Temporal dynamics of the mirror neurons effect and its stimuli dependent modulation: TMS study

Abstract

The study of mirror neurons (MN) has made significant progress since human studies commenced. However, when using transcranial magnetic stimulation (TMS), there are inconsistencies in the literature regarding stimulus presentation, duration of presentation, and timing of TMS pulses.

The study assessed the effects of stimuli presentations, using both pictures and videos of hand movements. To accomplish this, single-pulse TMS was applied to the dominant primary motor cortex (M1) during varying time frames (0, 320, 640 ms). Motor evoked potentials were then recorded from the FDI (index finger) and ADM (little finger) muscles of 29 healthy participants via adhesive electrodes. Subjects’ hands were positioned perpendicular to each other, and visual stimuli were presented under three varying conditions. The TMS coil was accurately repositioned using an Axilum Cobot robotic arm and navigation stimulation system to maintain consistency throughout the experiment.

The aim of this study is to provide a comprehensive analysis of stimulus presentation and stimulation timeframes to achieve optimal settings. This paper describes the two most commonly used stimulus modalities, namely, picture and video [1–4], and the frequently employed timeframes for TMS: from movement initiation (picture and video condition) to offset (post-video condition), with different timings (0, 320, and 640 ms) [1, 2, 5]. Notably, the stimulation at the offset of the movement is a novel concept in literature. We conducted three distinct three-way repeated measures ANOVAs employing independent variables. The collected data indicate that the two types of stimulation during the onset of movement, i.e., photograph and video, display varying changes over time. At 320 ms, MEPs increase for the related muscles while nonrelated muscles exhibit inhibitory effects at 640 ms. In the condition of stimulation during movement offset (post-video), this double dissociation is present across all stimulation time frames. Hence, the majority of mirror response can be attributed to inhibition of nonrelated muscles. This study displays the temporal progression of the mirror effect and its impact on both related and unrelated muscles throughout time.

The obtained data illuminates unresolved inquiries in human mirror neuron research and details the impacts of diverse stimuli presentations and TMS stimulation durations. With this information, an ideal protocol can be established to examine the human mirror neuron system tailored to specific research needs. Furthermore, these outcomes can foster the creation of enhanced rehabilitation protocols for patients with movement disorders in clinical settings.

Keywords

mirror neurons system / transcranial magnetic stimulation / primary motor cortex / motor evoked potentials

References

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[7] Received: 15.05.2023 Accepted: 26.11.2023 Published online: 20.01.2024

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