Because of the importance of vision to the conduct of space missions, it was suggested in the early literature that the 0-g environment could alter visual capabilities (Cooper, 1963; O'Lone, 1965; Berry, 1970). During the initial phases of the space program, astronauts demonstrated seemingly high capability to distinguish terrestrial reference points such as rivers, groups of people, and vehicles.
 Such observations fueled the suspicion that O g might actually enhance perception (White, 1965a).
American studies of visual functioning in space have failed to demonstrate any significant changes in visual perception as a result of 0-g. During the Gemini program, Duntley, Austin, Harris, and Taylor (1968) tested astronauts' acuity during preflight, in flight, and postflight, using devices calibrated to the sensitivity of a clinical wall chart test. No differences were observed in any phase. In addition, visual acuity of astronauts under ordinary viewing conditions from their capsule window was tested. Rectangular patterns were constructed in large fields in Laredo, Texas, and in Carnarvon, Australia, and the astronauts were asked to identify patterns arranged in a 4 X 3 matrix. Again, visual acuity in space paralleled that of 1-g, and no changes were observed across days of testing.
Observations taken aboard early Soviet spacecraft suggested that 0-g, rather than enhancing perception, might result in visual losses, at least during the early days of a mission. Soviet investigators assessed the ability of cosmonauts to judge the direction and number of certain focusing patterns of dashed lines under standard conditions. Decrements for all crewmembers were reported, with losses averaging 20%. Reductions in color perception during spaceflight have also been reported. Measurements made on Vostok 2 and Soyuz 9 indicated a 25% diminution of color intensity, with losses particularly marked with purple, light blue, and green (Popov and Boyko, 1967). Contrast sensitivity decrements of as much as 40% have also been reported. In spite of these changes, Soviet investigators have expressed the view that vision in weightlessness is reliable and presents no obstacle to the conduct of a space mission (Nicogossian and Parker, 1982). The recent Soviet French flight aboard the orbiting station Salyut 7 explored the possibility that the role of vision in human spatial orientation and muscular control is altered in 0 g (Pishchik, 1982). Data from these experiments are still under analysis and no conclusions can be drawn at present.
There are factors other than weightlessness which could influence visual functioning in space. High contrast effects and the requirement to adapt to rapidly changing brightness levels (as in rapid Earth orbits) render space a special visual environment. Another factor is the presence of high energy, high atomic number particle radiation. Variable form, low level light flashes have been reported by many astronauts following the initial sighting by one of the astronauts aboard Apollo 11. These lights were perceived as  "spot or starlike flashes" in 66% of the cases reported, as a "streak, sharp or dashed line" in 25% of the sightings, and as "clouds" in 8% of the reports (Pinsky, Osborne, Bailey, Benson, and Thompson, 1974). Such sightings occurred during periods of darkness in the spacecraft. They reportedly occurred in one eye at a time and even with both eyes closed. It is believed that these perceptions are generated by particle radiation traversing the head or eyes and triggering a retinal response. These findings highlight the question of duration effects in space. Although such light flashes are of relatively little consequence during short term missions, we do not know what their significance to the visual system might be during longer exposures.
Appreciable vibration can be another deterrent to the optimal functioning of the visual system in space. At least in some cases, vibration can be sufficiently intense to interfere with visual tasks, such as reading instruments (Grether, 1971a). Even though vibration effects may last only a few minutes, they can pose problems for crew performance of critical tasks, such as those associated with the liftoff. On the return flight, vibration during reentry and recovery apparently has not been a problem. However, short lived oscillation combined with deceleration pulse may be a problem of renewed interest with respect to winged aerospace vehicles of the Shuttle type. Here the aircraft is flown to ground by a human pilot following a ballistic reentry, and the effects of initial vibration could influence the successful performance of landing tasks.
Failure to find consistent perceptual changes in the weightless environment suggests that visual changes in space, if they exist, are not pronounced. However, further data, especially those emphasizing brightness and color sensitivity, dark adaptation, and accommodation, along with assessments of radiation and vibration effects, would help complete the picture of visual perception in the space environment.