravity signals into control programs. However, the adaptation to the uncommon gravity conditions was not complete following the first experience; finer tuning of the control system to both hyper- and microgravity continued over the measurement interval, presumably with a longer observation period being necessary before a stable performance can be reached. Received: 23 April 1998 / Accepted: 20 December 1998     

Egocentric and allocentric alignment tasks are affected by otolith input

Gravicentric visual alignments become less precise when the head is roll-tilted relative to gravity, which is most likely due to decreasing otolith sensitivity. To align a luminous line with the perceived gravity vector (gravicentric task) or the perceived body-longitudinal axis (egocentric task), the roll orientation of the line on the retina and the torsional position of the eyes relative to the head must be integrated to obtain the line orientation relative to the head. Whether otolith input contributes to egocentric tasks and whether the modulation of variability is restricted to vision-dependent paradigms is unknown. In nine subjects we compared precision and accuracy of gravicentric and egocentric alignments in various roll positions (upright, 45°, and 75° right-ear down) using a luminous line (visual paradigm) in darkness. Trial-to-trial variability doubled for both egocentric and gravicentric alignments when roll-tilted. Two mechanisms might explain the roll-angle-dependent modulation in egocentric tasks: 1) Modulating variability in estimated ocular torsion, which reflects the roll-dependent precision of otolith signals, affects the precision of estimating the line orientation relative to the head; this hypothesis predicts that variability modulation is restricted to vision-dependent alignments. 2) Estimated body-longitudinal reflects the roll-dependent variability of perceived earth-vertical. Gravicentric cues are thereby integrated regardless of the task's reference frame. To test the two hypotheses the visual paradigm was repeated using a rod instead (haptic paradigm). As with the visual paradigm, precision significantly decreased with increasing head roll for both tasks. These findings propose that the CNS integrates input coded in a gravicentric frame to solve egocentric tasks. In analogy to gravicentric tasks, where trial-to-trial variability is mainly influenced by the properties of the otolith afferents, egocentric tasks may also integrate otolith input. Such a shared mechanism for both paradigms and frames of reference is supported by the significantly correlated trial-to-trial variabilities.     

Contribution of neck proprioception to subjective vertical perception among experts in physical activities and untrained women

The purpose of this study was to investigate the influence of physical training on subjective vertical perception with the different head positions in order to explore the involving of the neck proprioception. Visual field dependence was assessed using a rod and frame test on women practising judo or dance (international level) or no specific physical activity. Tests were performed with head upright or tilted head to disturb the Z-axis egocentric reference frame. A cluster analysis determined the distribution of visual field independent (VFI) and visual field dependent (VFD) participants. The first result showed no head position effect for the group of judoists (6 degrees +/-5 degrees ; 7 degrees +/-5 degrees ) and dancers (4 degrees +/-2 degrees ; 5 degrees +/-3 degrees ) but a significant effect for untrained participants (5 degrees +/-2 degrees ; 7 degrees +/-3 degrees ): their visual vertical perception was more disturbed with tilted head than with head upright. A variability exists among experts and would necessitate further analysis. The second result showed no influence of the head position for all VFD participants, whereas for VFI participants significant difference between upright and tilted head appeared both for experts (3 degrees +/-1 degrees ; 4 degrees +/-2 degrees ) and untrained participants (3 degrees +/-1 degrees ; 5 degrees +/-2 degrees ). In this research, whatever physical activity level, the VFI participants would mainly use the Z-axis reference frame and rely on proprioceptive information. VFD among experts and VFI among untrained participants suggest that proprioceptive reference frame of neck may not provide alone according the groups an appropriate postural control.     

Effects of spaceflight on ocular counterrolling and the spatial orientation of the vestibular system

We recorded the horizontal (yaw), vertical (pitch), and torsional (roll) eye movements of two rhesus monkeys with scierai search coils before and after the COSMOS Biosatellite 2229 Flight. The aim was to determine effects of adaptation to microgravity on the vestibulo-ocular reflex (VOR). The animals flew for 11 days. The first postflight tests were 22 h and 55 h after landing, and testing extended for 11 days after reentry. There were four significant effects of spaceflight on functions related to spatial orientation: (1) Compensatory ocular counterrolling (OCR) was reduced by about 70% for static and dynamic head tilts with regard to gravity. The reduction in OCR persisted in the two animals throughout postflight testing. (2) The gain of the torsional component of the angular VOR (roll VOR) was decreased by 15% and 50% in the two animals over the same period. (3) An up-down asymmetry of nystagmus, present in the two monkeys before flight was reduced after exposure to microgravity. (4) The spatial orientation of velocity storage was shifted in the one monkey that could be tested soon after flight. Before flight, the yaw axis eigenvector of optokinetic afternystagmus was close to gravity when the animal was upright or tilted. After flight, the yaw orientation vector was shifted toward the body yaw axis. By 7 days after recovery, it had reverted to a gravitational orientation. We postulate that spaceflight causes changes in the vestibular system which reflect adaptation of spatial orientation from a gravitational to a body frame of reference. These changes are likely to play a role in the postural, locomotor, and gaze instability demonstrated on reentry after spaceflight.     

