Acta Astronautica 63 (2008) 828–832www.elsevier.com/locate/actaastro

Perception of depth inmicrogravity during parabolic flight

Gilles Clémenta,∗, Corinna Lathanb, Anna LockerdbaCentre de Recherche Cerveau et Cognition, UMR 5549 CNRS-UPS, Faculté de Médecine de Rangueil, Toulouse F-31062, France

bAnthroTronix Inc., Silver Spring, MD 20910, USA

Received 15 May 2007; received in revised form 19 December 2007; accepted 6 January 2008Available online 15 February 2008

Abstract

This investigation examined depth and distance perception of 3-D objects presented in stereoscopic vision. The results obtainedin normal gravity and microgravity in parabolic flight were compared. This experiment was performed during an ESA campaignof parabolic flight on the Airbus A300 Zero-G on six free-floating subjects. The subjects were first presented with a 3-D cubeand were instructed to adjust one dimension (width, length, or height) of the cube so that all three dimensions had the sameperceived length. The subjects perceived a normal cube to be taller, thinner, and shallower in microgravity compared to normalgravity. Consequently, when they adjusted its dimension so that it looked “normal” to them, they made it shorter, wider, anddeeper. A simple distance perception test confirmed that the subjects underestimated the distance of objects in the depth plane inmicrogravity. These results confirm a role of the gravitational reference in the 3-D perception of the objects and the environment.© 2008 Elsevier Ltd. All rights reserved.

Keywords: Visual perception; Depth; Distance; Microgravity-induced physiological effects; Weightlessness

1. Introduction

Depth perception is the ability to perceive the worldin three dimensions by evaluating the distance straightahead of the viewer’s eye, toward or into an object orsurface. Depth perception combines binocular cues,such as stereopsis, accommodation and convergence, aswell as monocular cues, such as motion parallax,changes in color, contrast or texture, and atmosphericand linear (also called “geometrical”) perspective.Under normal conditions, these different depth cues leadto a single, coherent, three-dimensional (3-D) percep-tion of our environment [1]. However, binocular cuesare less useful when objects are more than a few metersaway because the images on the retina become more

∗Corresponding author. Tel.: +33562173779.E-mail address: gilles.clement@cerco.ups-tlse.fr (G. Clément).

0094-5765/$ - see front matter © 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.actaastro.2008.01.002

identical the farther the objects are from the eyes.Motion parallax is useful only when the observer is inmotion. In the absence of atmosphere and with differ-ent lighting conditions affecting color and contrast, asin space flight, linear perspective is the most reliableof cues for depth perception.

Linear perspective is related to the fact that par-allel lines converge with increasing distance, suchas roads, railway lines, and electric wires. Similarly,when objects of known distance subtend a smaller andsmaller angle, they are interpreted as being furtheraway. This is because objects of the same size but invarying distances cast different retinal image sizes.This size-constancy rule gives us indirect cues aboutthe distance of objects of known absolute or relativesizes. Many geometric visual illusions, such as Ponzo,Müller–Lyer, and the reversed-T, occur because of theserules [1].

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Previous studies by our group indicated that thestrength of these geometric illusions was decreased dur-ing head tilt relative to gravity [2] or during short-termmicrogravity [3], suggesting that the gravitational ref-erence played a role in 3-D perception. Because mostof these geometric illusions are related to perspectivedepth cues, we hypothesized that 3-D perception wasaltered in microgravity. To further address this hypoth-esis, this experiment was designed to investigate visualperception of 3-D objects in microgravity.

2. Methods

This experiment was performed during the 44thcampaign of parabolic flights of the European SpaceAgency onboard the Airbus A300 Zero-G in Bordeaux(France) in October 2006. This aircraft is capable offlying parabolic trajectories during which the entirecabin is in microgravity (less than 10−2 g) for about20s. Six subjects (4 women, 2 men, aged 27–50y)participated in this experiment. Informed consent wasobtained from all subjects. The ethics board of both theEuropean Space Agency and Hôpital Cochin in Paris(CCPPRB) approved this study. All flight participantspassed the equivalent of a US Air Force Class III med-ical examination, and reported that they had normal orcorrected-to-normal vision with no known vestibulardeficits.

