Tool morphology constrains the effects of tool use on body representations.

What factors constrain whether tool use modulates the user's body representations? To date, studies on representational plasticity following tool use have primarily focused on the act of using the tool. Here, we investigated whether the tool's morphology also serves to constrain plasticity. In 2 experiments, we varied whether the tool was morphologically similar to a target body part (Experiment 1, hand; Experiment 2, arm). Participants judged the tactile distance between pairs of points applied to their tool-using target body surface and forehead (control surface) before and after tool use. We applied touch in 2 orientations, allowing us to quantify how tool use modulates the representation's shape. Significant representational plasticity in hand shape (increase in width, decrease in length) was found when the tool was morphologically similar to a hand (Experiment 1A), but not when the tool was arm-shaped (Experiment 1B). Conversely, significant representational plasticity was found on the arm when the tool was arm-shaped (Experiment 2B), but not when hand-shaped (Experiment 2A). Taken together, our results indicate that morphological similarity between the tool and the effector constrains tool-induced representational plasticity. The embodiment of tools may thus depend on a match-to-template process between tool morphology and representation of the body.