Visually-induced tilt during parabolic flights

Summary A helmet-mounted visual display system was used to study visually induced sensations of self-motion (vection) about the roll, pitch and yaw axes under normal gravity condition (1g) and during the microgravity and hypergravity phases of parabolic flights aboard the NASA KC-135 aircraft. Under each gravity condition, the following parameters were investigated: (1) the subject's perceived body vertical with eyes closed and with eyes open gazing at a stationary random dot display; (2) the magnitude of sensations of body tilt with respect to the subjective vertical, while the subject viewed displays rotating about the roll, pitch and yaw axes; (3) the magnitude of vection; (4) latency of vection. All eleven subjects perceived a definite up and down orientation throughout the course of the flight. During the microgravity phase, the average magnitudes of perceived body tilt and self-motion increased significantly, and there was no significant difference in vection latency. These results show that there is a rapid onset of increased dependence on visual inputs for perception of self-orientation and self-motion in weightlessness, and a decreased dependence on otolithic and somatosensory graviceptive information. Anti-motion sickness drugs appear not to affect the parameters measured.     

Otolith signals contribute to inter-individual differences in the perception of gravity-centered space

The aim of the present study was to investigate (1) the relative contribution of the egocentric reference as well as body orientation perception to visual horizon percept during tilt or during increased gravito-inertial acceleration (GiA, hypergravity environment) conditions and (2) the role of vestibular signals in the inter-individual differences observed in these perceptual modalities. Perceptual estimates analysis showed that backward tilt induced (1) an elevation of the visual horizon, (2) an elevation of the egocentric estimation (visual straight ahead) and (3) an overestimation of body tilt. The increase in the magnitude of GiA induced (1) a lowering of the apparent horizon, (2) a lowering of the straight ahead and (3) a perception of backward tilt. Overall, visual horizon percept can be expressed as the combination of body orientation perception and egocentric estimation. When assessing otolith reactivity using off-vertical axis rotation (OVAR), only visual egocentric estimation was significantly correlated with horizontal OVAR performance. On the one hand, we found a correlation between a low modulation amplitude of the otolith responses and straight ahead accuracy when the head axis was tilted relative to gravity. On the other hand, the bias of otolith responses was significantly correlated with straight ahead accuracy when subjects were submitted to an increase in the GiA. Thus, straight ahead sense would be dependent to some extent to otolith function. These results are discussed in terms of the contribution of otolith inputs in the overall multimodal integration subtending spatial constancy.     

Human ocular torsion during parabolic flights: an analysis with scleral search coil

Summary Rotation of the eyes about the visual axis is known as ocular torsion. A lateral inclination (a roll) of the head induces ocular torsion in the opposite direction, a response known as ocular counterrolling. For six subjects, we recorded the static (head still) and dynamic (head in oscillatory roll motion) ocular torsion in normal 1 g condition and also during the microgravity and hypergravity periods of parabolic flight, using the electromagnetic scleral search coil technique. With the head still, the direction and magnitude of torsion that occured in response to microgravity and hypergravity differed substantially from one individual to another, but there was a significant difference in torsional magnitude between the microgravity and hypergravity periods, for all static head positions including the upright position. Under normal 1 g conditions, counterrolling compensated for about 16% of (voluntary) static head roll, while dynamic counterroll was much larger, up to 36% of head roll at 0.55 Hz. With increasing frequency of head oscillation between 0.33 Hz and 0.55 Hz, the gain of counter rolling increased and there was no change in the phase relationship. The gain of dynamic counterroll (in response to voluntary head rolling) was not significantly less in hypogravity, suggesting that on the ground at these frequencies the contribution of gravity and gravity receptors to this reflex is redundant: this reflex is probably driven by the semicircular canals. In some subjects, the torsional displacement in microgravity is accompanied by micro-torsional oscillatory motion.     

