What are the differences between bipolar channels and monopolar channels for EEG?

Question

In EEG, each electrode records a biopotential which must be compared to a reference biopotential in order to obtain a channel of measurement. Two types of channels are the so-called bipolar channels and monopolar channels. In the case of bipolar channels, each working electrode uses a different reference electrode, whereas in the case of monopolar channels, each working electrode uses the same electrode as reference.

From an amplifier-technical point of view, reusing a signal can degrade it a bit, so that is indeed bad for the monopolar channels. However, is there any fundamental difference between monopolar and bipolar channels? What if every two channels have a common reference electrode?

At this point I feel the need to mention there is another question mentioning the difference between the two, but it doesn't ask whether there is a conceptual difference between the two, nor does it compare their advantages and disadvantages (in addition to the one accepted answer being rather short in my opinion).

Additionally, this paper mentions that bipolar recordings are generally known to be more robust to noise than monopolar recordings: why would that be the case?

Answer 1

In general, closely situated electrodes work well to eliminate environmental noise, at least when a differential amplifier is used. Environmental noise is a major problem in case of scalp-recorded signals, think of the electromagnetic distortions caused by electronical devices situated in the vicinity of your recordings. But also muscle artifacts are a nuisance, which can also be eliminated by using closely spaced recording electrodes and differential amplification. This type of electrode placement is called a bipolar recording, as opposed to monopolar recordings where the reference electrode is situated further away (Saab, 2009).

With differential amplification, the potential difference across the electrodes is amplified. Suppose environmental noise is present, or a muscle artifact, then the hopes is that this artifact is similar across closely spaced electrodes. In case of EEG, electrode A, ideally, records a part of the brain situated directly underneath it. An adjacent electrode B records the cortical area next to electrode A. The signal (EEG) is then said to be location specific. Ideal artifacts (though virtually nonexistent) are similar across electrodes. Hence, in a simplified example, when the differential is amplified across electrodes A and B then signal A = A-B, where the signals are different, yet the artifacts are similar (Saab, 2009).

Many other electrophysiological recordings besides EEG are recorded using bipolar recordings, because artifacts are too pronounced to allow a monopolar setup (e.g., Diez et al, 2010; Stronks et al., 2009; Stronks et al., 2016). Monopolar setups generally result in larger amplitude signals, yet often the artifacts become much more pronounced, and the signal-to-noise ratios are more favorable in a bipolar electrode configuration.

<sub>References
- Diez et al., Annu Int Conf IEEE Eng Med Biol Soc (2010): 5803-6
- Saab, Biofeedback (2009); 36(4): 128–33
- Stronks et al.,IOVS (2013); 54:3891–901
- Stronks et al., Doc Ophthalmol (2016); 132:1–15</sub>