Magnetic stimulation selectively affects pre-stimulus EEG microstates

Different electrophysiological (EEG) correlates may provide specific important assessment of the period that anticipates an imperative stimulus. Previous study of our group showed that a local (i.e. parietal) anticipatory EEG marker (i.e. the event related de-synchronization of the alpha rhythms; ERD) is selectively affected when transcranial magnetic stimulation (TMS) is delivered over crucial nodes belonging to well-known human networks involved in different cognitive domains. Here, we investigated whether such distinction is also present in the whole brain activity as seen through the pre-stimulus microstate’s topography, representing a global and reference-free measure of the neural activity. First, when subjects received a pseudo-stimulation (sham), we found two distinct pre-stimulus topographies during perceptual or memory task, respectively. Second, we reported that, during the visuo-spatial attention task, stimulation of left intraparietal sulcus (IPS), but not left angular gyrus (AG), significantly modifies the topography observed in the Sham condition. Conversely, stimulation of AG, but not IPS, changes the topography observed in the Sham condition during a semantic memory task. These findings provide the first causal evidence for the task and region specificity of the pre-stimulus EEG microstates, thus proposing this EEG index as of particular interest for the assessment of the period that precedes a predictable event.