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Influence of Visual Presentation of Finger Movements on Motor Responses Induced by Transcranial Magnetic Stimulation. An Effect Linked with a Possible Reaction of the Mirror Neuron System
Mirror neuron research has come a long way from their discovery in primates to modern experiments in humans. However, many methodological questions remain regarding the method and timing of stimulus presentation in such studies. What is the optimal way to present motor stimuli? Is it possible to evaluate the temporal dynamics of the mirror neuron effect using transcranial magnetic stimulation at defined time points? The study reported here examined different types of stimulus presentation (photos and videos of hand movements) combined with single-pulse transcranial magnetic stimulation (TMS) of the primary motor cortex (M1) of the dominant hemisphere in different time windows (0, 320, 640 msec). Motor evoked potentials (MEP) were recorded from the first dorsal interosseous (FDI) and the abductor digiti minimi (ADM) muscles using adhesive electrodes placed on the hand muscles in 11 healthy subjects at rest with the hands positioned orthogonally during observation of visual stimuli in three different presentation conditions. The study results showed that video stimuli produced the strongest mirror effect when the TMS stimulus was presented 320 msec after presentation of the movement. This effect was specific to particular muscles. The data obtained here will help to formulate an optimal protocol for studying the mirror neuron system in humans and will contribute to its further clinical use in investigations and rehabilitation.