Greentek Gelfree-S EEG Cap

khofstadterkhofstadter Colchester, U.K.
edited May 2019 in Headware

Hello!

If/when you have a moment could you please check this video (below posts) where I test an EEG cap?


image


It would be good to know whether you think it looks reliable or
not. 

Has any one used this cap before?

http://www.greenteksensor.com/eeg-caps/gel-free-eeg-electrode-cap/



Thanks, k

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Comments

  • wjcroftwjcroft Mount Shasta, CA
    K, wow.

    Where did you purchase, and what is the cost?

    I've used saline based EEG for many years, with home build equipment. But this looks like a big win if the price is reasonable.

    In general, most users don't need the added cost of sintered Ag-AgCl electrodes, usually these are very expensive. But if they've found a way to do it affordably, amazing. 

    I have yet to look at your video, will add some comments.

    Great find, thanks for reporting this.

    William

  • wjcroftwjcroft Mount Shasta, CA
    edited July 2018
    K, yeah looks decent.

    Can you compare amplitudes of the same sites you are using between the new cap and say gold cup with paste? Generally saline systems using Ag-AgCl are interchangeable with gel or paste based systems. And Ag-AgCl is actually superior to gold in terms of stability / signal quality.

    Please add some more info on pricing and dealers. Do they have a US seller?

    William
  • wjcroftwjcroft Mount Shasta, CA
    Here is their paper showing the electrode structure. Is this the same you have?


    The 'S' in the product name, I'm pretty sure stands for 'Saline'. There is a long history of these type systems. Here is my page showing some of the other commercial units as well as the home built using velcro.


    The most widely known manufacturer of these in the US, is EGI, with their Sensor Net systems. 

    Can you explain how the saline is applied, and what kind of maintenance of the electrodes is done before and after sessions? Do the sintered electrodes need replacing periodically? Sponges, etc.? 

    If this has good pricing it will be a big win.

    Regards,

    William

  • wjcroftwjcroft Mount Shasta, CA
    This system can also be used with the same OpenBCI Ag-AgCl ear clips used for the Ultracortex. So in that case you don't need to put reference and ground on the scalp. If you want to ensure the same kind of moist contact with the ears, you can use a small piece of chamois cloth between the ear lobe skin and ear clip. That's explained in my saline site page above.

  • khofstadterkhofstadter Colchester, U.K.
    hi William,

    the cap was bought in 2017 June from China for $415 inc.10% student discount. Customer service was 5*s. It was shipped from China to the UK, arrived fast.
    I only started using it today. The quality of the unit is in my opinion is very good, but then I haven't really used anything like this before.

    The link the to cap is:
    http://www.greenteksensor.com/eeg-caps/gel-free-eeg-electrode-cap/
    Instructions with more info:
    http://www.greenteksensor.com/wp-content/uploads/2017/02/GREENTEK-GELFREE-CAP-USE-INSTRUCTION.pdf
    // so I think the cap is different to the one in the paper you linked above;

    Today I was suggested that 50hz in the FFT plot looks still too high even after the notch filter is applied.

    Now I got interested in measuring impedance. Do I understand this right, can I measure this physically as well as with software?

    Good idea to use the earclips instead - I now need to buy some.

    I might even try putting a gold cup electrode next to a saline electrode to see the difference in reading.

    Thanks for the links, I'll study your google pages.

    Best, k



  • wjcroftwjcroft Mount Shasta, CA
    edited June 2019
    The EGI saline nets are used very widely in clinical and research applications. So I would have confidence that this Greentek sensor system produces similar results,


    I really don't think you need to bother with the gold cup comparison. They've done some of that in the paper pdf link previously given.

    Yes the GUI can report impedance. Pressing on the little 'ohm' symbols to the left of each time series will turn that on.


    EGI found that with modern EEG amps, skin-electrode impedance is not really an issue. Cyton ADS1299 amplifier has 1 gigaohm (1*10^9 ohms) input impedance, which is excellent, and even higher/better than they used in the EGI paper,


    ----

    I looked through the user instructions. How easy have you found it, in terms of applying, then storing? Perhaps you could post (Google Drive) a photo or so of what the electrodes look like up close, disassembled, etc. Did they supply 24 electrodes? I think you got the sintered Ag-AgCl. There might be an option for lower cost plain Ag-AgCl. I don't believe EGI uses sintered, just plain Ag-AgCl.

