The effect of water immersion on perception of the visual vertical.

Judgements of the apparent vertieality of a single visible line of light were compared under normal support conditions and under water. During head, body and trunk tilts up to 40° the visual vertical was not influenced by water immersion. Greater degrees of lateral body tilt (up to 180°) resulted in slightly greater departures of the visual from the gravitational vertical during immersion relative to terrestrial performance. It was suggested that this was due to the reduction of information from surface pressure receptors in the trunk. The mean visual aftereffects following head, body and trunk tilts were not affected by water immersion in any consistent manner. It was concluded that visual orientation constancy is only marginally reduced by immersion in water.