The Perception of Upright under Lunar Gravity

Maintaining an upright posture on the moon is not easy. NASA documents abound with examples of astronauts falling on the moon during the visits between 1969 and 1972 (19,20). Even on the most recent moon visit (Apollo 17, 1972), Astronaut Harrison Schmidt fell over as he worked on the lunar surface.1 However, although regarded as highly desirable (7), few studies have investigated the perceptual consequences of lunar gravity. Clark et al. (3) modeled expected changes in orientation perception during landing on the moon. Paloski et al. (23) and Johnston et al. (14) provide overviews of the perceptual and physiological risks associated with lunar and other environments likely to be encountered during space exploration. But, although there are anecdotal reports of perceptual and balance disturbances in lunar gravity, few controlled experiments have been conducted to explore the consequences of lunar gravity on human self-orientation perception. Postural stability and many aspects of perception require an accurate representation of upright which can be used as a reference with which to align the body for maximum stability (12). The perception of upright is determined by a combination of the internal representation of the body, visual orienting cues (such as the horizon) and the direction of gravity. The direction of gravity is carried within the otolithic division of the vestibular system and also by somatosensory cues (see 16 for a review). All these cues to upright appear to add linearly as a weighted vector sum to provide a “best estimate” of upright (5). Errors in the representation of upright can lead to postural instability (11). The perceived upright could be altered on the moon if lunar gravity was below the threshold needed for it to exert an influence on the perceived upright or if the visual cue, whether or not gravity still played a direct role, was assigned a changed weighting in response to the diminished gravity cue. For whole-body linear acceleration, the vestibular threshold is around 0.1 m.s -2 (although studies have reported values ranging from 0.014 to 0.25 m.s -2 (8) and so the lunar value of 1.6 m.s -2 is well above threshold. This is compatible with Homick and Miller’s conclusion (10) that lunar gravity is an adequate stimulus for the otolith organs to define a gravitationTHE PERCEPTION OF UPRIGHT UNDER LUNAR GRAVITY