Facilitation of tactile processing during action observation of goal-directed reach and grasp movements.

Our perception of sensory events can be altered by action, but less is known about how our perception can be altered by action observation. For example, our ability to detect tactile stimuli is reduced when our limb is moving, and task-relevance and movement speed can alter such tactile detectability. During action observation, however, the relationship between tactile processing and such modulating factors is not known. Thus, the current study sought to explore tactile processing at a task-relevant location during the observation of reaching and grasping movements performed at different speeds. Specifically, participants observed videos of an anonymous model performing movements at a slow [i.e., peak velocity (PV): 155 mm/s], medium (i.e., PV: 547 mm/s), or fast speed (i.e., PV: 955 mm/s). To assess tactile processing, weak electrical stimuli of different amplitudes were presented to participants' right thumbs when the observed model was at their starting position and prior to any movement, or when the observed model's limb reached its PV. When observing slow movements, normalized perceptual thresholds were significantly lower/better than for the premovement stimulation time. These data suggest that the movement speed can modulate tactile processing, even when observing a movement. Furthermore, these findings provide seminal evidence for tactile facilitation at a task-relevant location during the observation of slow reaching and grasping movements (i.e., speeds associated with tactile exploration). Previous work has highlighted the relationship between touch processing and movement speed during action, but the current study sought to understand this relationship during action observation of reaching and grasping movements. Here, we provide seminal evidence that tactile perceptual thresholds at the thumb are reduced compared with rest when observing the slowest movement speeds. Thus, tactile processing was facilitated at a task-relevant location during the observation of movements with speeds associated with tactile exploration.