Similar circuits in the brain are engaged during the performance and observation of identical actions. Such engagement manifests in priming effects, where observation of an action leads to faster production of that action and slower production of an action involving a different movement of the same effector (e.g. observed finger flexion vs. produced finger extension), or a completely different effector (e.g. observed hand action vs. produced leg action). Here, we asked whether priming occurs for actions involving identical muscle groups where the degree of muscle contraction in observed actions was the same or different to that underlying an instructed response and whether patterns of muscle activation were also affected. Participants held an unseen rubber ball between their forefinger and thumb and responded to colour cues instructing a hard or a soft squeeze, whilst EMG activity from the first dorsal interosseous and the abductor pollicis brevis was recorded. The colour cues were superimposed on videos depicting a hard or soft squeeze of an identical rubber ball. Thus, there were two congruent (observe hard, produce hard; observe soft, produce soft) and two incongruent (observe hard, produce soft; observe soft, produce hard) conditions. Results showed that reaction time was slowed and EMG activity was modulated in the direction of the difference between observed and instructed squeezing movements. Hence, neural circuits underlying action observation are sensitive not only to differences in the actual muscle groups underlying observed actions but also to different extents of activation of the same muscle groups.