    The paper they posted (previous link) shows plastic electrode holders with hollow "comb tips" at the scalp side, so as to go through thick hair. It looks like they dropped that feature, and the scalp side is now flat? Almost all dry contact EEG these days use comb tips.

    William

  • wjcroftwjcroft Mount Shasta, CA
    edited July 2018
    Sintered electrodes are known to be subject to easy contamination.


    Found this manual for the EGI,


    So on page 41: "Inside each sensor is a sintered silver/silver chloride or silver chloride–plated carbon pellet connected by a 1-meter-long insulated lead wire.." So they might use either type. Sintered lasts longer, but has tighter maintenance requirements.

  • khofstadterkhofstadter Colchester, U.K.
    Thanks William,

    I did some more tweaking and perhaps the EEG looks better now, but I'm not certain.
    I added some unedited videos to a Youtube list, the impedance cane be seen in here.

    Usually between 15-20KOhm, is this OK?

    Greentek (Bennie) has replied in an email concerning proper usage, contamination, etc. I'll find time to make another video this week. He also wrote that the cap I use is first generation and that new they have a 'better', third generation one.

    Could you please let me know the exact name of the earclips I could order? It would be good to see how they work instead of the GND and REF electrodes on the cap.

    Thanks, k

  • wjcroftwjcroft Mount Shasta, CA
    edited July 2018
    Impedance you mention is fine for the Cyton.

    FRI is the source of the comb / cone electrodes used in the Ultracortex. Here is their ear clip, seems a bit pricey, there may be other ones, look around.


    The ones I use are mentioned on my saline sensor website.


    With the STENS, you also need a female ECG type snap connector, one for each earclip.
  • khofstadterkhofstadter Colchester, U.K.
    Hi William and forum,

    Here a quick video on the setup:

    I will make some more screen-recordings with impedance tests, 50hz checks, EEG tests asap.

    Thanks! k
  • wjcroftwjcroft Mount Shasta, CA
    Kris, great video, thanks for recording that.

    Regarding the 50 hz mains noise: this can sometimes be reduced by getting any transformers or power bricks out of your vicinity. I've even seen the noise go down by simply taking the power brick off the same table with the laptop, and setting the brick on the floor. Since my table had some metal frame components, those were picking up the EMF emissions from the brick and making them stronger / wider in area affected. Setting the brick on the floor eliminated that coupling effect.

    One small note on your pronunciation of the "electrode head" plastic piece. In the video you pronounce 'head' several times. But initially (before I corrected my hearing) -- it sounded like you were saying 'hat' (short 'a' sound, rhymes with 'cat'). Instead of 'head', short e sound, which rhymes with 'bed'. It was fine once I realized my ear tuning bias.  :-)

    Thanks again for bringing this cool system to our awareness.

    William

  • khofstadterkhofstadter Colchester, U.K.
    Many thanks William,

    I'll added 'moving power brick' to the ground on the list. This is what I have at the moment to test:

    move electrodes around - make sure skin is wet;
    shoes (maybe different ones or no shoes is better);
    mics on the computer - does it have feedback?
    sweaty head?;
    cable lengths;
    orientation - move around;
    switch off all plugs/extension leads;
    unplug the laptop and run it on batteries;
    unplug other cables from laptop;
    move power brick to the ground;
    touch the laptop case when connected to power and when not;
    take a few steps away;
    shielding (?)
    add more water to the electrodes;
    check dongle in computer or USB slitter; 

    Can you add to this?

    So far I think best is to keep the electrodes wet.

    Also, Is impedance and 50hz hum related? I guess not, but I'm not sure.

    Thank you!

    p.s: yes, my pronunciation needs to improve! Sorry :)

  • wjcroftwjcroft Mount Shasta, CA
    Kris, hi.

