Single-pulse transcranial magnetic stimulation (spTMS) studies report that movement observation facilitates corticospinal excitability in primary motor cortex (M1) in a muscle-specific manner. However, motor evoked potentials (MEPs) elicited by spTMS are known to reflect the summation of several descending volleys in corticospinal neurons which are evoked via mono- and polysynaptic inputs (so-called indirect waves or I-waves). It is unclear which of these components contribute to the muscle-specific modulation of M1 during action observation. The interactions between different I-waves are reflected in the facilitatory peaks elicited with a short-intracortical facilitation (SICF) protocol when two pulses are sent to M1 at precise intervals (i.e., 1.3, 2.5 or 4.1 ms). Here, we explored the modulation of early and late SICF peaks during action observation by measuring highly specific MEP amplitude changes measured in two muscles (index, FDI and little finger, ADM) while participants observed two different actions (precision and whole-hand grip). Our results demonstrate that both early (1.3 ms) and late (2.5 and 4.1 ms) SICF peaks are modulated in the context of movement observation. However, only the second peak (ISI 2.5 ms) was significantly associated with the muscle-specific modulation of corticospinal excitability as measured with spTMS. This late SICF peak is believed to reflect the activity cortico-cortical pathways involved in the facilitation of muscle-specific representations in M1. Thus, our findings suggest that movement observation leads to widespread activation of different neural circuits within M1, including those mediating cortico-cortical communication.

中文翻译：

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动作观察期间间接输入到主运动皮层的肌肉特定调制。

单脉冲经颅磁刺激（spTMS）研究报告说，运动观察以特定于肌肉的方式促进初级运动皮层（M1）的皮质脊髓兴奋性。但是，已知由spTMS引起的运动诱发电位（MEP）反映了皮质脊髓神经元中几个下降的截线的总和，这些截击是通过单突触和多突触输入（所谓的间接波或I波）诱发的。目前尚不清楚这些成分中的哪一个在观察动作过程中对M1的肌肉特异性调节有贡献。当两个脉冲以精确的间隔（即1.3、2.5或4.1毫秒）发送到M1时，不同的I波之间的相互作用会反映在通过短皮质内促进（SICF）协议引发的促进峰中。这里，我们通过测量在两块肌肉（食指，FDI和小指，ADM）中测量到的高特异性MEP幅度变化来探索动作观察过程中SICF早期和晚期峰的调制，而参与者观察到两种不同的动作（精确和全手握持）。我们的结果表明，在运动观察的背景下，早期（1.3毫秒）和晚期（2.5和4.1毫秒）SICF峰均被调制。但是，只有第二个峰（ISI 2.5 ms）与用spTMS测量的皮质脊髓兴奋性的肌肉特异性调节显着相关。认为该晚期SICF峰反映了参与促进M1中的肌肉特异性表示的活性的皮质-皮质途径。因此，我们的发现表明，运动观察会导致M1内不同神经回路的广泛激活，