How far apart do ground, ref, and signal electrodes have to be?

Typically I've seen ground and reference attached to the earlobes but if I were to place all three electrodes (ground, reference, and signal assuming a single-channel system) right next to each other, would there be any deterioration to the signal quality?

Comments

  • wjcroftwjcroft Mount Shasta, CA

    Hi HP,

    It's generally not a good idea to 'short' ground and reference together. However I have seen some systems (NeurOptimal) that place the electrodes side by side on the same ear.

    You definitely do not want to place the active / signal channel electrode near ground or reference, because EEG is a differential signal and this would produce very low amplitude EEG.

    In quite a few electrode caps, they place ground on the midline, for example at AFz or CPz, because ground (Bias) in many systems injects a counter-mains signal to reduce mains artifacts. Putting it on the midline gets this radiating more evenly.

    Similarly reference can be also placed on the midline at an alternate unused site. And the cap in the Shop does this.

    William

  • edited February 21

    Hi William,

    Thank you for the quick response! That makes sense. I see that the cEEGrid has the ground, reference, and signal fairly close together in their around ear electrode placement. Would this cause noisy signals?

  • wjcroftwjcroft Mount Shasta, CA

    They mention ground and ref on R4a and R4b,

    So in that configuration, you cannot get larger distance because of the constraint of the shape. Yet, they still got decent signals.

    https://www.frontiersin.org/articles/10.3389/fnhum.2017.00163/full

    I've not looked in depth at the paper, do they use the right ear ground and ref to differentially measure left ear signals? I assume so because the figure caption does not mention a left ear ground and ref.

  • edited February 21

    That’s interesting, would make sense to use both left and right ears to get a reference baseline. I’ll look into the paper more, thank you!

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