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The present study was designed to explore the changes in motor performance and motor resonance after multiple sessions of action observation (AO) training. Subjects were exposed to the observation of a video showing finger tapping movements executed at 3Hz, a frequency higher than the spontaneous one (2Hz) for four consecutive days. Motor performance and motor resonance were tested before the AO training on the first day, and on the last day. Results showed that multiple sessions of AO training induced a shift of the speed of execution of finger tapping movements toward the observed one and a change in motor resonance. Before the 3Hz-AO training cortical excitability was highest during the observation of the 2Hz video. This motor resonance effect was lost after one single session of 3Hz-AO training whereas after multiple sessions of 3Hz-AO training cortical excitability was highest during the observation of the 3Hz video. Our study shows for the first time that multiple sessions of AO training are able not only to induce performance gains but also to change the way by which the observer's motor system recognizes a certain movement as belonging to the individual motor repertoire. These results may encourage the development of novel rehabilitative protocols based on multiple sessions of action observation aimed to regain a correct movement when its spontaneous speed is modified by pathologies or to modify the innate temporal properties of certain movements.

Objective To test whether action observation combined with peripheral nerve electrical stimulation was able to evoke plasticity in the primary motor cortex (M1). Methods The stimulation protocol consisted in the observation of a video showing repetitive thumb-index tapping movements (AO) combined with peripheral electrical nerve stimulation (PNS) delivered on the median nerve (AO–PNS). M1 excitability, measured by means of transcranial magnetic stimulation, was compared with that assessed after AO and PNS alone. Results M1 excitability increased after AO–PNS, whilst no modifications occurred after AO and PNS alone. The increased M1 excitability after AO–PNS was long-lasting (45 min) and specific for the stimulated muscle. Conclusions This study described an innovative stimulation paradigm that exploited the mirror neuron system to induce plasticity in M1. However, this occurred only when action observation was combined with afferent signals coming from periphery. Significance This study supports the literature proposing the mirror neuron system as neural substrate for rehabilitation and opens a debate on the rehabilitative treatments that employ AO to improve patients’ motor functions. Indeed, these results suggest that AO has to be combined with afferent inputs from periphery to evoke plasticity in the human motor system.

The present study tested whether and how motor experience with a specific tool affects motor representation of a specific movement. To this aim, we considered a group of expert tennis players and a control group of athletic individuals without tennis experience. Participants were asked to execute 20 single forehands into the wall with a tennis racket (movement execution – ME) and, afterward, to produce a kinesthetic image of themselves while executing the same movements (motor imagery – MI). During MI participants handled one of the following tools: a tennis racket, a tennis-like racket and an umbrella. Results showed that the duration of the real and the imagined movements were almost similar when participants of both groups held the tennis rackets. In contrast, when tennis players handled the tools not specific for tennis the duration of the imagined movements increased significantly compared to the MI duration with a tennis racket. On the opposite, the handled tool did not modulate MI performances of the control group. In conclusion, this study showed that motor representation of subjects who developed motor skills associated to tool-use is reliant on the object used to practice movements. This finding suggests that, although MI mainly relies on the activity of cortical motor regions, non-motor information – as the use of the tool to practice movement – strongly affects the MI performance.