Motor resonance is modulated by an object’s weight distribution

Abstract

Transcranial magnetic stimulation (TMS) studies showed that corticospinal excitability (CSE) is modulated during observation of object lifting, an effect termed ‘motor resonance’. Specifically, motor resonance is driven by movement features indicating object weight, such as object size or observed movement kinematics. We investigated in 16 humans (8 females) whether motor resonance is also modulated by an object’s weight distribution. Participants were asked to lift an inverted T-shaped manipulandum with interchangeable center of mass after first observing an actor lift the same manipulandum. Participants and actor were instructed to minimize object roll and rely on constrained digit positioning during lifting. Constrained positioning was either collinear (i.e. fingertips on the same height) or noncollinear (i.e. fingertip on the heavy side higher than the one on the light side). The center of mass changed unpredictably before the actor’s lifts and participants were explained that their weight distribution always matched the one of the actor. Last, TMS was applied during both lift observation and planning of lift actions. Importantly, our results revealed that CSE was similarly modulated during lift observation and planning: when participants observed or planned lifts in which the weight distribution was asymmetrically right-sided, CSE, recorded from the thumb muscles, was significantly increased compared to when the weight distribution was left-sided. Moreover, this increase seemed to be primarily driven by the observed and planned thumb positioning when lifting the right-sided asymmetrical weight distribution. In conclusion, our results suggest that complex intrinsic object properties such as weight distributions can be encoded by an individual’s motor system during both observation and planning of lifting actions.