Skip to page content Mission Information


Visual-Otolithic Interactions in Microgravity (9301126)
Principal Investigator
Research Area:
Species Studied
Scientific Name: Homo sapiens Species: Human

On Earth, when a ball drops it accelerates in a straight line toward the ground. This is an example of a linear acceleration, and these kinds of accelerations are very common in everyday life. On the Earth's surface, two major sources of linear acceleration exist. One is related to the Earth's gravity. Gravity significantly affects most of our movement (motor) behavior (it has been estimated that about 60% of our musculature is devoted to opposing gravity), and it provides a constant reference for up and down. It is present under all conditions on Earth, and it forms one of the major pillars of spatial orientation. Other sources of linear acceleration arise in the side-to-side, up-and-down, or front to- back translations that commonly occur during walking or running, and from the centrifugal force that we feel when going around turns or corners. As Einstein noted, all linear acceleration is equivalent, whether it is produced by gravity or motion; and when we are in motion, the linear accelerations sum. The body responds to the resultant, and we tend to align our long body axis with the resultant linear acceleration vector, called the gravitoinertial acceleration (GIA) vector. For example, when a person is either walking or running around a turn, the inward linear acceleration is added to the upward gravitational acceleration to form a GIA that is tilted in toward the center of the turn. Unconsciously, the head, body, and eyes are oriented so that they tend to align with the GIA. The angle of tilt of these body parts depends on the speed of turning. Simply put, people align with gravity when standing upright and tilt into the direction of the turn when in motion. If they don't, they lose balance and fall. Perhaps the most graphic illustration of the importance of body tilt toward the resultant linear acceleration vector is provided by motorcycle drivers, who tilt their machines 30-45 degrees into the direction of the turn to maintain their balance. People do the same when walking or running around a curve.

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.

++ -- View more

Bellossi F, Clement G, Cohen B, Cork M. EDEN: a payload dedicated to neurovestibular research for Neurolab. Acta Astronautica. 1998. Jan-Apr; 42(1-8): 59-67. []

Clement G, Berthoz A, Cohen B, Moore S, Curthoys I, Dai M, Koizuka I, Kubo T, Raphan T. Perception of the spatial vertical during centrifugation and static tilt. In: Buckey J Jr., Homick J, eds. The Neurolab spacelab mission: Neuroscience research in space. Houston, TX: National Aeronautics and Space Administration; 2003:5-10. NASA Special Publication SP-2003-535. [NTRS]

Clement G, Moore ST, Raphan T, Cohen B. Perception of tilt (somatogravic illusion) in response to sustained linear acceleration during space flight. Experimental Brain Research. 2001. June; 138(4): 410-8. []

Cohen B, Clement G, Moore S, Curthoys I, Dai M, Koizuka I, Kubo T, Raphan T. Adaptation to linear acceleration in Space (ATLAS) experiments: equipment and hardware. In: Buckey J Jr., Homick J, eds. The Neurolab spacelab mission: Neuroscience research in space. Houston, TX: National Aeronautics and Space Administration; 2003:279-283. NASA Special Publication SP-2003-535. [NTRS]

Highstein SM, Cohen B. Neurolab mission. Curr Opin Neurobiol. 1999 Aug; 9(4): 495-9. []

Moore S, Clement G, Raphan T, Curthoys I, Koizuka I, Cohen B. The human response to artificial gravity in a weightless environment: results from the Neurolab centrifugation experiments. El-Genk ed. Conference Proceedings (CP504) of the Space and Applications International Forum-2000, Albuquerque, New Mexico, Jan 30-Feb 3 2000. American Institute of Physics; 2000.

Moore S, Diedrich A, Biaggioni I, Kaufmann H, Raphan T, Cohen B. Artificial gravity: a possible countermeasure for post-flight orthostatic intolerance. Acta Astronautica. 2005. May-Jun;56(9-12):867-76.[]

Moore ST, Clement G, Dai M, Raphan T, Solomon D, Cohen B. Ocular and perceptual responses to linear acceleration in microgravity: alterations in otolith function on the COSMOS and Neurolab flights. J Vestib Res. 2003;13(4-6):377-93. []

Adaptation, physiological
Gravity perception
++ -- View more

Photo Gallery
+ View digital images

Data Information
Data Availability
Archive is complete. Data sets are not publicly available but can be requested.
Data Sets+ Request data

Perceived pitch tilit (degrees)
Perceived roll tilt (degrees)
Perceived tilt angle (degrees)

Mission/Study Information
Mission Launch/Start Date Landing/End Date Duration
STS-90 04/17/1998 05/03/1998 16 days

Additional Information
Managing NASA Center
Johnson Space Center (JSC)
Responsible NASA Representative
Johnson Space Center LSDA Office
Project Manager: Pamela A. Bieri
Institutional Support
National Aeronautics and Space Administration (NASA)
Alternate Experiment Name
Body Rotation Device (BRD)
Adaptation to Linear Acceleration in Space (ATLAS)
ESA Developed Elements for Neurolab (EDEN)
Visual and Vestibular Investigation System (VVIS)
Proposal Date
Proposal Source