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]

Referential montages are only as good as their reference electrode. The reference electrode is intended to be distant from the potential focus and inactive, but sometimes it may be very active. This is called an active reference; it misleads the reader into false localization. An active reference occurs if the reference electrode lies close to the potential focus or becomes contaminated by artifactual potentials. If the reference is at or near the focus, typically there is a unidirectional reflection (often with varying amplitudes) in all the channels. If the reference is contaminated by an artifactual potential, typically there is a phase reversal over channels that overlie a potential focus. Central references are prone to contamination with sleep potentials and ear references are prone to contamination with EKG and temporalis artifact. Switch to an alternative referential montage if you suspect an active reference. This is easy with a digital display.