Tool-use extends Peripersonal Space and Body Schema

Using a tool to reach objects placed in the far space modifies our action space, extending the limits of the Peripersonal Space (PPS), i.e. the space surrounding the body where stimuli from different sensory modalities are integrated. In the present study we investigated whether the extension of PPS accompanies to a change in the perceived dimension of the body, compatibly with an incorporation of the tool into the Body Schema (BS). We assessed PPS around the forearm by means of an audio-tactile integration task and the representation of the forearm by means of a tactile distance confrontation task before and after a training consisting of using a 1m long tool to reach far objects. Tool-use extended PPS representation along the tool axis: the point in space where audio-tactile stimuli interacted shifted to a farther location after tool-use. A similar effect was present in the task assessing the BS: after tool-use, subjects perceived their forearm narrower and longer, such as their arm took the shape of the tool. Therefore, even a brief training with a tool induces an extension of the representations of both the arm and the space surrounding it, suggesting a possible incorporation of the tool into the BS.