THE EFFECT OF OCULAR TORSIONAL POSITION ON PERCEPTION OF THE ROLL-TILT OF VISUAL-STIMULI

Perceived postural orientation during whole-body roll-tilt is commonly inferred from settings of a visual line to the perceived gravitationa l horizontal or vertical. This inference assumes that the change in oc ular torsional position (ocular counterrolling) which occurs during ro ll-tilt has no effect on the perceived orientation of the visual stimu lus. We investigated this assumption by measuring, during whole body r oll-tilt stimulation, settings of a visual line and a somatosensory ba r to the perceived gravitational horizontal and comparing the differen ce in these settings to the objectively measured ocular torsional posi tion for each subject. Two stimulus paradigms were used: one where the subject was given a roll-tilt stimulus and the eye torted, the other where there was eye torsion without a roll-tilt stimulus. In both para digms there was a very close relationship in magnitude and direction b etween the difference in the settings of the two perceptual indicators to gravitational horizontal and the objectively measured ocular torsi on. We conclude that change in ocular torsional position alone changes the perceived orientation of a visual line. The corollary is that set tings of a visual line cannot be used to infer perceived postural orie ntation directly. (C) 1997 Elsevier Science Ltd.