Inverse relationship between amplitude and latency of physiological mirror activity during repetitive isometric contractions.

Neuroscience. 2019. No. 406. P. 300–313.

Maudrich T., Kenville R., Nikulin V., Maudrich D., Villringer A., Ragert P.

Mirror Activity (MA) is a phenomenon that is characterized by involuntarily occurring muscular activity inhomologous contralateral limbs during unilateral movements. Even in neurologically healthy humans, MA of a smallextent has been described, which does not directly lead to visible movements, but nonetheless, it is still detectable withsurface electromyography (EMG) and therefore defined as physiological MA (pMA). The present study investigatedlatency- and amplitude-characteristics of pMA during repetitive unimanual isometric contractions with high but constantforce requirements (80% maximum force). Here, we show for thefirst time that pMA is not time-locked to the muscle onsetof voluntarily contracting hand muscles but starts with varying and dynamically changing latencies. Following consecu-tive isometric unilateral contractions, the latency of pMA progressively decreases accompanied by a progressive linearincrease in its amplitude possibly as a result of changes in inhibitory mechanisms involved in suppressing involuntarilyoccurring muscular activity. Overall, the latency and amplitude of pMA show a strong inverse relationship. Furthermore,based on the previously proposed hypothesis of motor overflow, we explored the possibility of pMA modulation throughanodal and cathodal transcranial direct current stimulation (tDCS) applied to the ipsilateral primary motor cortex (M1),relative to a voluntarily contracting hand. Neither anodal nor cathodal tDCS is able to modulate amplitude or latency ofpMA compared to sham tDCS. In conclusion, our results extend the existing knowledge of pMA occurring due to high-effort unilateral contractions with constant force requirements to the aspect of its latency and the inverse association withits amplitude.

Keywords: latency, tDCS, EMG, mirror activity, motor overflow

Publication based on the results of: Investigations of neuronal mechanisms of cognition and decision making: neuroimaging, computational models and non-invasive brain stimulation (2019)