Sensory and Motor Building Blocks of an Extra Robotic Digit Representation

Supernumerary limbs provide a unique opportunity to ask how the brain represents an entirely novel effector within an existing sensorimotor system. Here, we investigated how primary motor cortex (M1) represents a robotic extra digit (the Third Thumb; Dani Clode Design), and how this representation changes with experience. Using fMRI, we compared participants following short-term exposure with participants who had undergone longitudinal Third Thumb training. The device was controlled by the toes but operated alongside the biological fingers, while its sensory consequences were experienced on the hand. Following prolonged experience, greater activity was elicited in the M1 hand area both during Third Thumb control, and in response to its sensory consequences alone. Multivariate analyses showed that the Third Thumb representation became more distinct from its biological toe controller. Pattern component modelling further showed that the representation could be explained by a combination of its constituent motor and sensory patterns, but their relative contributions changed substantially with experience, from toe-dominated following short-term exposure to sensory-dominated following longitudinal training. These findings suggest that M1 can represent a novel effector through flexible recombination of existing sensorimotor representations, with experience altering how these pre-existing resources contribute to the emerging representation.