Electrical recording of neuronal activity has been the preferred means of analyzing single neurons and neuronal networks (Contreras, 2004; Llinas, 1988). Electrical signals produced by neurons can be detected at a distance from the source. Several recording tools apply to different spatial scales. At the mesoscale, where local neuronal populations can be analyzed, a popular method is extracellular recording using metal electrodes. An electrode placed inside a brain slice in vitro or inserted in the brain in vivo detects electrical signals produced by the surrounding cells. A wide range of neural phenomena can be observed, from the spiking activity of individual neurons (extracellular action potentials or EAPs; bandwidth: 300-3000 Hz) to the slower network oscillations of small populations (local field potentials or LFPs; bandwidth: 1-300 Hz). Additionally, the same electrode can be used to deliver electrical stimulation to a local area in the brain. While applying this method for brain recording and stimulation is relatively straightforward, the challenge lies in the analysis of recorded data. With hundreds of possible signal sources surrounding an electrode, the specificity and selectivity of such technique is poor. Thus, extracellular recording has been widely used for analyzing population activity.