Assessing cortical connectivity using TMS–EEG

The combination of brain stimulation by transcranial magnetic stimulation (TMS) with simultaneous electroencephalographic (EEG) recording has the potential to be of great value for understanding the cortico-cortical connections within brain networks and how they are linked to cognitive or motor functions. It can reveal how connectivity varies as a function of neuronal state, differing between individuals and patient groups. In this chapter, we will first provide an historical overview on the development of the TMS-EEG coregistration methodology and highlight the technical challenges that need to be faced for its application. We will then discuss the wide range of possible TMS-EEG coregistration employments and what new information may be gained on the dynamics of brain functions, hierarchical organization, and cortical connectivity, as well as on TMS action per se. An advance in the understanding of these issues is timely and promises to have a substantial impact on many areas of clinical and basic neuroscience.