The effect of water immersion on postural and visual orientation.

BACKGROUND

The subjective visual vertical (SVV) and the subjective horizontal body position (SHP) perceptually dissociate and may therefore be based on a differential participation of graviceptive references. This study focuses primarily on the effects that are caused by an alteration of somatosensory information on the skin surface induced by water immersion. We expect the SHP to be selectively affected during water immersion while the SVV remains unchanged.

METHODS

Four diving subjects took part in the experiments. An underwater apparatus (UWA) allowed for a determination of the SHP and the SVV during water immersion. On land, the adjustments of the SHP and SVV were made in a tilt chair apparatus (TCA).

RESULTS

Three of four subjects show a significant head-upward shift between 7 degrees and 20.3 degrees of the SHP when tested under water (p < 0.02). The adjustments of the SVV were compared within corresponding physical roll inclinations and did not differ with the exception of one subject who showed a significant change of the SVV toward the physical vertical.

CONCLUSIONS

The SHP and SVV perceptually dissociate on land and, as we determined for the first time, also under water. Water immersion leads to an increased roll tilt perception as measured by the SHP. However, a systematic influence on SVV could not be observed under water thus supporting the assumption that the SVV and SHP depend on gravitoreceptive information that is selectively altered during water immersion.