questions on Cyton SRB1/SRB2, BIAS and the OpenBCI electrode cap

DAL12DAL12
edited February 2020 in Headware

I'm currently working on a project and I am using the following tools:

However, I have some questions about how to use Daisy + Cython alongside the electrode cap, that haven't been answered by reading the documentation.

  1. In the electrode cap there is an electrode labelled as REF which is supposed to be used as reference and corresponds to CZ. What exactly does the software do with the signal collected by this electrode? Does it subtract it's signal from the signals collected from other channels?
  2. According to what I read in the documentation the y-splitter (connected to SRB1 and SRB2) is supposed to be connected to the reference (by using the cable labelled as REF in the electrode cap) and the BIAS to any other channel to provide noise cancelling. What is the difference between them (SRB1/2 and BIAS)? What does the software do differently with the signals collected by each one?
  3. What is the ideal channel to connect the BIAS to? Is it worth it using extra electrodes to connect it to the ear lobe?
  4. Does the software apply any sort of ocular, muscular or cardiac artefact removal algorithms or techniques?
  5. Is it possible to collect viable information without using the reference (I was thinking of choosing references and make calculations with them after collecting the data)?

Comments

  • wjcroftwjcroft Mount Shasta, CA
    edited February 2020

    Dora, hi, thanks for your questions.

    It does look like the documentation on the electrode cap needs a bit more clarification.

    1. Yes, the reference electrode in EEG is the common reference for all other channels and is subtracted with the differential amplifier in the Cyton, from the channel signals. So you are measuring the difference between reference and channel. On the cap in the store, I can see in the photos that the REF electrode is at position CPz in the diagram shown on the doc page. So no, it is not at position Cz as you state, but CPz instead. Actually it is your choice as to where you place your reference electrode, there are other choices available in EEG, such as ear lobes, "linked ear lobes", etc.
    2. No, the Y-splitter is connected to the TWO SRB2 pins, one on the Cyton mainboard, and the other on the Daisy. SRB1 is a different pin and not used in this case. Just follow the doc instructions and you will be ok. SRB2 pin is the default common reference pin. BIAS is covered in your next question.
    3. BIAS is the same as 'Ground' in other EEG systems. Again you can connect it wherever you like. But on this cap the Ground wire goes to position AFz on the map shown on the docs page. This is a fine and suitable place for Ground, and AFz is used for this ground connection on many other caps. No, BIAS on a cap like this usually goes to AFz and not ear lobes.
    4. No automatic artifact removal is done, that instead should be done by your software. The GUI does not do this for you.
    5. As mentioned there are other referencing schemes, and ways to "re-reference" from your data collection / acquisition reference, to any other reference. Such as, these methods. EEGLAB has a re-reference function.

    Regards, William

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