Miller LE, Bolognini N, Hayward V, Farnè A

It is unknown to what extent the human brain incorporates tools into its sensorimotor maps. To this aim, the present study combined behaviour, electrophysiology and neuronal modelling to demonstrate that the human brain treats a tool like an extended sensory ‘organ’. As with the body, participants can localise touches on a hand-held tool with near-perfect accuracy. Using electroencephalography, we found that where a tool was touched was almost immediately coded in the neural dynamics of primary sensorimotor cortices. This time course was close to maximally efficient, as quantified using a biologically plausible model of mechanoreceptors in the hand. A second electroencephalography experiment found that touches on the tool and arm surfaces were mapped at identical stages of processing. Multivariate decoding algorithms and cortical source reconstruction demonstrated that low-level arm-based mechanisms were repurposed to map touch on a tool. We thus reveal that an elementary mechanism the human brain uses for the sensory incorporation of tools consists in recycling primary sensory dynamics otherwise devoted to the arm.