<p>In general, closely situated electrodes work well to eliminate environmental noise, at least when a <strong><a href="https://www.electronics-tutorials.ws/opamp/opamp_5.html" rel="nofollow noreferrer">differential amplifier</a></strong> 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 <a href="https://www.bcia.org/files/public/CE%20Articles/Saabbiof-36-04-128-133.pdf" rel="nofollow noreferrer">(Saab, 2009)</a>.</p>
<p>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 <a href="https://www.bcia.org/files/public/CE%20Articles/Saabbiof-36-04-128-133.pdf" rel="nofollow noreferrer">(Saab, 2009)</a>.</p>
<p>Many other electrophysiological recordings besides EEG are recorded using bipolar recordings, because artifacts are too pronounced to allow a monopolar setup (e.g., <a href="https://pubmed.ncbi.nlm.nih.gov/21096910/" rel="nofollow noreferrer">Diez <em>et al</em>, 2010</a>; <a href="https://iovs.arvojournals.org/article.aspx?articleid=2189069" rel="nofollow noreferrer">Stronks <em>et al</em>., 2009</a>; <a href="https://pubmed.ncbi.nlm.nih.gov/26743880/" rel="nofollow noreferrer">Stronks <em>et al.</em>, 2016</a>). 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.</p>
<p><sub><strong>References</strong><br />
<strong>-</strong> <a href="https://pubmed.ncbi.nlm.nih.gov/21096910/" rel="nofollow noreferrer">Diez <em>et al</em>., <em>Annu Int Conf IEEE Eng Med Biol Soc</em> (2010): 5803-6</a><br />
<strong>-</strong> <a href="https://www.bcia.org/files/public/CE%20Articles/Saabbiof-36-04-128-133.pdf" rel="nofollow noreferrer">Saab, <em>Biofeedback</em> (2009); <strong>36</strong>(4): 128–33</a><br />
<strong>-</strong> <a href="https://iovs.arvojournals.org/article.aspx?articleid=2189069" rel="nofollow noreferrer">Stronks <em>et al</em>.,<em>IOVS</em> (2013); <strong>54</strong>:3891–901</a><br />
<strong>-</strong> <a href="https://pubmed.ncbi.nlm.nih.gov/26743880/" rel="nofollow noreferrer">Stronks <em>et al</em>., <em>Doc Ophthalmol</em> (2016); <strong>132</strong>:1–15</a></sub></p>

CC BY-SA 4.0
2023-05-14T13:50:37.633Z
If I understand correctly, closer together means you can pick up localized artifacts on both thus enabling the differential amplifier to remove them (good), but then you also pick up similar activity from nearby areas due to volume conduction (not good).
David Cian
CC BY-SA 4.0
2023-05-14T13:52:52.203Z
Also, shouldn't the reference be in a spot with no brain activity? Is one of the two electrodes used as reference or is there still a dedicated reference electrode? If the former is true, does it make sense, since brain activity at the reference moves the reference thus affecting the difference between the two?
David Cian
CC BY-SA 4.0
2022-01-07T20:02:53.860Z
2022-01-07T20:02:53.860Z
AliceD
<p>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.</p>
<p>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?</p>
<p>At this point I feel the need to mention there is <a href="https://psychology.stackexchange.com/q/20648/26809">another question</a> 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).</p>
<p>Additionally, <a href="https://ieeexplore.ieee.org/document/5627451" rel="nofollow noreferrer">this paper</a> mentions that bipolar recordings are generally known to be more robust to noise than monopolar recordings: why would that be the case?</p>

<neuroscience><eeg><electrophysiology>
What are the differences between bipolar channels and monopolar channels for EEG?
David Cian
# What are the differences between bipolar channels and monopolar channels for EEG?

(Asked by: username_0 on 2022-01-07)

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](https://psychology.stackexchange.com/q/20648/26809) 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](https://ieeexplore.ieee.org/document/5627451) mentions that bipolar recordings are generally known to be more robust to noise than monopolar recordings: why would that be the case?

**Tags:** neuroscience, eeg, electrophysiology


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## 1 answers

### Answer by username_1 on 2022-01-07. Score: 3 | Accepted answer

In general, closely situated electrodes work well to eliminate environmental noise, at least when a **[differential amplifier](https://www.electronics-tutorials.ws/opamp/opamp_5.html)** 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)](https://www.bcia.org/files/public/CE%20Articles/Saabbiof-36-04-128-133.pdf).

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)](https://www.bcia.org/files/public/CE%20Articles/Saabbiof-36-04-128-133.pdf).

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](https://pubmed.ncbi.nlm.nih.gov/21096910/); [Stronks *et al*., 2009](https://iovs.arvojournals.org/article.aspx?articleid=2189069); [Stronks *et al.*, 2016](https://pubmed.ncbi.nlm.nih.gov/26743880/)). 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.

**References**  
**-** [Diez *et al*., *Annu Int Conf IEEE Eng Med Biol Soc* (2010): 5803-6](https://pubmed.ncbi.nlm.nih.gov/21096910/)  
**-** [Saab, *Biofeedback* (2009); **36**(4): 128–33](https://www.bcia.org/files/public/CE%20Articles/Saabbiof-36-04-128-133.pdf)  
**-** [Stronks *et al*.,*IOVS* (2013); **54**:3891–901](https://iovs.arvojournals.org/article.aspx?articleid=2189069)  
**-** [Stronks *et al*., *Doc Ophthalmol* (2016); **132**:1–15](https://pubmed.ncbi.nlm.nih.gov/26743880/)

> **Comments:**
> **username_0**: If I understand correctly, closer together means you can pick up localized artifacts on both thus enabling the differential amplifier to remove them (good), but then you also pick up similar activity from nearby areas due to volume conduction (not good).  
> **username_0**: Also, shouldn't the reference be in a spot with no brain activity? Is one of the two electrodes used as reference or is there still a dedicated reference electrode? If the former is true, does it make sense, since brain activity at the reference moves the reference thus affecting the difference between the two?  

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