Studies done in an aircraft that produce brief periods of microgravity suggest that, in the absence of gravity, people rely more on body cues than vision to tell them which way is up. This study examined whether the same is true on the International Space Station (ISS). Scientists noted that the ISS can be "tricky" because it contains different modules that are not all in a straight line. An astronaut in one module who perceives the floor to be where their feet are may become disoriented when they float into another module. Crewmembers often go through a right angle to go between one module and another, therefore whatever corresponds to the ground in one module will not necessarily match in another module. Scientist believed that these issues may also affect astronauts during spacewalks. When the astronauts go outside of the spacecraft, they must adjust their orientation and use whatever visual cues they have. The specific objective of this study is to conduct experiments during long-duration microgravity conditions to better understand how humans first adapt to microgravity and then re-adapt to normal gravity conditions upon return to earth.
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This experiment involved comparisons of preflight, flight, and postflight perceptions and mental imagery, with special reference to space flight related decreases in the vertical component of perception. Seven crewmembers performed the test while ground-based controls were used for comparison. The test involved having subjects view a computer screen through a cylinder that blocks all other visual information. The subjects were presented with background images with different orientations relative to their bodies. On top of these images were a superimposed letter that could be either a “p” or a “d” depending on its orientation. Subjects indicated which letter they saw and the investigators measured the transition points where the letters change from a “p” to a “d” and back again. The researchers used this to determine the relative importance of visual and body cues to the subjects' perception of up.
A trend for a reduction in visual influence was observed inflight with lower-than-baseline levels maintained throughout six months in orbit. Visual influence was still lower than baseline levels several months after returning to Earth. The investigators conclude that sensory weightings are altered by long-term exposure to microgravity and do not recover within six months of return to normal gravity.