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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 and here.

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?
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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).

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.

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    $\begingroup$ 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? $\endgroup$
    – Vivek Subramanian
    Sep 9, 2018 at 22:20
  • $\begingroup$ The reference is subtracted from the recording electrode. The ground is used for both $\endgroup$
    – AliceD ♦
    Sep 10, 2018 at 7:35
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    $\begingroup$ 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 $\endgroup$
    – S.A.
    Sep 13, 2018 at 12:07

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