During the microgravity phase of parabolic flight, thesubjects maintained a seated-like posture and were posi-tioned in mid-air by an operator to eliminate orientation-related tactile cues. The test area was surrounded by anet to allow the test subjects to free-float without therisk of bumping into obstacles (for a schematic of thefree-floating procedure, see [3]). Testing began when astable free-floating position was achieved, which usu-ally occurred approximately 2–3s into the parabola.

During the tests, the free-floating subjects wore acommercial off-the-shelf virtual reality goggle (Z8003DVisor, from eMagin Corporation, Hopewell Junction,NY, USA) connected to a laptop PC. These gogglesdisplayed separate figures to each eye, thus allowingstereoscopic vision, in an otherwise dark environment.These interactive figures subtended a viewing angle ofabout 25.

In the first experiment, subjects were initially pre-sented with a 3-D Necker’s cube with one dimension(height, width, or depth) clearly smaller or larger thanthe other two (Fig. 1). The subjects were instructed toadjust the length of that particular dimension by meansof a finger trackball until all three dimensions had thesame perceived length, i.e., the cube looked “normal”.

Fig. 1. Experiment 1. The top drawing represents a normal 3-D cubewith perspective lines converging to a single vanishing point locatedright and above. Bottom row: the subjects were initially presentedwith cubes with smaller or larger dimensions than normal, whichthey could modify in width, length, or depth by means of a trackball.

The test was repeated six times for each dimension.The dimensions to be adjusted were presented in ran-dom order. For each trial, the size differential betweenthe adjusted dimension and the other cube dimensionswas calculated. Size differential is expressed in percent-age as: (adjusted dimension–actual dimension)/actualdimension ∗100. A t-test was used for evaluating thesignificance of the difference in this measurement be-tween normal gravity and microgravity.

In the second experiment, subjects were presentedwith three blocks. Two blocks appeared in the frontalplane (perpendicular to the subject’s line of sight) atthe same distance, whereas the third block appearedto be farther away from the others because of a per-spective effect (Fig. 2). Subjects were instructed toadjust the spacing between blocks 1 and 3 so that itwas equivalent to the distance between blocks 1 and 2.Each test was repeated six times in random order. Foreach trial the distance differential between blocks 1–3and blocks 1–2 was calculated for each figure. Distancedifferential is expressed in percentage as: (adjusteddistance–actual distance)/actual distance ∗100. A t-testwas used for evaluating the significance of the differ-ence in this measurement between normal gravity andmicrogravity.

All displayed figures obeyed to the rule of a one-pointperspective, with a vanishing point on the horizon whereall the receding lines converged (Fig. 1). Horizontaledges were shown as horizontal lines perpendicular tothe line of sight and parallel to the ground line. Verticaledges were shown as vertical lines.

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Fig. 2. Experiment 2. Three blocks were presented. The subjectshad to move block 3 with a trackball, so that the distance betweenblocks 1 and 3 was equivalent to the distance between blocks 1 and2. Distance adjustments were performed in the horizontal (Test A)or vertical (Test B) direction, or in depth (Test C).

Three out of the six subjects tested in this experimenthad a previous experience of parabolic flight. Five sub-jects took prophylactic anti-motion sickness medica-tion (a combination of scopolamine and amphetamine)before boarding the plane. None of the test subjectsmanifested symptoms of motion sickness while per-forming the experiment. All subjects were also testedin normal gravity on board the plane when it wasflying straight and level. Consequently, the differencebetween the measurements obtained in normal gravityand microgravity cannot be attributed to the effects ofthe anti-motion sickness medication.

