Oscillatory TMS-EEG-Responses as a Measure of the Cortical Excitability Threshold

Abstract

Transcranial magnetic stimulation (TMS) is a non-invasive tool to perturb brain activity. In TMS studies, the stimulation intensity (SI) is commonly normalized to the resting motor threshold (rMT) that produces muscle responses in 50% of stimulations applied to the motor cortex (M1). Since rMT is influenced by spinal excitability and coil-to-cortex distance, responses recorded from the cortex, instead of a peripheral muscle, could provide a more accurate marker for cortical excitability. Combining TMS with electroencephalography (EEG) enables the measurement of brain-wide cortical reactivity to TMS. We quantified TMS-induced changes in oscillatory power and the phase of EEG with event-related spectral perturbation (ERSP) and inter-trial coherence (ITC). We studied the SI-dependency of ERSP and ITC responses by stimulating the dominant M1 of ten healthy volunteers using single-pulse TMS with 150 pulses at 60%, 80%, 100%, and 120% of rMT. We found SI-dependent ERSP and ITC responses in M1, most notably with the wide-band (8-70 Hz) early ITC responses averaged 20-60 ms after TMS. With approximately linear SI-dependence, the early ITC response was consistent between SIs (intraclass correlation = 0.78, ). Our results reveal the potential of oscillatory EEG responses, in place of rMT, as a measure of the cortical excitability threshold in M1.

Toward the establishment of neurophysiological indicators for neuropsychiatric disorders using transcranial magnetic stimulation‐evoked potentials: A systematic review

“…After adjusting for the individual RMT of each pulse type, the following findings were found: (i) GMFP was higher with monophasic pulses (P < 0.001); (ii) local mean field power was higher with longer pulse width (P = 0.015); (iii) early TEP polarity was modulated depending on the current direction (P = 0.01). Saari et al, 2018 70 This study provides support for earlier intracranial electrical stimulation findings as it demonstrates cerebellar involvement in the alteration of core frequencies linked to cognitive and emotive aspects of human behavior. 78 Paired-pulse TMS: Intracortical facilitation -M1 and DLPFC ICF is a pp-TMS paradigm composed of a subthreshold conditioned stimulus and a suprathreshold test stimulus that produces MEP facilitation at ISI of 7 to 30 ms, 33 thought to be associated with both GABA(A) 34 and NMDA 35 receptors.…”

Phase clustering in transcranial magnetic stimulation-evoked EEG responses in genetic generalized epilepsy and migraine

“…Several studies show no difference in resting motor threshold between people with migraine and controls [11][12][13][14][15]. Combining TMS with EEG offers new options to assess cortical excitability, bypassing sensory and motor areas [16,17]. Previous TMS-EEG studies in epilepsy investigating TMS-evoked potential and the epileptiform EEG discharges triggered by TMS have identified aberrant excitability and connectivity [18][19][20][21][22][23][24] The only TMS-EEG study in JME to date found increased amplitude potentials in JME compared to controls, and increased amplitude of late peaks when participants with JME were sleep deprived, demonstrating cortical hyperexcitability (Del Felice 2011).…”

“…One novel way of assessing cortical excitability using TMS-EEG is by determining the uniformity of phase angles across trials in EEG responses, [17]. On a single electrode, the phase of TMS evoked responses align between trials shortly after the TMS pulse.…”

“…On a single electrode, the phase of TMS evoked responses align between trials shortly after the TMS pulse. Empirical evidence from computational models and in vivo studies suggests that a more excitable brain results in more synchronised phases in the higher frequency bands and reduced synchronisation in the lower frequency bands (8-14 Hz) (Saari et al, 2018). Measures of inter-trial phase clustering could thus be a measure of cortical excitability.…”

Can Biological Quantum Networks Solve NP‐Hard Problems?

“…Moreover, by stimulating the brain with external magnetic pulses-transcranial magnetic stimulation (TMS)-and recording EEG spectra and/or fMRI images, one can get information about correlations in time and space. [20,[121][122][123][124][125][126] Tononi's information integration theory (IIT) [22,23] treats consciousness as a global irreducible state of a complex system characterised by a single parameter describing the information content. The parameter is effectively obtained by averaging the EEG output from repeated TMS pulses and zip-compressing the output data.…”