Introduction

The EEG, along with its event-related aspects, reflects the immediate mass action of neural networks from a wide range of brain systems, and thus provides a particularly direct and integrative noninvasive window onto human brain function. During the 80 years since the discovery of the human scalp EEG, our neurophysiological understanding of electrical brain activity has advanced at the microscopic and macroscopic level and has been linked to physical principles, as summarized in standard textbooks. The present introduction builds upon these texts but focuses on spatial aspects of EEG generators, many of which are applicable to both spontaneous and event-related activity. In particular, it is critical for the purpose of electrical neuroimaging to know which neural events are detectable at which spatial scales. As we will show, the spatial characterization of the neural EEG generators, and the advances in spatial signal processing and modeling converge in important aspects and provide a sufficiently sound basis for electrical neuroimaging. Because of the unique high temporal resolution of the EEG, electrical neuroimaging not only concerns the possible neuronal generator of the scalp potential at one given moment in time, but also the possible generators of rhythmic oscillations in different frequency ranges. In fact, understanding the intrinsic rhythmic properties of cortical or subcortical–cortical networks can help to constrain electrical neuroimaging to certain frequency ranges of interest and to perform spatial analysis in the frequency domain.