Physiology of Interregional Connectivity in the Human Brain

The purpose of this study is to understand the physiology of connectivity between cortical regions in the human brain in healthy participants and in patients with white matter lesions. Specifically, the investigators will examine the effects of paired associative stimulation (PAS) which consists in delivering brief (\< 1 ms) current pulses separated by a short millisecond-level time interval ("asynchrony") to two cortical areas. The used techniques are all non-invasive and considered safe in humans: transcranial magnetic stimulation (TMS), electroencephalography (EEG), magnetic resonance imaging (MRI), and functional MRI (fMRI). Based on prior literature in animals and human studies, it is hypothesized that PAS may increase or decrease effective connectivity between the stimulated areas depending on the asynchrony value. The main outcome measure is source-resolved EEG responses evoked by single-pulse TMS; this is a more direct measure of neuronal changes occurring at the targeted cortical area than motor evoked potentials (MEPs) or sensor-level EEG responses used in previous studies.