Effects of varying gravity levels in parabolic flight on the size-mass illusion.

When an observer lifts two objects with the same weight but different sizes, the smaller object is consistently reported to feel heavier than the larger object even after repeated trials. Here we explored the effect of reduced and increased gravity on this perceptual size-mass illusion. Experiments were performed on board the CNES Airbus A300 Zero-G during parabolic flights eliciting repeated exposures to short periods of zero g, 0.16 g, 0.38 g, one g, and 1.8 g. Subjects were asked to assess perceived heaviness by actively oscillating objects with various sizes and masses. The results showed that a perceptual size-mass illusion was clearly present at all gravity levels. During the oscillations, the peak arm acceleration varied as a function of the gravity level, irrespective of the mass and size of the objects. In other words we did not observe a sensorimotor size-mass illusion. These findings confirm dissociation between the sensorimotor and perceptual systems for determining object mass. In addition, they suggest that astronauts on the Moon or Mars with the eyes closed will be able to accurately determine the relative difference in mass between objects.