3. Results

3.1. Experiment 1

In normal gravity, the subjects were able to adjust theheight and the depth of the cube so that both matchedthe other dimensions with an error of less than 0.5%(Table 1). However, on average, the subjects made acube with a width that was smaller (−3.92%) than thelength of the other edges. In microgravity, once adjustedby the subjects, the cube’s width matched quite well thelength of the other edges (0.56%), but its height wassmaller (−3.22%), and its depth was longer (3.95%)than the other edges. Consequently, the difference be-tween normal gravity and microgravity indicates that a“normal” cube seen in microgravity looks about 3.5%taller, 4.5% thinner, and 3.5% less deep than whenthat same “normal” cube is displayed in normal grav-ity. These differences, however, were only significantfor the width and depth dimensions of the cube. Thesechanges in the apparent dimensions of the cube suggestthat subjects interpret the vanishing point given by theperspective cues as being lower in microgravity com-pared to normal gravity (Fig. 3).

Table 1Results of the tests illustrated on the bottom row of Fig. 1

Width Height Depth

1G −3.92 ± 5.36 0.43 ± 5.12 0.47 ± 4.230G 0.56 ± 5.61 −3.22 ± 5.86 3.95 ± 5.55

Difference 4.47 ± 2.33 −3.65 ± 5.68 3.48 ± 3.55p< 0.05 p = 0.07 p< 0.05

Values represent the size differential (in percentage) between theadjusted and the normal length of the width, height, and depth ofa 3-D cube (mean of six trials in six subjects ±SD) in normalgravity and in microgravity, and computed difference (0G–1G) andsignificance between the two gravity conditions.

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Fig. 3. The changes in the apparent dimensions of a 3-D cube inmicrogravity (Adjusted), with a smaller height, larger width, andlonger depth than normal (Normal), suggest that the subjects placedin microgravity erroneously interpret the vertical position of thevanishing point given by the perspective cues.

Table 2Results of distance tests A, B, and C illustrated in Fig. 2

Test A Test B Test C

1G −23.0 ± 15.7 −21.1 ± 15.4 4.1 ± 24.40G −41.2 ± 21.4 −29.0 ± 12.1 −7.7 ± 27.3

Difference −18.2 ± 22.3 −7.9 ± 11.2 −11.8 ± 29.2p< 0.05 p< 0.05 p = 0.105

Values represent the distance differential (in percentage) betweenblocks 1 and 3 with respect to the reference distance between blocks1 and 2. Mean of six trials in six subjects ±SD in normal gravity(1G) and in microgravity (0G), and computed difference (0G–1G)and significance between the two gravity conditions.

3.2. Experiment 2

In normal gravity, when subjects spaced the twoblocks in the picture (frontal) plane horizontally (TestA) or vertically (Test B), so that the distance betweenthem was equivalent to the apparent distance of theblock seen in perspective (see Fig. 2), they placed themat a shorter distance (−21.1% and −23%, respectively)(Table 2). When they adjusted the distance of the blockseen in the depth plane so that it corresponded to theperceived distance between the other blocks (Test C)they placed it farther away (4.1%). For all three tests,the perceived distance of the block in depth was re-duced by 7.9–18.2% in microgravity relative to normalgravity. This decrease was only significant for the ad-justment in the horizontal (Test A) and vertical (Test B)directions.

4. Conclusion

These results support the hypothesis that visual per-ception of 3-D objects is altered during short-term

microgravity. A normal cube looks taller, thinner,and shallower in microgravity than in normal gravity.The distance of an object in the depth plane is alsounderestimated.

The tests used in these two experiments eliminatedthe contribution of monocular depth cues such as con-trast, color, texture, motion parallax, and atmosphericperspective. However, 3-D cues were provided bybinocular disparity and the use of linear perspective, in-cluding receding points converging toward a vanishingpoint. However, the observed changes in 3-D percep-tion were presumably not due to an adaptation in theconvergence or accommodation responses, because thefigures were actually in 2-D and the exposure to micro-gravity was too short to change properties of the extra-or intra-ocular muscles. Rather, the observed changespoint at an alteration of the use of linear perspectivecues for 3-D perception in absence of a gravitationalreference.

