Event-Related Potentials (ERPs) are changes in the ongoing electrical activity of the brain (Electroencephalograms, or EEGs) which are caused by the specific occurrence of a cognitive, motor or perceptual event. Any changes in EEG due to the demands of the task are amplified, averaged and extracted as ERP waveforms (see Figure 1). These wave-forms are measured as the difference between the electrical activity of a baseline reference electrode attached to an electrically inactive site, such as the mastoid bone below the ear or the naison on the nose, and the electrical activity of the areas of the brain covered by the electrodes. These changes allow neuroscientists to determine what areas of brain are being stimulated at a given time (and therefore which brain areas are involved in a given process), precisely when these areas become activated and what happens in these areas when people make an error.