Mu-ERD reflects action understanding, but the effect is small

Changes in mu-ERD while participants watch an actor execute a goal in which the goal is discernable early based on the type of grip used to grasp a coffee mug (unambiguous) or is discernable late, only after the goal is executed (ambiguous). [Display omitted] •We address recent criticisms suggesting that mu-ERD does not reflect action understanding/mirror neuron activity.•Mu-ERD may reflect several different underlying motor and cognitive processes related to motor control and planning.•We observed a small effect for mu-ERD and action understanding. Since the mid-2000’s, many researchers have provided evidence that mu-ERD measured at the motor cortex may reflect the collective activation of upstream brain regions associated with the human mirror system during action observation paradigms; however, several recent papers have called these findings into question. Our study represents an effort to address these criticisms. In our study, participants watched videos in which the type of grip an actor used to grasp a coffee mug either conveyed the goal with 100 % certainty (unambiguous-goal trials), or offered no predictive information (ambiguous-goal trials). If mu-ERD indexes action understanding, then we predicted that mu-ERD should increase while participants watched the actor grasp the mug for unambiguous-goal trials, but not for ambiguous-goal trials. During the intervals where participants watched the actor execute the goal, mu-ERD for unambiguous-goal trials should remain steady, whereas mu-ERD for ambiguous-goal trials should now increase. Conversely, if mu-ERD does not index action understanding, and instead reflects general motor processes associated with action (such as the activation of population vectors in M1 or planning processes), then mu-ERD should show no difference across conditions. Across most comparisons, we found that mu-ERD mostly reflected general motor processes; however, there was a small effect when participants overserved unambiguous-goal trials while watching the actor execute the goal suggesting that mu-ERD does reflect mirroring, but the effect is small.