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Abstract

We perceive objects in external space, yet information is initially coded according to the stimulated surface. For example, visual information is coded in retinotopic coordinates, that can then be remapped into external coordinates such that perceived objects remains stable over time. For tactile information, this process is not straightforward as the information can be mapped into gaze-centred or external coordinates. Here we investigate influences on reference frame selection for passive tactile stimulation on the hand: we presented circular patterns of motion to palm and fingertip for different effector orientations. The direction of motion is ambiguous and therefore its interpretation can define the adopted reference frame. Two reference frames dominated fingertip stimulation: gaze-centred when the effector was upright and external when the effector was perpendicular to the stimulating object. The reference frame was consistent across rotations around the vertical axis. On the palm, however, perceived motion direction oscillated between clockwise and counter-clockwise interpretations both within and across orientations. To understand if this was due to spatial acuity, we conducted an additional experiment using a gap detection measure. The results suggest that this palm bi-stability cannot be accounted for by surface-dependent spatial acuity. The difference may be linked to the different functional role of palm and fingertip. Perceiving tactile objects in external space requires remapping of the stimulation into external and gaze centred coordinates. The fingertip data show weak tactile spatial constancy across orientation changes, similar to viewpoint changes in vision. Interestingly, perception on the palm appears to be more bi-stable.