Reference frames and haptic perception of orientation: Body and head tilt effects on the oblique effect

The aim of this study was to examine the effect of body and head tilts on t he haptic oblique effect. This effect reflects the more accurate processing of vertical and horizontal orientations, relative to oblique orientations. Body or head tilts lead to a mismatch between egocentric and gravitational axes and indicate whether the haptic oblique effect is defined in an egoce ntric or a gravitational reference frame. The ability to reproduce principa l (vertical and horizontal) and oblique orientations was studied in upright and tilted postures. Moreover, by controlling the deviation of the haptic subjective vertical provoked by postural tilt, the possible role of a subje ctive gravitational reference frame was tested. Results showed that the hap tic reproduction of orientations was strongly affected by both the position of the body (Experiment 1) and the position of the head (Experiment 2). In particular, the classical haptic oblique effect observed in the upright po sture disappeared in tilted conditions, mainly because of a decrease in the accuracy of the vertical and horizontal settings. The subjective vertical appeared to be the orientation reproduced the most accurately. These result s suggest that the haptic oblique effect is not purely gravitationally or e gocentrically defined but, rather, depends on a subjective gravitational re ference frame that is tilted in a direction opposite to that of the head in tilted postures (Experiment 3).