question on using BOTH top and bottom pins for EEG?

Alexandr56Alexandr56 New York
edited January 30 in Cyton

I was recently going through my clubs old documentation and they had electrodes listed like this "C4_Cz" Researching it a bit more it seems like they were using both the top and bottom electrode pins for N1P-N8P. When looking through Cyton documentation i couldn't find anything about using both the top and bottom electrode connection pins. I was wondering if there is any reason to use both the top and bottom pins. Do signals come in with less noise? For reference were using two ear clips for the bias and SRB.

Comments

  • wjcroftwjcroft Mount Shasta, CA

    Hi Alex,

    Most EEG is done with a single electrode (typically ear lobe or AFz or CPz) acting as the common reference for all the active channels. In the default configuration of the Cyton ADS1299 chip, all of the top pins (IN1P through IN8P) are bussed together and appear as the SRB2 pin (bottom SRB pin). Then the scalp channels are connected to the bottom pins, IN1N through IN8N. With this default configuration it would be a mistake to connect anything to the top pins as they are already being bussed to SRB2.

    Another type of EEG 'montage' is called sequential / bipolar, but it is only rarely used in certain medical conditions. It is not ever used in the BCI / neurofeedback field.

    https://openbci.com/forum/index.php?p=/discussion/1840/cyton-bipolar-montage

    In that case, yes both pins would be used on each channel and in addition the ADS1299 must be configured in the GUI to turn off the SRB2 bussing feature as well as turning off the Bias injection.

    Regards, William

  • Alexandr56Alexandr56 New York
    edited January 31

    Ah thats starting to make sense thanks for the response! For reference im using the eletrodes that are labeled:
    O2_Oz
    C4_Cz
    F4_Cz
    Fp2_fpz
    Fp1_fpz
    f2_Cz
    C3_Cz
    O1_Oz
    Ill probably be only using the first parts as the second parts were referred to as "Reference" electrodes in a old training video. Just to confirm there no way or reason to connect the Cz Oz fpz pins for a motor imagery data collection project along side these ones. As for reference electrodes i always see two earlobe clips for most projects set up one on Bias one on SRB is this the proper way as it seems like signals are cleaner when using them both.

    Heres a picture of how they had set up their headset.

  • wjcroftwjcroft Mount Shasta, CA
    edited January 31

    If the notation you show is 'electrode position' _ 'electrode reference'

    Then the general observation is that electrodes close together, such as Fp1 Fpz and Fp2 Fpz, will have very low amplitude because of their close proximity. You just cannot get much potential differences over close locations like that.

    I'm not seeing any advantage in using this custom EEG montage. Better to stick with the ear reference. It is standard in the field. Some caps use either AFz or CPz as a reference instead of the ear.

  • Ah ok thanks for the help. Ill be using the openbci fabric cap for this I dont think they have those but they do have some kind of ground and bias pins that ill use. Either way thanks for the help!

  • wjcroftwjcroft Mount Shasta, CA

    The cap in the shop 'labels' (on the tag) CPz as the reference. The 'Ground' connection on the cap goes to another location, I forgot which. You can still use your ear clips and place reference on one of those. Actually for the Bias injection position, midline works better as far as the mains cancelation. So you could use CPz or other 'z' site.

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