Contribution of Visual Motion Cues from a Held Tool to Kinesthesia

Incorporation of a tool into the body schema is well established. Here, we assessed whether visual signals originating from the tool provide relevant cues for the perception of arm movements, as would signals originating from the arm holding it. Kinesthetic illusions were investigated by passively moving one arm (via a robotized manipulandum) and therefore the tool (a rake), using the mirror paradigm, with the reflected part being limited to the tool, the arm, or both. Illusory movements concerned the other arm, remaining static and hidden behind the mirror. In Experiments 1 and 3, participants held the same tools in their hands. Results showed that seeing the displacement of the reflected tool in the mirror induced kinesthetic mirror illusions in the hidden arm, similarly to seeing the reflected arm itself, though slightly reduced in terms of strength and occurrence frequency. In Experiment 2, participants held either the same objects in their hands (the rakes) or different ones (a rake, the image of which was reflected in the mirror, and a ball in the other hand). Results showed that mirror vision of the moving tool was not sufficient for mirror illusions to occur, the same tool in the two hands being an essential condition. Finally, in Experiment 3, we showed that neither prior practice nor active tool use was necessary for the tool mirror illusion to occur. Altogether, these results demonstrate that the visual cues originating from the held-tool are integrated for sensing arm movement.