Accuracy of spatial localization depending on head posture in a perturbed gravitoinertial force field

Spatial orientation is crucial when subjects have to accurately reach memorized visual targets. In previous studies modified gravitoinertial force fields were used to affect the accuracy of pointing movements in complete darkness without visual feedback of the moving limb. Target mislocalization was put forward as one hypothesis to explain this decrease in accuracy of pointing movements. The aim of this study was to test this hypothesis by determining the accuracy of spatial localization of memorized visual targets in a perturbed gravitoinertial force field. As head orientation is involved in localization tasks and carrying relevant sensory systems (visual, vestibular and neck muscle proprioceptive), we also tested the effect of head posture on the accuracy of localization. Subjects (n=10) were seated off-axis on a rotating platform (120 degrees s(-1)) in complete darkness with the head fixed (head-fixed session) or free to move (head-free session). They were required to report verbally the egocentric spatial localization of visual memorized targets. They gave the perceived target location in direction (i.e. left or right) and in amplitude (in centimeters) relative to the direction they thought to be straight ahead. Results showed that the accuracy of visual localization decreased when subjects were exposed to inertial forces. Moreover, subjects localized the memorized visual targets more to the right than their actual position, that was in the direction of the inertial forces. With further analysis, it appeared that this shift of localization was concomitant with a shift of the visual straight ahead (VSA) in the opposite direction. Thus, the modified gravitoinertial force field led to a modification in the orientation of the egocentric reference frame. Furthermore, this shift of localization increased when the head was free to move while the head was tilted in roll toward the center of rotation of the platform and turned in yaw in the same direction. It is concluded that the orientation of the egocentric reference frame was influenced by the gravitoinertial vector.     

Judging beforehand the possibility of passing under obstacles without motion: the influence of egocentric and geocentric frames of reference

Previous studies have shown that the perception of the earth-based visual horizon, also named Gravity Referenced Eye Level (GREL), is modified by body tilt around a trans-ocular axis. Here, we investigated whether estimates of the elevation of a luminous horizontal line presented on a screen in otherwise darkness and estimates of the possibility of passing under are identically related to body tilt in absence of motion. Results showed that subjects overestimated the elevation of the projected line, whatever their body orientation. In the same way, subjects also overestimated their capacity of passing under the line. Both estimates appeared as a linear function of body tilt, that is, forward body tilt yielded increased overestimations, and backward body tilt yielded decreased overestimations. More strikingly, the linear effect of body tilt upon these estimates is comparable to that previously observed for direct GREL judgements. Overall, these data strongly suggest that the perception of the elevation of a visible obstacle and the perception of the ability of passing under in otherwise darkness shared common processes which are intimately linked to the GREL perception. The effect of body tilt upon these perceptions may illustrate an egocentric influence upon the semi-geocentric frame of reference required to perform the task. Possible interactions between egocentric and geocentric frames of reference are discussed.     

Moving weightless objects

When we move grasped objects, our grip force precisely anticipates gravitational and inertial loads. We analysed the control of grip forces during very substantial load changes induced by parabolic flights. During these flight manoeuvres, the gravity varies between hypergravity associated with a doubling of normal terrestrial gravity and a 20-s period of microgravity. Accordingly, the contribution of the object's weight to the load changed from being twice the normal value to being absent. Two subjects continuously performed vertical and horizontal movements of an object equipped with grip force and acceleration sensors. Whereas, during vertical movements performed under normal and hypergravity, a load force maximum occurred at the lower turning point and a minimum at the upper turning point, the load force pattern was completely changed under microgravity. In particular, the upper turning point was also associated with a load force maximum. Analysis of the grip forces produced by the two subjects revealed that the grip forces underwent the same characteristic changes as the load forces. Thus, subjects were able to adjust grip forces in anticipation of arm movement-induced fluctuations in load force under different and novel load conditions. Adaptation to changing levels of gravity was also obvious when the vertical and horizontal movements were compared: grip forces depended heavily on movement direction during normal and hypergravity but not during microgravity. The predictive coupling of grip force and load force was observed even during transitions between gravity levels, indicating rapid adaptation to changing load conditions. To account for the striking preservation of the normal characteristics of grip force control, we suggest that a highly automatized, extremely flexible sensorimotor mechanism firmly implemented within the central nervous system can cope with even massive changes in the environmental conditions.     

