Elsevier

Neuroscience

Volume 406, 15 May 2019, Pages 300-313
Neuroscience

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

https://doi.org/10.1016/j.neuroscience.2019.03.029Get rights and content
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Highlights

  • Involuntary mirror activity in healthy adults starts after varying latencies which change dynamically.

  • The amplitude of mirror activity increases linearly following repetitive contractions.

  • Amplitude and latency of mirror activity are inversely related.

  • Amplitude and latency of mirror activity seem to be unaffected by conventional tDCS.

Abstract

Mirror Activity (MA) is a phenomenon that is characterized by involuntarily occurring muscular activity in homologous contralateral limbs during unilateral movements. Even in neurologically healthy humans, MA of a small extent has been described, which does not directly lead to visible movements, but nonetheless, it is still detectable with surface electromyography (EMG) and therefore defined as physiological MA (pMA). The present study investigated latency- and amplitude-characteristics of pMA during repetitive unimanual isometric contractions with high but constant force requirements (80% maximum force). Here, we show for the first time that pMA is not time-locked to the muscle onset of voluntarily contracting hand muscles but starts with varying and dynamically changing latencies. Following consecutive isometric unilateral contractions, the latency of pMA progressively decreases accompanied by a progressive linear increase in its amplitude possibly as a result of changes in inhibitory mechanisms involved in suppressing involuntarily occurring 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 through anodal 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 of pMA 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 with its amplitude.

Abbreviations

A
Vargha–Delaney effect size of stochastic superiority
ATS
ANOVA-type statistic
CF
crossed facilitation
CMCT
central motor conduction time
CNS
central nervous system
FDI
first dorsal interossei muscle
IHF
interhemispheric facilitation
IHI
interhemispheric inhibition
LQ
laterality quotient
MA
mirror activity
MCT
motor conduction time
MDF
median frequency
MM
mirror movements
MVC
maximum voluntary contraction
M1
primary motor cortex
fNIRS
functional near-infrared spectroscopy
pMA
physiological mirror activity
PMCT
peripheral motor conduction time
tDCS
transcranial direct current stimulation
TMS
transcranial magnetic stimulation

Key words

mirror activity
motor overflow
latency
EMG
tDCS

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