<blockquote>
  <p>For the unipolar configuration, why can't we just use Earth ground as
  our reference instead of having a reference electrode or performing an
  average of measurements on all the electrodes?</p>
</blockquote>

<p>The ground has a different function than the reference. The reference is the electrode that the voltage is recorded against; a voltage is a potential difference so you need two electrodes. The ground is used for common mode rejection. The primary purpose of the ground is to prevent power line noise from interfering with the small biopotential signals of interest (source: <a href="https://www.biopac.com/knowledge-base/ground-vs-reference-for-eeg-recording/" rel="nofollow noreferrer">Biopac</a>). </p>

<blockquote>
  <p>For the bipolar configuration, if the electrodes are too close to each
  other, wouldn't we measure very low voltages since much of the signal
  appearing on one electrode would also be picked up by the other?</p>
</blockquote>

<p>Yes. The smaller the distance, the better the rejection of unwanted signals. But with that, you're also risk to throw away the baby with the bath water. </p>

CC BY-SA 4.0
2018-09-09T22:20:20.327Z
Thank you for your response, @AliceD. Why can't ground be used both for rejecting a common signal *and* as a reference? For example, if ground and the signal on an EEG lead were fed into the inverting and noninverting inputs of a differential amplifier, respectively, wouldn't the output be the difference between the lead and ground, with the common mode signal (such as 60 Hz noise) greatly attenuated/removed? Or is it important to have a reference electrode to pick up artifacts that show up on all all electrodes but not on ground?
Vivek Subramanian
CC BY-SA 4.0
2018-09-10T07:35:13.453Z
The reference is subtracted from the recording electrode. The ground is used for both
AliceD
CC BY-SA 4.0
2018-09-13T12:07:42.947Z
The amplifier might record the signal in such a way: `Signal = (electrode - ground) - (reference - ground)`, which simplifies to `Signal = electrode - ground - reference + ground` .. and then you see that the ground signal disappears (the common mode is rejected): `Signal = electrode - reference`
S.A.
CC BY-SA 4.0
2018-09-09T22:04:18.197Z
2018-09-09T22:04:18.197Z
AliceD
<p>When performing EEG, a number of electrodes are placed on the scalp to measure the electric potential difference between those locations and a reference. In the unipolar configuration, the reference is often the signal on another electrode (and is common to all measurement electrodes) or an average of the all measurement electrodes. In the bipolar configuration, the reference is specific to each measurement electrode (and is often another electrode located nearby). For examples, see <a href="https://youtu.be/86zIa3pGM50" rel="nofollow noreferrer">here</a> and <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5479869/" rel="nofollow noreferrer">here</a>.</p>

<p>Questions:</p>

<ol>
<li>For the unipolar configuration, why can't we just use Earth ground as our reference instead of having a reference electrode or performing an average of measurements on all the electrodes?</li>
<li>For the bipolar configuration, if the electrodes are too close to each other, wouldn't we measure very low voltages since much of the signal appearing on one electrode would also be picked up by the other?</li>
</ol>

<measurement><eeg><electrophysiology><signal-processing>
Reference signal when measuring EEG
Vivek Subramanian
# Reference signal when measuring EEG

(Asked by: username_0 on 2018-09-09)

When performing EEG, a number of electrodes are placed on the scalp to measure the electric potential difference between those locations and a reference. In the unipolar configuration, the reference is often the signal on another electrode (and is common to all measurement electrodes) or an average of the all measurement electrodes. In the bipolar configuration, the reference is specific to each measurement electrode (and is often another electrode located nearby). For examples, see [here](https://youtu.be/86zIa3pGM50) and [here](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5479869/).

Questions:

1. For the unipolar configuration, why can't we just use Earth ground as our reference instead of having a reference electrode or performing an average of measurements on all the electrodes?
2. For the bipolar configuration, if the electrodes are too close to each other, wouldn't we measure very low voltages since much of the signal appearing on one electrode would also be picked up by the other?

**Tags:** measurement, eeg, electrophysiology, signal-processing


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## 1 answers

### Answer by username_1 on 2018-09-09. Score: 2

> For the unipolar configuration, why can't we just use Earth ground as
> our reference instead of having a reference electrode or performing an
> average of measurements on all the electrodes?

The ground has a different function than the reference. The reference is the electrode that the voltage is recorded against; a voltage is a potential difference so you need two electrodes. The ground is used for common mode rejection. The primary purpose of the ground is to prevent power line noise from interfering with the small biopotential signals of interest (source: [Biopac](https://www.biopac.com/knowledge-base/ground-vs-reference-for-eeg-recording/)).

> For the bipolar configuration, if the electrodes are too close to each
> other, wouldn't we measure very low voltages since much of the signal
> appearing on one electrode would also be picked up by the other?

Yes. The smaller the distance, the better the rejection of unwanted signals. But with that, you're also risk to throw away the baby with the bath water.

> **Comments:**
> **username_0**: Thank you for your response, @username_1. Why can't ground be used both for rejecting a common signal *and* as a reference? For example, if ground and the signal on an EEG lead were fed into the inverting and noninverting inputs of a differential amplifier, respectively, wouldn't the output be the difference between the lead and ground, with the common mode signal (such as 60 Hz noise) greatly attenuated/removed? Or is it important to have a reference electrode to pick up artifacts that show up on all all electrodes but not on ground?  
> **username_1**: The reference is subtracted from the recording electrode. The ground is used for both  
> **username_2**: The amplifier might record the signal in such a way: `Signal = (electrode - ground) - (reference - ground)`, which simplifies to `Signal = electrode - ground - reference + ground` .. and then you see that the ground signal disappears (the common mode is rejected): `Signal = electrode - reference`  

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