Egocentric and allocentric reference frames for catching a falling object

When programming movement, one must account for gravitational acceleration. This is particularly important when catching a falling object because the task requires a precise estimate of time-to-contact. Knowledge of gravity’s effects is intimately linked to our definition of ‘up’ and ‘down’. Both directions can be described in an allocentric reference frame, based on visual and/or gravitational cues, or in an egocentric reference frame in which the body axis is taken as vertical. To test which frame humans use to predict gravity’s effect, we asked participants to intercept virtual balls approaching from above or below with artificially controlled acceleration that could be congruent or not with gravity. To dissociate between these frames, subjects were seated upright (trunk parallel to gravity) or lying down (body axis orthogonal to the gravitational axis). We report data in line with the use of an allocentric reference frame and discuss its relevance depending on available gravity-related cues.     

Afferent input-associated reduction of muscle activity in microgravity environment

Responses of electromyogram (EMG) of soleus, lateral portion of gastrocnemius (LG) and tibialis anterior (TA), and both afferent and efferent neurograms at the L(5) segmental level of the spinal cord, to altered gravity levels created by the parabolic flight of a jet airplane were investigated in adult rats. The EMG activity in antigravity soleus muscle gradually increased when the gravity was elevated from 1-G to 1.5-G (+23%) and 2-G (+67%) during the ascending phase of parabolic flight. The activity decreased approximately 72% from the 1-G level immediately when the rat was exposed to microgravity. The EMG level was maintained low during the 20-s microgravity, but it was restored immediately once the gravity level was increased to 1.5-G and then 1-G during the descending and recovery phase. The EMG level of LG also increased gradually when the gravity level was elevated and the level then decreased when the rat was exposed to microgravity (P>0.05). However, the activity level during the 20-s microgravity was identical to that obtained at 1-G. The EMG level of TA even increased insignificantly in response to the exposure to microgravity. The responses of afferent neurogram were similar to those of soleus EMG, even though the magnitude of the reduction of integrated neurogram level in response to microgravity exposure was small (approximately 26% vs. 1-G level) relative to that of soleus EMG. The level of efferent neurogram was also decreased, but only approximately 9% vs. 1-G level, during the 20-s microgravity. The data in the current study suggest that the afferent input is closely associated with the gravity-dependent muscular activity.     

Evaluation of the vestibular evoked myogenic potential during parabolic flight in humans

The purpose of this study was to investigate how gravity affects the vestibular evoked myogenic potential (VEMP). Eight healthy subjects (seven men, one woman; age range 19–45 years) participated in experiments in which three different gravity levels [microgravity (MG), normal gravity (NG), and hypergravity (HG)] were imposed during a parabolic flight procedure. The VEMP was evoked in response to an intense mono-aural click while the subjects kept the sternocleidomastoid (SCM) muscle contracted bilaterally. Background electromyographic activity of the SCM during the test was corrected. The p13–n23 amplitude was significantly greater under MG than under NG or HG. There was no difference in p13 latency between the three gravity levels. Possible mechanisms related to this phenomenon are discussed.     

Perceptual reversal of bi-stable figures in microgravity and hypergravity during parabolic flight

This experiment investigated whether the perception of depth-reversible figures is altered when the observer is in microgravity or hypergravity. A set of five bi-stable ambiguous figures was presented to ten participants in 1 g, 0 g, and 1.8 g during parabolic flight. The figures included static images such as the Necker cube; kinetic depth displays such as a moving plaid and a sphere cluster of moving dots appearing to rotate in one of two directions; and a silhouette photograph. For each stimulus figure, subjects reported which of the two possible perceptual configurations they saw first and then continuously indicated when perceptual reversals occurred for durations ranging from 20 to 30 s. The same first percept was reported in 1 g, 0 g, and 1.8 g. The time delay for the first reversal between the two possible image interpretations was longer and the number of reversals was fewer in 0 g as compared to 1 g for four of the five figures. The opposite effects were seen when going from 0 g to 1.8 g. These findings confirm that, consistent with a multisensory approach to three-dimensional form perception, gravity has a clear effect on the interpretation of depth-based stimuli and this gravity-based component interferes with visual perception stability.     

Egocentric references and human spatial orientation in microgravity

This article describes the results of the ellipses experiment conducted during the second French-Soviet spaceflight (project Aragatz). The realization of oriented motor tasks, on the basis of internal body representation and without visual feedback, was chosen as a paradigm for studying the determinants of spatial orientation under weightlessness. The process of drawing ellipses in the air, using arm movements with axes parallel or perpendicular to the longitudinal body axis, was studied under normal gravity and in weightlessness, and recorded using a video computer motion-analyzing system (Kinesigraph). On Earth, the experiments were performed in standing and lying positions, and in flight, in the erect position with the feet fixed to the floor. In general, performance of the task in microgravity was not disturbed. Under conditions of spaceflight, the longitudinal ellipse was inclined forward in accordance with the inclination of the whole body relative to the fixed feet. On Earth, the angle between the long axes of longitudinal and transverse ellipses deviated from 90° by 20–30°. The same deviation persisted under microgravity conditions. The distinctive features of ellipses traced by individual subjects were also preserved. It is concluded that an egocentric reference system ensures normal performance of sensorimotor tasks in the absence of a gravitational reference.     

