Assessing cortical network properties using TMS–EEG

Abstract

The past decade has seen significant developments in the concurrent use of transcranial magnetic stimulation (TMS) and electroencephalography (EEG) to directly assess cortical network properties such as excitability and connectivity in humans. New hardware solutions, improved EEG amplifier technology, and advanced data processing techniques have allowed substantial reduction of the TMS-induced artifact, which had previously rendered concurrent TMS-EEG impossible. Various physiological artifacts resulting from TMS have also been identified, and methods are being developed to either minimize or remove these sources of artifact. With these developments, TMS-EEG has unlocked regions of the cortex to researchers that were previously inaccessible to TMS. By recording the TMS-evoked response directly from the cortex, TMS-EEG provides information on the excitability, effective connectivity, and oscillatory tuning of a given cortical area, removing the need to infer such measurements from indirect measures. In the following review, we investigate the different online and offline methods for reducing artifacts in TMS-EEG recordings and the physiological information contained within the TMS-evoked cortical response. We then address the use of TMS-EEG to assess different cortical mechanisms such as cortical inhibition and neural plasticity, before briefly reviewing studies that have utilized TMS-EEG to explore cortical network properties at rest and during different functional brain states.

Investigating TMS–EEG Indices of Long-Interval Intracortical Inhibition at Different Interstimulus Intervals

“…TMS-EEG facilitates a direct assessment of the cortical response to stimulation, removing the confounding influence of variations in spinal cord excitability that are known to effect conventional MEP measurements. In addition, combining these two methods provides significantly more information about the local and global response to ppTMS than can be derived from the MEP [21,22]. For LICI, previous studies utilising TMS-EEG in M1 have identified cortical indices of inhibition within TMS-evoked EEG potentials (TEPs), with specific TEP peaks produced by the test stimulus being reduced in amplitude when a conditioning stimulus is applied 100 ms earlier [23].…”

Assessment and modulation of cortical inhibition using transcranial magnetic stimulation

“…Furthermore, because of possible 'filtering' of cortico-spinal synapses and the motor endplate, a direct correlation between MEP amplitude and cortical excitability remains controversial. Accordingly, efforts are currently focusing on a more direct analysis of cortical inhibition, which is independent of the stimulated area of the cortex, using TMS-evoked electroencephalographic (EEG) potentials (Rogasch and Fitzgerald, 2013;Premoli et al, 2014). This attempt is supported by other techniques, e.g.…”

Non-invasive induction of plasticity in the human cortex: Uses and limitations

“…TEPs provide quantifiable markers of the cerebral neurophysiological state (Carlo Miniussi & Thut, 2010;Rogasch & Fitzgerald, 2013) and, in a similar fashion to MEPs, are influenced by TMS intensity (Kahkonen, Wilenius, Komssi, & Ilmoniemi, 2004;) and coil orientation (Bonato, Miniussi, & Rossini, 2006). While the amplitude of TEPs is greatest in the stimulated cortical area, it is possible to measure TEPs at cortical sites distant to the site of stimulation, potentially providing a useful measure of connectivity (Ilmoniemi et al, 1997;Kahkonen, Komssi, Wilenius, & Ilmoniemi, 2005;Komssi et al, 2002;Komssi et al, 2004).…”