The combination of transcranial magnetic stimulation (TMS) and electroencephalography (EEG) provides a unique means of manipulating ongoing processes within the brain, and observing the consequences on electrophysiology or participant behaviour1,2,3.
With a rapidly developing consensus on how to complete successful TMS-EEG studies, the technique is ﬁnding its use in explorations of how TMS pulses interact with the natural frequencies beneath the site of stimulation to provide evoked responses, and how such responses may differ between health and disease4,5. More recently, drug interventions have been combined with TMS-EEG to reveal how speciﬁc neural circuits — i.e. NMDA — may be responsible for particular components in TEPs, and the impact of drug interventions or TMS pulses on ongoing oscillations within a site of interest6,7. TMS can also be applied whilst participants complete a task during EEG, with studies revealing that visual attention can alter TMS-induced alpha oscillations and the N40 component of the TEP8.
Considerations for TMS-EEG studies
Owing to the nature of TMS-EEG, many of the initial challenges of combining these two powerful techniques were methodological: related to the non-neural consequences of applying TMS pulses whilst EEG is taking place simultaneously9. Signiﬁcant process has since been made in dealing with these non-neural consequences — artefacts — produced by TMS pulses during EEG by carrying out data collection carefully, or by dealing with the TMS-induced artefact with offline processing techniques.
There are now a number of widely-known and used countermeasures to successfully avoid artefacts that are produced by applying TMS during EEG. Perhaps surprisingly, the biggest complication in conducting TMS-EEG studies is not related to the large electric ﬁeld induced during TMS, but, rather, a result of the current induced within the scalp, which can be removed using independent components analysis (ICA), or minimised by carefully placing the TMS coil above a scalp location near the midline10.
Another unavoidable consequence of applying TMS during EEG recording is that the clicking sound produced by transcranial magnetic stimulation is measured by the EEG11. This clicking sound can be masked, but careful attention must be paid to the acoustic waveform that is used to mask it: recent debates emphasise the critical important and ensuring that the sound is properly masked in TMS-EEG datasets12,13 — something that can be achieved by mixing the time-varying frequency content of the clicking sound with white noise, for instance.
One ﬁnal consideration when combining TMS and EEG is that some stimulators — particularly those delivering monophasic pulses — can produce a period of decay after a high-amplitude voltage measurement caused by a TMS pulse: some EEG caps contain rotatable TMS-EEG electrode leads that can minimise the duration of this decay.
- New insights into rhythmic brain activity from TMS-EEG studies.. Thut G., Miniussi C.. Trends in Cognitive Sciences, 13.. (April 2009), pp. 182-189.
- A review of combined TMS-EEG studies to characterise lasting effects of repetitive TMS and assess their usefulness in cognitive and clinical neuroscience.. Thut G., Pascual-Leone A.. Brain Topography, 22. (2009).. (October 2009)
- Mechanisms underlying long-interval cortical inhibition in the human motor cortex: a TMS-EEG study.. Rogasch, N. Daskalakis Z., Fitzgerald B.. Journal of Neurophysiology, 109.. (January 2013), pp. 89-98.
- Natural Frequencies of Human Corticothalamic Circuits.. Rosanova M., Casali A., Bellina V., Resta F., Mariotti M., Massimini M.. Journal of Neuroscience, 29.. (June 2009), pp. 7679-7685.
- Clinical utility and prospective of TMS-EEG.. Tremblay S., Rogasch N., Premoli I., Blumberger D., Casarotto S., Chen R., Di Lazzaro V., Farzan F., Ferrarelli F., Fitzgerald P., Hui J., Ilmoniemi R., Kimiskidis K., Kugiumtzis D., Lioumis P., Pascual-Leone A., Pellicciari M., Rajji., Daskalakis Z.. Clinical Neurophysiology, 130.. (May 2019), pp. 802-844.
- TMS-evoked EEG potentials from prefrontal and parietal cortex: reliability, site specificity, and effects of NMDA receptor blockade.. Rogasch, N., Zipster C., Darmani G., Mutanen T., Biabani M., Zrrenner C., Desideri D., Belardinelli P., Müller-Dahlhaus F., Ziemann U.. bioRxiv.. (January 2019).
- The impact of GABAergic drugs on TMS-induced brain oscillations in human motor cortex.. Premoli I., Bergmann T., Fecchio M., Rosanova M., Biondi A., Belardinelli P., Ziemann U.. Neuroimage, 163.. (December 2017), pp. 1-12.
- Attention Modulates TMS-Locked Alpha Oscillations in the Visual Cortex.. Herring JD., Thut G., Jensen O., Bermann T.. Journal of Neuroscience, 35.. (October 2015), pp. 14435-14447.
- Short-Latency Artifacts Associated with Concurrent TMS-EEG.. Rogasch N., Thomson R., Daskalakis Z., Fitzgerald P.. Brain Stimulation, 6.. (November 2013), pp. 868-876.
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