    The cap instructions are wise when they advise using a q-tip or other soft stick type object, to move the hair away from the electrode cap hole, so that more scalp is exposed. This means the saline goes through fewer layers of hair to reach the skin.

    re: mains noise in general, just try changing your equipment and subject location around to various parts of the room or other rooms. In addition to the power bricks and transformers, extension cords, etc. There are also conduits inside the walls and floors that you might happen to be next to. Your shoes are not a concern.

    Running the laptop on battery can help sometimes, but is not a requirement for reducing mains noise. The mains noise is propagated as a radiant field, surrounding the bricks or wires. So just getting a few feet away from the source usually reduces levels substantially. Shielding usually is not required.

    re: impedance and mains noise, yes this can contribute, especially if the impedance is in the high range, say over 100K.

    Cheers,


  • khofstadterkhofstadter Colchester, U.K.
    Hi William,

    Thanks for all you help. I'll need to study some physics soon.

    I summed my thoughts:



    Impedance has been between 15-20KOhm usually since I started with this project.

    thanks, k

  • wjcroftwjcroft Mount Shasta, CA
    Bennie Deng at Greentek, sent Kris and I this email with a new tutorial.

    ----

    Dear Krisztian,

    I'm really sorry for having you both wait so long for my tutorial video. Really sorry.

    Yesterday we made a very "simple" video, showing you how to install the electrodes and how to prepare the scalp to get a low impedance. Please refer to this video:



    As the OpenBCI board we have is not the latest, we cannot check the impedance, so we just used our BrainProducts amplifier to demonstrate that if preparing the scalp and installing the electrodes correctly, you could get the impedance down in very short time. 

    After checking the impedance we then switched to OpenBCI board and did some tests, we recorded eyes open, eyes closed and eyes blinking EEG, and you could see the signal quality is good and AC 50Hz Noise is also quite low. 

  • khofstadterkhofstadter Colchester, U.K.
    edited January 2019
    Hello William and forum,


    I am currently trying to understand good practice for referencing. With the current NF protocols I plan to read Fz and Pz.

    With the Gelfree Greentek cap I have REF between Cz and Pz and GND between Fz and Fp1/Fp2 on the 10-20 system. So Pz is much closer to REF than Fz. Can this be an issue?

    Thanks! k


  • wjcroftwjcroft Mount Shasta, CA
    Kris, hi.

    Here is a common practice with neurofeedback protocols. They use what is called "linked ears" for reference. You use a 'Y' cable to connect both the left and right ear clips wires together, and that becomes your reference. Then Ground / Bias can be placed anywhere on the head. Typically on a midline location. The AFz (10-10 map) you are using is actually a cap location for ground in the common Electro-cap used in neurofeedback, QEEG, etc.

    You are correct, placement of the reference will influence signal amplitudes. So linked ears would give more consistent results than CPz. Particularly if you are comparing Fz and Pz band amplitudes.

    Regards,

    William
  • khofstadterkhofstadter Colchester, U.K.
    edited January 2019
    Many thanks William,

    Is it possible to use another cap location for REF to achieve better results? Perhaps Cz, which seems to be the same distance from Fz and Pz? Then, I would simply need to connect Cz to SRB2 on the board.

    I am not sure, but I imagine if I connected two ear-clips with the Y cable to SRB2 to reference, perhaps the way these ear-clips measure is not the same how the saline electrodes do on this cap - or am I worried to much here?

    Thank you! k
  • wjcroftwjcroft Mount Shasta, CA
    Yes, in your situation Cz would be better than CPz. However, note that with the reference on the head, you are going to get somewhat lower signal amplitudes that if you used one or both ear lobes as reference. That might be ok in your situation. The 'standard' used in neurofeedback is linked ears, and should work fine with whatever ear clips you have.

    But in all honesty, the original way you were using it, with reference on one ear, and ground the other, would be almost as good as linked ears. Linked ears is mostly encouraged because it is more neutral way for measuring signal level from L/R hemispheres, for example consider measuring C3 vs C4 or F3 vs F4. If you only use one ear as reference, the site closer to the ear will have lower amplitude. But in your case, measuring Fz and Pz, those are on the midline. So using one ear as reference will not favor one side over the other.

