These findings suggest that the dopaminergic state contributes to the effects of action observation, and this training may be a promising approach in the rehabilitation of bradykinesia in PD.

Several investigations suggest that actual and mental actions trigger similar neural substrates. Motor learning via physical practice results in long-term potentiation (LTP)-like plasticity processes, namely potentiation of M1 and a temporary occlusion of additional LTP-like plasticity. However, whether this neuroplasticity process contributes to improve motor performance through mental practice remains to be determined. Here, we tested skill learning-dependent changes in primary motor cortex (M1) excitability and plasticity by means of transcranial magnetic stimulation (TMS) in subjects trained to physically execute or mentally perform a sequence of finger opposition movements. Before and after physical practice and motor-imagery practice, M1 excitability was evaluated by measuring the input-output (IO) curve of motor evoked potentials. M1 LTP and long-term depression (LTD)-like plasticity was assessed with paired-associative stimulation (PAS) of the median nerve and motor cortex using an interstimulus interval of 25 ms (PAS25) or 10 ms (PAS10), respectively. We found that even if after both practice sessions subjects significantly improved their movement speed, M1 excitability and plasticity were differentially influenced by the two practice sessions. First, we observed an increase in the slope of IO curve after physical but not after MI practice. Second, there was a reversal of the PAS25 effect from LTP-like plasticity to LTD-like plasticity following physical and MI practice. Third, LTD-like plasticity (PAS10 protocol) increased after physical practice, whilst it was occluded after MI practice. In conclusion, we demonstrated that MI practice lead to the development of neuroplasticity, as it affected the PAS25- and PAS10- induced plasticity in M1. These results, expanding the current knowledge on how MI training shapes M1 plasticity, might have a potential impact in rehabilitation.

The cerebellum is involved in the generation of essential tremor (ET) and cerebellar timing function is altered in patients with ET showing an increased variability of rhythmic hand movements. Using a sensor-engineered glove, we evaluated motor behaviour during repetitive finger tapping movements in 15 patients with ET and in 11 age- and gender-matched normal subjects. In addition, we investigated whether, in patients with ET, an inhibitory repetitive transcranial magnetic stimulation (1 Hz-rTMS) over lateral cerebellum was able to change timing properties and motor behaviour. Patients with ET showed a longer touch duration (TD) and a lower inter tapping interval (ITI) than normal subjects. The temporal variability of the movement (coefficient of variation of ITI) was increased in patients with ET. Neither clinical rating scale or tremor measurements correlated with any parameter of motor performance in the ET group. 1 Hz-rTMS over ipsilateral lateral cerebellum transiently affected the performance of patients with ET, by reducing TD values and normalizing ITI values. After 1 Hz-rTMS, the coefficient of variation of ITI was restored to values similar to those of normal subjects. We postulate that the strategy to increase TD, probably adopted to allow a better perception of movement, can affect ITI and its variability. The results support the idea that the cerebellum plays a central role in the selection of motor strategy of rhythmic finger movements, particularly in terms of temporal organization of movement.

The hand blink reflex is a subcortical defensive response, known to dramatically increase when the stimulated hand is statically positioned inside the defensive peripersonal space (DPPS) of the face. Here, we tested in a group of healthy human subjects the hand blink reflex in dynamic conditions, investigating whether the direction of the hand movements (up-to/down-from the face) could modulate it. We found that, on equal hand position, the response enhancement was present only when the hand approached to (and not receded from) the DPPS of the face. This means that, when the hand is close to the face but the subject is planning to move the hand down, the predictive motor system can anticipate the consequence of the movement: the "near" becomes "far." We found similar results both in passive movement condition, when only afferent (visual and proprioceptive) information can be used to estimate the final state of the system, and in motor imagery task, when only efferent (intentional) information is available to predict the consequences of the movement. All these findings provide evidence that the DPPS is dynamically shaped by predictive mechanisms run by the motor system and based on the integration of feedforward and sensory feedback signals.

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