In order to embark on the study of so complex organ as the brain, we select, observe, and measure one of the various forms of energy produced and used by the brain—electric, magnetic, chemical, thermal, and metabolic—and a hierarchical level of analysis—microscopic, mesoscopic, and macroscopic—each with its characteristic space-time scales. By choosing to read our book, we infer that readers have ... [Show full abstract] chosen to analyze the electroencephalogram from the scalp (EEG), the electrocorticogram from cortical surfaces (ECoG, Fig. 6.1), and the local field potentials from the depth of the brain (LFP) in any or all accessible forms and locations (Lopes da Silva 1993; Basar 1998). Then we characterize and classify the phenomena that we want to analyze and understand. We have begun with time series analysis of single channel recordings; now we undertake the spatial analysis of signals from arrays of channels. To that end, we require some basic concepts that we introduce in this chapter, with references to detailed treatments in other chapters.