what is the relationship between high impedance on EEG channels and 50/60hz noise?

khofstadterkhofstadter Colchester, U.K.
in Electrodes
Hello! 

What is the relationship between high impedance on EEG channels and 50/60hz noise?

For instance, can a high impedance on one EEG channel e.g. on PZ add to the amplitude of the 50/60hz noise?

Or, can a not well 'grounded' GND channel add to the impedance of a EEG channel e.g. PZ? 

Thank you! k


Comments

  • wjcroftwjcroft Mount Shasta, CA
    Kris, hi.

    Did you read the paper by EGI scientist Tom Ferree?


    EGI makes clinical and research grade saline electrode "geodesic mesh" caps.


    The Greentek saline cap you are using has many similarities to what EGI is doing. Hence if your measured impedance (with Cyton) is 40K ohms or less, you should be fine. Could probably be even somewhat higher, since the ADS1299 chip has a 1 gigaohm input impedance. The higher the amp input impedance, the less important to comply with the (previous generation EEG equipment) requirement for 5K ohm skin impedance. At one point EEG equipment had input impedances in the low megohms range (say 1 to 10 megohms). This is no longer the case with amps that have input impedance in the hundreds or thousands of megohms.

    Yes, if your skin impedances are way out of range, they will increase noise. But also be sure to use a notch filter at the mains frequency.

    The Bias/Ground lead on the ADS1299 is not intended to be connected to a mains wall ground pin. But it does help to center the differential amps, and in the case of 'Bias', inject some inverse signal to counteract common mode noise. Your scalp ground connection is usually best positioned on the midline. Electro-cap uses AFz position. 

    Regards,

    William

  • khofstadterkhofstadter Colchester, U.K.
    Thanks William, 

    I tried to read this paper (again), but the physics at the moment in there are too high for me. Hopefully I can make some time in the future to understand more. 

    Thanks for all the info. Yes, the ground is on AFz on the Greentek S1 cap, for REF I use Cz. I only seem to have two active electrodes: P3 and F4. 
    http://www.greenteksensor.com/eeg-caps/gel-free-eeg-electrode-cap/

    More questions soon.. 

    Cheers, k


  • wjcroftwjcroft Mount Shasta, CA
    Kris, hi.

    What Ferree's paper is saying, in summary, is that skin impedance can be much higher than the old 5 Kohm standard -- provided that the EEG amplifier is a modern 'high input impedance" type. This is true for OpenBCI, as the ADS1299 is 1 gigaohm input impedance. Older amps could be in the low hundreds of megohms.

    Results: There was no significant change in amplitude of any EEG frequency as scalp-electrode impedance increased from less than 5 kohm (abraded skin) to 40 kohm (intact skin). As expected, 60 Hz noise increased linearly as a function of the absolute impedance and impedance mismatch between the measurement and reference electrodes.

    Conclusion: With modern high input-impedance amplifiers and accurate digital filters for power line noise, high-quality EEG can be recorded without skin lesions.

    Your Greentek saline cap, I'm pretty sure is all passive electrodes, no actives. Active electrodes have a small circuit board behind each location, and additionally require special power supply and cabling.

    Regards,

    William

  • khofstadterkhofstadter Colchester, U.K.
    thanks William, 

    yes, I the Greentek has passive electrodes, no small circuits on the cap. 

    I was told that science papers ideally want you to have your impedance around or lower than 5 Kohm. Is this still true or was this relevant only in the past? 

    Cheers, k
  • wjcroftwjcroft Mount Shasta, CA
    edited July 2019
    As Ferree points out in the paper (summary in my last post), 5K 'standard' was created when EEG amplifiers had relatively poor input impedance. Ferree works for EGI, which makes saline based geodesic mesh caps for clinical and research use. My understanding is that the groups using the EGI caps are satisfied with the skin impedance provided by their caps.


    EGI is now part of Philips group, which has huge investments in medical equipment and EEG.
  • khofstadterkhofstadter Colchester, U.K.
    Thanks, k
  • wjcroftwjcroft Mount Shasta, CA
    https://www.google.com/search?q=philips+eeg

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