This chapter describes localization using bipolar and referential montages and includes the concepts of active reference and how to select a suitable reference when using referential montages. Localization is the art of locating the site of maximal potential on the cortical surface, presumably the origin of a waveform. This depends on the montage and rules of polarity. Certain regions such as sulcal depths, inferior surfaces, and very small potentials may be surface EEG negative. Bipolar montages use phase reversals and referential montages use maximum amplitude for localization. An apparent phase reversal on a referential montage suggests the possibility of a horizontal dipole or an active reference. Active reference occurs when the reference electrode lies close to the potential focus or becomes contaminated by artifact leading to false localizations. Therefore, it is important to select relatively inert electrodes as reference electrodes when using a referential montage. [145 words/844 characters]

Localization is the art of locating the site of maximal potential, presumably the origin of a waveform of interest. The EEG is relatively blind to certain cortical regions such as the sulcal depths, inferior surfaces of the brain, and infoldings such as the insula. Localization depends on the montage in use and the rule of polarity. Phase reversal is the key to localization with a bipolar montage. Maximal amplitude is the key to localization with a referential montage. Phase reversals are usually but not always adjacent. Nonadjacent phase reversals are separated by an isopotential channel (near flat line). Foci located at the end of a chain in a bipolar montage don’t show a phase reversal. An apparent phase reversal on a referential montage may indicate a horizontal dipole or an active reference.