Hands as tools: how manual behavior shapes actions and spontaneous and task-evoked brain activity

Humans establish interactions with external objects through stereotypical upper-limb movements. These regularities may generate probabilistic representations about the body and internal models (i.e., prior) for adaptive sensorimotor control. Recent studies show that, in the absence of any task, spontaneous brain activity patterns resemble those evoked by the execution of ecological hand movements. These observations suggest that, even at rest, the brain preserves a hand representation, likely for efficient motor control. Through hands, humans manipulate tools that can be incorporated into the body schema (i.e., brain representation of the body). Using neuroimaging studies, we tested whether spontaneous activity patterns more strongly resemble patterns evoked by the observation of visual stimuli depicting hands vs non-hands and regular vs perturbed object-related arm and hand movements. Then, in two behavioral studies, we explored whether humans could embody a bionic tool (i.e., experience it as part of the body) and thus if this would affect behavior and the body schema. Results showed that spontaneous activity patterns code for the visual representation of human hands in somatomotor brain regions and for regular upper-limb movements in the dorsal attention network. Furthermore, we found that the virtual grafting of a bionic tool elicits a sense of embodiment like or even stronger than its natural counterpart (i.e., a virtual hand) and that tool use can alter the body representation through changes in muscular intensity and kinematics parameters. We suggest that hand shape and regular movements are more represented in spontaneous activity than control stimuli, likely due to replay mechanisms for processing and interpreting information to which we are regularly exposed. Our studies also indicate that the natural use of bionic tools can change human behavior, opening new research and application possibilities, especially for amputees struggling to embody prosthetic limbs.