Intensive tool-practice and skillfulness facilitate the extension of body representations in humans.

The brain׳s representation of the body can be extended to include objects that are not originally part of the body. Various studies have found both extremely rapid extensions that occur as soon as an object is held, as well as extremely slow extensions that require weeks of training. Due to species and methodological differences, it is unclear whether the studies were probing different representations, or revealing multiple aspects of the same representation. Here, we present evidence that objects (cotton balls) held by a tool (chopsticks) are rapidly integrated into the body representation, as indexed by fading of the cotton balls (or 'second-order extensions') from a positive afterimage. Skillfulness with chopsticks was predictive of more rapid integration of the second-order cotton balls held by this tool. We also found that extensive training over a period of weeks augmented the level of integration. Together, our findings demonstrate integration of second-order objects held by tools, and reveal that the body representation probed by positive afterimages is subject to both rapid and slow processes of adaptive change.