Sensorimotor predictions and tool use: Hand-held tools attenuate self-touch.

Human survival requires quick and accurate movements, both with and without tools. To overcome the sensorimotor delays and noise, the brain uses internal forward models to predict the sensory consequences of an action. Here, we investigated whether these sensory predictions are computed similarly for actions involving hand-held tools and natural hand movements. We hypothesized that the predictive attenuation of touch observed when touching one hand with the other would also be observed for touches applied with a hand-held tool. We first show that when touch is applied to the left index finger with the right index finger, the perceived force sensation is attenuated, only when the fingers are aligned in a manner that simulates direct physical contact and not when a distance of 25cm is introduced between the hands. We then show that touch applied to the left index finger with a tool held in the right hand at a distance of 25cm produces full sensory attenuation, similar to direct finger-to-finger contact. Finally, we show that touch is attenuated only when the tip of the tool is aligned with the receiving left index finger and not when the tip is placed at a distance of 25cm. Collectively, these results suggest that tool use and natural limb movements share the same computational mechanism for sensory predictions. We submit that the brain uses effector-independent forward models: touch is predicted based on the anticipated position of the current effector (i.e., the tip of the tool) rather than the body part per se.