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Brain Stimul. 2019 Jan - Feb;12(1):110-118. doi: 10.1016/j.brs.2018.09.009. Epub 2018 Sep 21.

EEG-triggered TMS reveals stronger brain state-dependent modulation of motor evoked potentials at weaker stimulation intensities.

Author information

1
Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe University, Frankfurt am Main, Germany; Department of Neurology & Stroke, and Hertie Institute for Clinical Brain Research, University of Tübingen, Germany.
2
Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe University, Frankfurt am Main, Germany.
3
Department of Neurology & Stroke, and Hertie Institute for Clinical Brain Research, University of Tübingen, Germany. Electronic address: ulf.ziemann@uni-tuebingen.de.
4
Department of Neurology & Stroke, and Hertie Institute for Clinical Brain Research, University of Tübingen, Germany.

Abstract

BACKGROUND:

Corticospinal excitability depends on the current brain state. The recent development of real-time EEG-triggered transcranial magnetic stimulation (EEG-TMS) allows studying this relationship in a causal fashion. Specifically, it has been shown that corticospinal excitability is higher during the scalp surface negative EEG peak compared to the positive peak of μ-oscillations in sensorimotor cortex, as indexed by larger motor evoked potentials (MEPs) for fixed stimulation intensity.

OBJECTIVE:

We further characterize the effect of μ-rhythm phase on the MEP input-output (IO) curve by measuring the degree of excitability modulation across a range of stimulation intensities. We furthermore seek to optimize stimulation parameters to enable discrimination of functionally relevant EEG-defined brain states.

METHODS:

A real-time EEG-TMS system was used to trigger MEPs during instantaneous brain-states corresponding to μ-rhythm surface positive and negative peaks with five different stimulation intensities covering an individually calibrated MEP IO curve in 15 healthy participants.

RESULTS:

MEP amplitude is modulated by μ-phase across a wide range of stimulation intensities, with larger MEPs at the surface negative peak. The largest relative MEP-modulation was observed for weak intensities, the largest absolute MEP-modulation for intermediate intensities. These results indicate a leftward shift of the MEP IO curve during the μ-rhythm negative peak.

CONCLUSION:

The choice of stimulation intensity influences the observed degree of corticospinal excitability modulation by μ-phase. Lower stimulation intensities enable more efficient differentiation of EEG μ-phase-defined brain states.

KEYWORDS:

Brain-state dependent brain-stimulation; Corticospinal excitability; EEG-TMS; Input-output curve; Motor evoked potential; Sensorimotor μ-rhythm

PMID:
30268710
DOI:
10.1016/j.brs.2018.09.009
[Indexed for MEDLINE]

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