Each of our eyes, like a camera lens, sees the world in2-D. Like the artists who employ subtle exaggerationsand enhancements of classic perspective to suggestdepth perception, the central nervous system operatesby exaggerating and enhancing perspective in generat-ing a single new 3-D image out of the two 2-D imagesit receives. The fact that the perspective features of a2-D figure provide cues for apparent distance judg-ment is demonstrated by the occurrence of geometricillusions such as the reversed-T, Ponzo, andMüller–Lyerillusions. These illusions are due to a misjudgmentof size produced by perspective cues of depth [1]. Inrecent experiments, we have demonstrated that thestrength of these geometric illusions based on perspec-tive decreased in subjects with the head and body tiltedrelative to the gravitational reference [2] and when free-floating in microgravity [3]. The results of the presentexperiments confirm these previous observations andsuggest that perspective depth cues are less salient inmicrogravity.

Why is the perspective effect altered in micrograv-ity? Linear or geometric perspective obeys well-definedrules. For example, perspective lines typically con-verge at a vanishing point on an imaginary horizontalline at eye level corresponding to the straight-aheaddirection. In the absence of a gravitational reference,such as in microgravity, it is more difficult to define ahorizontal line [4]. Also, previous studies have shownsignificant deviations in the vertical position of the eyein microgravity due to the stimulation of the otolith or-gans by changes in the amplitude of the gravito-inertialforces [5], which could alter the apparent directionof the “straight ahead”. In our first experiment, if the

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subjects in microgravity perceived the straight-aheaddirection, hence the vanishing point, as being lowerthan in normal gravity, then this would explain whythey adjusted the 3-D cube with a larger width, smallerheight and a longer depth than normal when placed inmicrogravity (Fig. 3).

Although space research has not demonstrated con-sistent deficits in 3-D perception [6–8], many astronautshave reported that they tend to underestimate the dis-tance and size of objects [9]. This was particularly evi-dent on the lunar surface [10], as often occurs on Earthin the clear air of the desert. An overall distortion ofperspective, similar to the persistent “fish-eye” effectreported by divers [11], has also been reported by crewmembers returning to Earth after long-duration flights[12]. It is unclear if these illusions are direct effects ofreduced gravity or due to other factors of the space en-vironment, such as high contrast effects, confinement tocramped quarters, and the absence of known landmarks,such as trees, vehicles, buildings, in the crew member’sintermediate space. Nevertheless, these errors in visualperception and misperceptions of size, distance, andshape represent potentially serious operational prob-lems. For example if a crew member does not accuratelygauge the distance of a target, such as a docking portor an approaching vehicle, then the speed of this targetcan be misevaluated. In fact, it is believed that such anerror was a contributing cause to the collision betweena Russian Progress supply spacecraft1 and the RussianMir space station in June 1997 [14]. Also, disturbancesin the mental representation of objects and the surroundmay influence the ability of astronauts to accurately per-form perceptual-motor and perceptual-cognitive taskssuch as those involved in robotic control.

A series of experiments has recently been de-signed to allow further identification of 3-D perceptionduring long-duration spaceflight [15]. This NASA/ESA-funded research effort includes motor tests com-plemented by psychophysics measurements, designedto investigate the mental representation of spatialcues and distinguish the effects of cognitive versusperceptual-motor changes due to microgravity expo-sure. Identifying lasting abnormalities in the perceptionof distance will establish the scientific and technicalfoundation for development of preflight and in-flighttraining and rehabilitative schemes, enhancing astronaut

1 Because the Mir radar was turned off and the Progress was notvisible out of the Mir’s windows for laser range measurements atappropriate times, the Mir commander’s sole source of range rateinformation was the changing angular size and position of Mir ona TV monitor from the vantage point of the Progress vehicle [13].

performance of perceptual-motor and perceptual-cognitive tasks.

Acknowledgments

The authors would like to thank the people from ESA,Novespace, CEV, and MEDES who helped accommo-dating these experiments in the Airbus A300 Zero-G,and the flight participants who served as test subjects.This project was supported by ESA, CNES, and CNRS.

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