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In a typical 10-20 system there are 21 electrodes placed on the scalp. However, this does not mean there are 21 distinct "channels" or voltage sources.

I've heard that some of these electrodes are "reference" electrodes, while others are "active" electrodes. Sometimes electrodes can setup in a "bipolar" or "differential" fashion.

  • How many actual channels (that is, distinct input sources) will there be?
  • What's the difference between reference and active channels?
  • How does this bipolar/differential setup work differently than a "normal" setup?

Update

Here is my updated understanding based on @Christiaan's answer below:

time (t)    F2v    F3v        F2-F3         F3-F2
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1           2      1            1             -1
2           3      6           -3              3
3           5      3            2             -2

So, if my understanding is correct, then at time t=1, the voltage of the F2 electrode might be, say, 2 (units; volts, microvolts, whatever), and the voltage of the F3 electrode might be 1. If F2 is the arbitrary active electrode, then the potential difference between F2 and F3 is 2 - 1 = 1. But, if F3 was the active electrode (and F2 the reference), then the potential difference would be the inverse (that is, 1 - 2 = -1).

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enter image description here
EEG 10-20 system. source: Wikipedia

  • How many actual channels are there?

21 in the figure, i.e., the number of active electrodes.

  • What's the difference between reference and active channels?

The active electrode is the electrode under investigation. Basically this is arbitrary. Consider electrode F2 and F3. When you measure the potential difference between them and F2 is the active electrode and F3 the reference, the signal will be exactly identical as in the reverse situation, only the polarity (the sign of the voltage) is reversed. That's all there is to it. The active electrode is typically the electrode linked to the location you wish to record. The reference is elsewhere.

  • What is the difference between a bipolar and a normal (unipolar/monopolar) setup?

In a regular unipolar setup, each of the electrodes is either measured against a distant reference (e.g. the earlobe) or against the aggregate of all the electrodes. Either way, the reference is distant. This means that the signal will be high, but artifacts will be large too, because artifacts from both the electrodes will be merged into the signal. Consider a reference in the neck; neck musculature activity will add artifacts (EMG) to the signal. Likewise, a reference in the chest area will add ECG to your signal.

In a bipolar setup adjacent electrodes are recorded. This means that large artifacts occurring through eye blinks or whatsoever are recorded by both electrodes and as their polarity is opposite, the large artifact responses will be cancelled and the signal is much cleaner. However, the downside of the coin is that commonalities in the signal, which will be larger when the electrodes are closer will also be subtracted and hence your signal will decrease accordingly.

I have used bipolar setups when measuring eERGs and eCAPs, i.e., electrically evoked activity. The electrical stimuli generated large artifacts and actually so large that their amplitudes were approximately similar between active and reference electrodes. The reduction in artifact by far outweighed the loss in signal. It depends on the situation you are at.

An authoritative work in this field is listed below.

Reference
- Pivik et al., Psychophysiol (1993); 30: 547-58

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    $\begingroup$ As always, thanks @Christiaan (+1)! And, as always, a few followup questions for you (if you don't mind)! (1) Please see the update to my question. Regarding your explanation of active vs. reference electrodes, can you please confirm that my understanding (based off the table I provided) is correct? $\endgroup$
    – smeeb
    Apr 7, 2016 at 12:50
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    $\begingroup$ (2) Whereas I now understand the cons of unipolar setups, I am failing to understand the cons of the bipolar setup. Using the table I provided above, are you saying that - under a bipolar setup - the electrodes are close together and hence will have similar voltages? Hence F2 and F3 might both measure very close to one another (say, F2=2.6 and F3=2.5), hence the potential difference between them (+/- 0.1) will be very small? Is this what you're saying?! $\endgroup$
    – smeeb
    Apr 7, 2016 at 12:50
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    $\begingroup$ ad(1): My pleasure! The table is kind of OK as a simplification scheme, but you should understand a voltage is always a potential difference. On 1 electrode there is no such thing as a voltage of -3. Voltages exist across points. An electrode placed on the positive pole of a battery and the reference in the air yields 0V. $\endgroup$
    – AliceD ♦
    Apr 7, 2016 at 12:53
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    $\begingroup$ ad(2): The signal may be smaller in bipolar. The noise as well, simply because the electrodes (may) have overlapping areas of interest. $\endgroup$
    – AliceD ♦
    Apr 7, 2016 at 12:54
  • $\begingroup$ Sorry I forgot the +1 @Christiaan, I added it! Also +1 for both comments above (thanks!). So it sounds like in my #2 followup above that my understanding is correct (that is, that because the electrodes are close, their voltages will be nearly equal and opposite, which makes using their data more difficult). One last followup question here, I promise! $\endgroup$
    – smeeb
    Apr 7, 2016 at 13:00

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