Rationale: Electrophysiological potentials such as electroencephalography (EEG) and nerve conduction studies are registered through a differential amplifier. The EEG machine compares each input to a reference. Input 1 (G1) is compared to input 2 (G2). In bipolar montage the resulting potential is the difference between G1 and G2, and the reference cancels out (G1/ref -G2/ref = G1-G2). In referential montage G2 is typically a common average reference. Other cortical potentials, external, and physiological artifacts frequently contaminate G2. The resulting EEG wave is the difference between G1 and G2 (G1/ref – G2 average/ref= G1-G2 average)
(Mayor, L. C., Burneo, J. G. Ochoa, J. G. Electroencephalography Handbook. (Ediciones Uniandes, 2013. Chapter 3). Ideally, G2 common reference should be close to zero. This study aims to determine that adding a low frequency filter to a referential electrode above the clinically relevant frequency band should remove contamination of G2, herein named the O reference.
Methods: This study designed the O reference by adding a series of two 300 Hz low frequency filters to a standard EEG electrode, aiming to block brain frequencies of clinical significance and potentials derived from external and physiological artifacts.
To test the hypothesis, the O reference electrode was connected at the investigator’s head FP1 location in the 10-20 system, standard FP2, and Cz electrodes. All EEG electrodes were connected to a Compumedics EEG machine at 500Hz sampling rate, 70 Hz HFF, 1 Hz LFF, notch filter off. The EEG was then compared from the FP2 and Cz using FP1 (O reference). Contamination was created by tapping sequentially FP1 and FP2. The presence of eye movement artifacts and 60 Hz contamination was then observed.
Results: This study showed complete elimination of 60 Hz contamination, eye movement artifacts and electrode movement artifacts at the O reference electrode.
No alteration of the input from G1 when the O reference was used. Figure 2.
Conclusions: The O reference is the ideal reference for a referential montage. In addition, the O reference prevented EEG contamination including 60 Hz, movements, and other physiological artifacts from the reference electrode.
Funding: No funding received