Different tool training induces specific effects on body metric representation.

Morphology and functional aspects of the tool have been proposed to be critical factors modulating tool use-induced plasticity. However, how these aspects contribute to changing body representation has been underinvestigated. In the arm bisection task, participants have to estimate the length of their own arm by indicating its midpoint, a paradigm used to investigate the representation of the metric properties of the body. We employed this paradigm to investigate the impact of different actions onto tool embodiment. Our findings suggest that a training requiring actions mostly with proximal (shoulder) or distal (wrist) parts induces a different shift in the perceived arm midpoint. This effect is independent of, but enhanced by, the use of the tool during the training and in part influenced by specific demands of the task. These results suggest that specific motor patterns required by the training can induce different changes of body representation, calling for rethinking the concept of tool embodiment, which would be characterized not simply by the morphology of the tools, but also by the actions required for their specific use.