Visual information is predominantly interpreted within an eye-centered reference frame. Tactile information, on the other hand, can be interpreted within different reference frames, i.e., local-surface, or whole-body centered. An important question is whether, given the different possibilities, each observer has a natural reference frame that they consistently adopt in different conditions, or they can freely adopt several reference frames and switch from one to another without cost. Recognition of ambiguous asymmetrical tactile letters (e.g., b, d, p, q) allows us to interrogate the different reference frames adopted by observers when interpreting tactile information. For such stimuli drawn on the skin, recognition requires assigning top-bottom, left-right and front-back axes to the letter. Across several experiments, participants had to recognize these letters when presented on different body surfaces, either with a freely adopted reference frame or an imposed one. In the unconstrained condition, participants consistently adopted one reference frame, with between-participant differences being evident. When adapted to a new reference frame, interpretation of the letters produced an important cost in response accuracies and latencies, indicating that the freely adopted reference frame corresponded to a natural reference frame rather than an arbitrary choice. This cost was greater for the freely-adopted egocentric (head- or body-centered) than for the non-egocentric (off-centered) reference frame. By training participants with a particular set of symbols, we then tested for generalization to novel stimuli and body surfaces. Our results have implications for the design of visuo-tactile sensory substitution devices and for understanding emergence of the distal attribution phenomenon.

tactile perception; sensory substitution; learning; generalization; distal attribution