Head orientation involvement in assessment of the subjective vertical during whole body tilt.

The aim of this research was to investigate the involvement of head tilt in the assessment of the subjective vertical during whole body tilt. For inclinations up to 28 degrees, it appears that the whole body tilt gives rise to more important deviations towards its direction (A-effect) than when the tilt was restricted to the head. A multiple regression analysis shows that errors in head-tilt condition partially account for the global errors observed in condition of whole body tilt. Other factors (relative to the degree of inclination) also played a role in the determination of the subjective vertical in whole body tilt but were less important than head tilt. These results are highly compatible with the assumption of head orientation as the major determinant in roll-tilt effects.