Object size effects on initial lifting forces under microgravity conditions

Individuals usually report for two objects of equal mass but different volume that the larger object feels lighter. This so-called size-weight illusion has been investigated for more than a century. The illusion is accompanied by increased forces, used to lift the larger object, resulting in a higher initial lifting speed and acceleration. The illusion holds when subjects know that the mass of the two objects is equal and it is likely that this also counts for the enlarged initial effort in lifting a larger box. Why should this happen? Under microgravity, subjects might be able to eliminate largely the weight-related component of the lifting force. Then, if persistent upward scaling of the weight-related force component had been the main cause of the elevated initial lifting force under normal gravity, this elevated force might disappear under microgravity. On the other hand, the elevated initial lifting effort in the large box would be preserved if it had been caused mainly by a persistent upward scaling of the force component, necessary to accelerate the object. To test whether the elevated initial lifting effort either persists or disappears under microgravity, a lifting experiment was carried out during brief periods of microgravity in parabolic flights. Subjects performed whole-body lifting movements with their feet strapped to the floor of the aircraft, using two 8-kg boxes of different volume. The subjects were aware of the equality of the box masses. The peak lifting forces declined almost instantaneously with approx. a factor 9 in the first lifting movements under microgravity compared with normal gravity, suggesting a rapid adaptation to the loss of weight. Though the overall speed of the lifting movement decreased under microgravity, the mean initial acceleration of the box over the first 200 ms of the lifting movement remained higher (P=0.030) in the large box (1.87±0.127 m/s²) compared with the small box (1.47±0.122 m/s²). Under normal gravity these accelerations were 3.30±0.159 m/s² and 2.67±0.159 m/s², respectively (P=0.008). A comparable trend was found in the initial lifting forces, being significant in the pooled gravity conditions (P=0.036) but not in separate tests on the normal gravity (P=0.109) and microgravity (P=0.169) condition. It is concluded that the elevated initial lifting effort with larger objects holds during short-term exposure to microgravity. This suggests that upward scaling of the force component, required to accelerate the larger box, is an important factor in the elevated initial lifting effort (and the associated size-weight illusion) under normal gravity.     

Changes in cerebral oxygenation during parabolic flight

Assessing changes in brain activity under extreme conditions like weightlessness is a desirable, but difficult undertaking. Results from previous studies report specific changes in brain activity connected to an increase or decrease in gravity forces. Nevertheless, so far it remains unclear (1) whether this is connected to a redistribution of blood volume during micro- or hypergravity and (2) whether this redistribution might account for neurocognitive alterations. This study aimed to display changes in brain oxygenation caused by altered gravity conditions during parabolic flight. It was hypothesized that an increase in gravity would be accompanied by a decrease in brain oxygenation, whereas microgravity would lead to an increase in brain oxygenation. Oxygenized and deoxygenized haemoglobin were measured using two near infrared spectroscopy (NIRS) probes on the left and right prefrontal cortex throughout ten parabolas in nine subjects. Results show a decrease of 1.44 μmol/l in oxygenized haemoglobin with the onset of hypergravity, followed by a considerable increase during microgravity (up to 5.34 μmol/l). In contrast, deoxygenized haemoglobin was not altered during the first but only during the second hypergravity phase and showed only minor changes during microgravity. Changes in oxygenized and deoxygenized haemoglobin indicate an increase in arterial flow to the brain and a decrease in venous outflow during microgravity.     

Mass-discrimination in weightlessness and readaptation to earth's gravity

Summary Five members of the first Spacelab mission (STS-9) were tested on several occasions for weight-discrimination before and after the flight, and for mass-discrimination under microgravity in flight. Thresholds for mass-discrimination were higher than for preflight weight-discrimination by a factor of about 1.8, and there was no clear evidence of improvement throughout the ten day mission. Too few tests were conducted to monitor the improvement during the first two days of flight, when adaptation to weightlessness may have occurred. Subjects reported perceptual aftereffects of body heaviness for two or three days after the flight. Their weight-discrimination thresholds were raised during this period, when they were re-adapting to normal gravity. Incomplete adaptation to altered arm weight can only partly explain the raised threshold for mass- discrimination in microgravity. Differences in the sensory information available with and without gravity are discussed.