TMS-EEG: A Tool for Measuring Cortical Excitability in All Areas of the Brain
TMS is also utilized to analyze cortical excitability by applying magnetic pulses to the motor cortex. Except for occipital areas of the brain whose stimulation can be objectively confirmed through the observation of phosphenes, the stimulation of other areas of the brain does not provide measurable evidence. Therefore, TMS cannot help analyze the cortical excitability of other brain regions. The combination of TMS with EEG was first proposed by Cracoo in 1989 to record cortical responses evoked by TMS in the contralateral hemisphere. This technique has been employed to study neurophysiological processes in different cortical areas. A major challenge in the TMS-EEG combination includes the artifacts produced by the extensive electromagnetic discharge emitted from the coil that saturates traditional and old amplifiers. The manufacture of new amplifiers that are not saturated by the magnetic field has recently resolved this problem. This has also helped with the offline removal of artifacts and the provision of clearer EEG signals.
TMS-evoked EEG potentials (TEPs) are like a wave that lasts about 300 milliseconds and consists of some peaks that occur following TMS pulses with different delay periods. Each of these components in TEP represents the sum of postsynaptic inhibitory and excitatory potentials from a large population of pyramidal neurons. Stimulation of M1 causes some peaks on EEG with different levels of N15, N45, P55, N100, P180, and N280. Early peaks, such as N15 and P30, reflect cortical excitability. By contrast, N45 is associated with the GABAA inhibitory process in pharmacological studies. In paired-pulse TMS paradigms, N100 is associated with the GABAB inhibitory activity. The morphology and physiology of TEPs are less clear following the stimulation of other cortical areas. The stimulation of the dorsolateral prefrontal cortex (DLPFC) causes a waveform with peaks at N40, P60, N100, and P185. In addition, the stimulation of DLPFC by paired-pulse TMS paradigms creates an N100 peak and activates the cortical inhibitory system mediated by GABAB. Further studies are required to obtain more information on the physiology of TEPs.
Facilities of Atieh
This laboratory is equipped with an amplifier (NEURO PRAX-TMS/tES) manufactured by NeuroConn. This device can record EEGs at a frequency ranging from 0.3 Hz to 80-1200 Hz. This device allows you to record EEG at the same time with tDCS, tRNS, and tACS, and most importantly TMS. This device consists of 32 channels and provides a maximum sampling rate of 4,206.
