Modality-dependent indication of the subjective vertical during combined linear and rotational movements.

Humans can discriminate whether a change in the direction of gravito-inertial force (GIF) is caused by body tilt or by linear translation. This ability, attributed to vestibular sensory fusion, is often examined by asking subjects to adjust an indicator to match their subjective earth-fixed vertical (SV). We used two different modalities, visual and haptic, to examine continuous adjustment during different combinations of roll rotation and linear translation on a hexapod motion device. We conclude that, in conditions of combined translational and rotational motion, the modality of indication plays a major role for the perception of verticality of the indicator.