TMS-EEG/EMG integration to study the excitability of M1

COEX 

Transcranial magnetic stimulation (TMS) is a non-invasive brain stimulation method that can activate cortical neurons. The TMS power needed to activate the neurons varies between cortical areas and individuals. This enables the use of TMS to study cortical excitability. However, the applicability of TMS is limited as it can only provide activation but not outcome measures. When combined with simultaneous electroencephalography (EEG) or electromyography (EMG), TMS evolves into a sophisticated neurophysiological method capable of studying cortical excitability and its dynamics. While TMS-EMG responses reflect the excitability of a limited population of neurons directly connected to pyramidal tract neurons, TMS-EEG responses reflect the activity of larger populations of neurons in the stimulated and connected areas. The advantage of the TMS-EEG/EMG methods is that TMS activates the cortical excitatory (glutamatergic) and inhibitory (gamma-aminobutyric acid-ergic, GABAergic) neurotransmitter systems at different time scales for EEG/EMG to measure them, allowing the investigation of both the excitatory and inhibitory excitability with these methods. Since the pathophysiology of several neuropsychiatric disorders is often associated with excitatory and/or inhibitory neurotransmitter systems, TMS-EEG/EMG methods offer potential biomarkers to study the presence or trajectory of a disease or response to a therapeutic intervention. Though promising findings, TMS-EEG/EMG methods still lack an understanding of which factors influence the outcome measures. This talk will provide an overview of the current state of TMS-EEG/EMG applications studying neuronal excitability and their limitations and possibilities, focusing on applications targeting M1. Data is presented on TMS-EEG/EMG excitability findings across the motor homunculus.