Methodology for Combined TMS and EEG

Abstract

The combination of transcranial magnetic stimulation (TMS) with simultaneous electroencephalography (EEG) provides us the possibility to non-invasively probe the brain's excitability, time-resolved connectivity and instantaneous state. Early attempts to combine TMS and EEG suffered from the huge electromagnetic artifacts seen in EEG as a result of the electric field induced by the stimulus pulses. To deal with this problem, TMS-compatible EEG systems have been developed. However, even with amplifiers that are either immune to or recover quickly from the pulse, great challenges remain. Artifacts may arise from the movement of electrodes, from muscles activated by the pulse, from eye movements, from electrode polarization, or from brain responses evoked by the coil click. With careful precautions, many of these problems can be avoided. The remaining artifacts can be usually reduced by filtering, but control experiments are often needed to make sure that the measured signals actually originate in the brain. Several studies have shown the power of TMS-EEG by giving us valuable information about the excitability or connectivity of the brain.

Excitability modulation of the motor system induced by transcranial direct current stimulation: A multimodal approach

“…Finally, the time-course of LMFP effects over the motor cortex highlight that the tDCS modulates, in a polarity-specific manner, not only the initial component of cortical response, that is the correlate of the direct activation of the stimulated area, but also the later components, that is the markers of the activity triggered by axonally conducted signals (Ilmoniemi and Kicic, 2010). The hypothesized mechanism underlying that time-course could be represented by a differential balance between the intracortical (presumably GABAergic) inhibition or an enhancement of the intracortical (presumably glutamatergic) excitation, or by a mix of both.…”

“…The TMS-induced artifact was not removed from the recording (see Veniero et al, 2009). It is important to note that if an EEG epoch was rejected, the corresponding EMG epoch was also rejected (Ilmoniemi and Kicic, 2010), resulting in a one-to-one correspondence between cortical and peripheral measures.…”

“…On this basis, we hypothesize that the anodal tDCS decreases the synaptic activation threshold, increasing the probability that the TMS will polarize the cell membranes and, consequently, recruit a larger neuronal population. In addition, it determines the increase in the opening of the voltage-sensitive ion channels and the facilitation of action potential induction (Ilmoniemi and Kicic, 2010). Moreover, long-lasting neuroplasticity effects might be linked to enduring transformations of synaptic strength, which are likely related to the intracellular calcium concentration changes (Hattori et al, 1990;Islam et al, 1995).…”

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].…”

Current clinical approach to patients with disorders of consciousness

“…Some studies have shown promising results in the integration of transcranial magnetic stimulation (TMS) with simultaneous EEG for assessment of brain connectivity and cortical interactions. 11,12 treatMent of coMatose patients…”