Does gravity play an essential role in the asymmetrical visual perception of vertical and horizontal line length?

The eye perceives the length of vertical and horizontal lines with an inher ent asymmetry. A vertical line having the same length as a horizontal one i s usually perceived to be longer. In this experimental investigation we tes ted the hypothesis that gravity has a direct role in producing the observed perceptual asymmetry. To this end we performed experiments in weightlessne ss during long orbital space flights onboard the MIR station. Subjects perf ormed a psychophysical task in which the length of a visually-presented ver tical line was adjusted to match the length of a horizontal reference. On E arth, almost all subjects produce errors in adjusting the length of the ver tical line, consistently under-estimating the length of the horizontal refe rence. The asymmetry of perception of the line lengths persisted in weightl essness. From these results we conclude that the phenomena of asymmetry of perception of the lengths of vertical and horizontal lines is not dependent on gravity, but is instead defined by proprieties of the system of interna l representation. (C) 2001 Published by Elsevier Science Ltd.