The effect of lunar gravity on perception: Ambient visual cues have less effect on orientation judgements than they do under normal gravity

Abstract

INTRODUCTION

We previously reported that during the brief periods of near-zero microgravity of parabolic flight there was a significant reduction in the influence of a tilted background visual scene on the perceived orientation of a foreground object (Dyde et al., 2006, http://journalofvision.org/6/6/183/). This was unexpected, as removing the gravity cue should result in a relative increase of the influence of the remaining cues. Is there something special about zero gravity, or would just reducing gravity have a similar effect?

METHODS

Parabolic flights were used to create periods of lunar (one sixth of earth's) gravity. Six subjects experienced an average of 38 parabolas, i.e. approximately 13 minutes of reduced gravity each. Control data were collected during periods of 1g level flight and during the 2g phases of parabolic flight. Perceived orientation was measured using the OCHART protocol (Dyde et al., 2006, Exp. Brain Res. 173: 612) in which a tilted character is identified as either a ‘p’ or ‘d’ to find its orientation of maximum ambiguity from which the perceptual upright (PU) is calculated. The effect of a visual background on the orientation of PU was measured using two backgrounds tilted +/− 112.5°. The difference in the orientation of PU between the two backgrounds is defined as the “visual effect.”

RESULTS

The visual effect was significantly smaller during lunar gravity than during level flight. The visual effect was also smaller during the 2g phases of parabolic flight than during level flight.

DISCUSSION

Modelling these data in terms of a vector sum of gravity, visual and body directions, suggests a decrease in the weighting accorded to ambient visual cues when making orientation judgements. Such a reduction is not predicted by cue combination theory.

Footnotes

 Supported by the Canadian Space Agency and grants from the Natural Sciences and Engineering Research Council of Canada to L.R. Harris and M.R. Jenkin.