    Cheers,
  • khofstadterkhofstadter Colchester, U.K.
    edited January 2019
    Thanks William. Could you please clarify this? :

    "However, note that with the reference on the head, you are going to get somewhat lower signal amplitudes that if you used one or both ear lobes as reference."

    With 'lower signal amplitudes' do you mean
    1) lower signal amplitudes of actual locations I'd like to measure i.e. Fz and Pz or
    2) lower signal amplitudes for the reference i.e. SNRB2?

    Thanks!
  • wjcroftwjcroft Mount Shasta, CA
    #1, because with the reference at Cz, it is only a short distance to Fz or Pz. The shorter the distance between the differential amplifier electrodes, the lower the microvolts. Because both + and - electrodes are getting inputs from the roughly the same cortical areas. 

    With reference at one or both ears, you are essentially 'off' the cortex, hence longest distance. (This is not actually the case, the ear lobe / ear is closer to temporal cortex.) But as I've said, linked ears is the most common reference in neurofeedback.

  • khofstadterkhofstadter Colchester, U.K.
    great - thanks.
    do you happen to have any links to resources, where I could study this, the types of referencing, etc. ?
    thanks, k
  • wjcroftwjcroft Mount Shasta, CA
    Shows effect of different montages,


    Wikipedia list of common montages,

  • khofstadterkhofstadter Colchester, U.K.
    perfect, thanks. k
  • wjcroftwjcroft Mount Shasta, CA
    Kris, hi.

    I was just reminded of your Greentek saline cap usage today, by receiving an update from this project working on a Lucid Dreaming application.


    Do you have any sense if the sponges were loaded up quite full with saline, that the EEG would remain usable over a night's 8 hour sleep time? Perhaps to keep the moisture in, one might also fasten some type of moisture barrier over top of the Greentek cap. Such as the loose fitting woman's shower caps.


    Obviously the impedance per electrode is going to vary as the sleeper rolls the head around on the pillow overnight, but with the saline sponge contact -- it might remain within reasonable range.

    Also you mentioned at one point you were going to be getting the latest Greentek version. How did that go?

    I'm going to point the lucid team here to your thread on the forum in case they want to follow up. Not sure if they have logins here yet.

    Regards,

    William

  • khofstadterkhofstadter Colchester, U.K.
    edited May 2019
    Thanks William, 

    Nice project. 

    I usually use the sponges for a couple of hours max. I'll try a bit longer next time... 

    I still plan to buy the new cap, probably this Summer. 

    @ Peter Michael Mross and @ Joachim Kildau - please email if you have any questions. 

    Cheers, k
  • PeetPeet Germany
    Hi Kris

    thank you! I have the possibility to test and use the S3 Version of the cap, an institution who supports the project is buying it for research purposes. However, china doesn't respond to their mails, I guess it will take some time ;D

    Regards
    Peter
  • khofstadterkhofstadter Colchester, U.K.
    Hi Peter, 

    my kris[at]khofstadter[dot]info email did not get through the 'Chinese Great Wall' - it landed Greentek's SPAM folder. 
    However they have received my email from tedor2[at]gmail[dot].com. 

    I can email them to look for your email in their SPAM folder if you'd like. If you'd like me to do this, please send me 

    - the email address you sent the email to them; 
    - time; 
    - subject; 

    This would help them to find your message. 

    Alles Gute, k
  • khofstadterkhofstadter Colchester, U.K.
    quick update on the Greentek cap:

    I managed to reduce the impedance by using more salt in the saline water. My previous calculations for 3.5% saline solution were wrong, now they are correct. 

    I also made some distilled water as it turned out that the water where we live is too hard. Maybe helps as well. 

    I have ordered the new generation of this cap, the Greentek S3 and are looking forward to testing and sharing results here. 

    Cheers, k
  • Peter_GammaPeter_Gamma

    This is for me personally the most attractive EEG cap I ve seen up to this date. Thank you for sharing your experience with it. khofstadter, especially our real-time demonstration of this cap is very impressive. I think there is a great potential for what can be done with this cap.

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