Sensorimotor processing is dependent on observed speed during the observation of hand–hand and hand–object interactions

Observing a physical interaction between individuals (e.g., observing friends shaking hands) or between an object and an individual (e.g., observing a teammate striking or being struck with a ball) can lead to somatosensory activation in the observer. However, it is not known whether the speed of the observed interaction modulates such somatosensory activation (e.g., observing a teammate being struck with a slow vs. a fast-moving ball). In three experiments, participants observed a hand or object interact with another hand or object, all presented with a slow- or fast-moving effector. To probe sensorimotor processes during observation, participants were asked to react to an auditory beep (i.e., response time [RT] task) at the moment of observed contact. If observed contact led to increased somatosensory activation, RTs would decrease due to statistical and/ or intersensory facilitation. In all three experiments, RTs were lower when observing fast compared to slow motion stimuli, regardless of the moving (i.e., hand or ball) and target stimulus (i.e., hand or leaf). Further, when only an object (i.e., leaf) was the target, RTs did not differ between the moving hand and moving ball condition. In contrast, when an object (i.e., ball) was used as the moving stimulus, the magnitude of the speed effect (i.e., fast – slow RT difference) was significantly larger when the ball contacted a hand as compared to a leaf. Overall, these results provide novel evidence for a relationship between the observed kinematics of an object–human interaction and the sensorimotor processing in the observer.