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| On the left, I'm trying ECG electrodes. On the right, I'm trying gold cup EEG electrodes. In both cases, I'm looking pretty sharp. |
Disposable ECG Electrodes: First, I decided to try some disposable ECG electrodes. These are cheap and really easy to use. They're not very good for using in your hair, but they're great for sticking on your forehead. For the reference and bias connections, I used an ECG electrode on the mastoid bone behind each of my ears. The picture below shows the pre-gelled, self-adhesive ECG electrodes that I used along with the clip-type ECG electrode wires (see this post for specific recommendations).
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| ECG Electrodes and Clip Leads That I Used on my Head |
Once I stuck three of these electrodes on my head (forehead and behind each ear), I connected the lead wires to my OpenBCI V1 board using my homemade adapter.
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| Connecting the ECG Leads to my OpenBCI V1 Board. |
After getting all connected, I activated the ADS1299's "Lead Off" excitation signal for the channel that was connected to the ECG electrode on my forehead. As discussed in my previous post, the excitation signal is a 6 nA AC current source that the ADS1299 toggles at 31.2 Hz. The flow of current through the electrode creates a voltage that can be measured by OpenBCI just like a normal EEG signal. I configured my OpenBCI board to digitize the data and send it to the PC. On the PC, I used the OpenBCI GUI to view the data in real time and to record it for post-test analysis.
A zoomed-in plot of the recorded waveform is shown on the left of the figure below. As you can see, it has a fairly large amplitude of 508 uVrms. This corresponds to an impedance of 120 kOhm. That's really big! Being surprised by that large value, I swapped the wires around so that I was measuring the electrode that I had been using as my reference (or "-") electrode. As seen in the waveform above on the right, I got a very similar value. That's not cool.
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| Example Waveforms Recorded While Using OpenBCI To Measure the Electrode-to-Skin Impedance of (Left) the ECG Electrode on my Forehead and (Right) the ECG Electrode behind my Left Ear. |
Zooming out so that you can see more of my recording, the figure below shows about a minute's worth of data. This is the full recording from which I made the excerpts shown above. The longer view below shows the story of me recording data for one electrode (up to about t = 119), of how the signal goes away as I unplug and swap the electrode connections (from t = 120 to t = 132), and then how the signal returns once I am connected to the other electrode. Again, you can see that I measured 120K from one electrode and 116K from the other electrode.
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| Zoomed-Out View of my Recordings Using the "Lead-Off Detection" Excitation while Using Disposable ECG electrodes. |
Gold Cup EEG Electrodes: Because I found the impedance of the ECG electrodes to be surprisingly high, I tried using some gold cup EEG electrodes. The picture below shows the electrodes and the conductive electrode paste that I used. Like with the ECG electrodes, I put one of these on my forehead, one on the bone behind my left ear for the EEG reference and one one the bone behind my right ear as the EEG bias.
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| Gold Cup EEG Electrodes and Ten20 Brand EEG Paste |
After putting on the electrodes, I activated the "Lead Off" excitation, like before. Some examples waveforms from the data that I recorded are shown below. As expected, the waveform shape is the same as seen before, but the amplitude is different, which reflects the fact that the impedance of the electrode-to-skin interface is different.
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| Waveforms Recorded from the Gold Cup Electrodes During the "Lead Off" Excitation. |
Notice that the three plots show decreasing amplitude, which means that I was getting better contact and less impedance in each case. What was happening?
Well, for the first waveform (the one on the left) was shows what I measured when I first attached the electrodes. It shows an RMS amplitude of 389 uV, which corresponds to an impedance of approximately 92 kOhm. This was still higher than I wanted, so I fiddled with the electrode and pushed it into my skin to try to make better contact. That's when I got the middle graph -- 230 uV and 54 kOhm. Finally, I pulled off the electrode, replaced the conductive paste, and really pressed and twisted the electrode against my skin. That's when I got the graph on the right -- 64 uV, which corresponds to 15 kOhm. That is more like the kind of value that I was hoping to see.
Below is a zoomed-out plot of the whole scenario with the gold cup electrode. Again, this is the full record from which I made the excerpts above. On the left side of the plot, you can see the 389 uV, 92 kOhm condition that I showed before. Then, you can see my multiple attempts at re-seating and re-pasting the electrode. Finally, at the end, I finally got to the 64 uV / 15 kOhm condition. So, while it does take some effort, it is possible to improve the electrical contact between the electrode and your skin.
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| Zoomed-Out View of my Recordings Using the "Lead-Off Detection" Excitation while Using the Gold Cup EEG Electrodes |
Why Were the ECG Electrodes So Bad? This experiment started with the ECG electrodes, which yielded a very high impedance of 120 kOhm. If I could only use the ECG electrodes, this high impedance value would have prompted me to remove the electrode, to scrub the skin (hard!) with alcohol and a rough pad, and then to attach a new electrode. Maybe this would have worked to lower the impedance, or maybe not. If it would not have helped, the problem could be that my ECG electrodes are really old. If you look really closely at my picture showing the electrodes, you'll see that the ECG packet in the background shows a date of "June 2012". Yikes! I have a friend who is developing a hacker-friendly EMG system (go FlexVolt!) who has mentioned to me that he has seen difficulty when using old ECG electrodes. So, I'm thinking that maybe disposable electrodes have a limited shelf life...and that 2 years is maybe too old.
Next Steps: With the impedance monitoring working on my OpenBCI board, I'm hoping that it will enable me to make more reliable EEG recordings. Hopefully, getting good low-impedance connections will increase my chances of detecting those low-level signals that have vexed me with their sometimes-I-see-them and sometimes-I-don't behavior. In particular, I'm thinking about those pesky Mu waves that have been hard for me! We'll see if this impedance checking can help...
Thanks for reading!
Follow-Up: Want to get the data from this post? Try downloading it from my github!
Follow-Up: Want to get the data from this post? Try downloading it from my github!
I am very grateful for all the engineering work that Vojin, Chip, James, Mike and others have put into the impedance calculation capability for the ADS1299 and our OpenBCI hardware. This is a type of feature that is generally only found on high end EEG equipment, so I see it as a significant plus for OpenBCI acceptance.
ReplyDeleteI've mentioned before that several prominent experts in the field of EEG neurofeedback have argued that skin impedances even in the range of 100K+ ohms are acceptable. This is because the input impedance of amps these days is so high, in the hundreds of megohms or even gigaohm. So considering the effect of that difference between 10K and 100K in how the EEG microvolt signal is effected -- the actual microvolt difference is miniscule. Yes you can see large differences in microvolts with the constant current injected signal used by the ADS1299 lead off circuit topology. But real EEG measured from the scalp is not operating in this constant current paradigm.
Here is a page from the book, Clinical Neurotherapy, the chapter by Dr. Richard Soutar. A very clear explanation of what is often a contentious issue. And it cites Robert Thatcher's painstaking numerical calculation as well, as further evidence.
http://www.lightfield.com/jpg/thatcher-impedance.png
As some more data points, I have done some DC impedance measurements of my Ag-AgCl saline sensor system, and also get impedances in the 100K range. This is completely normal for a saline system which uses no skin abrasion. Skin abrasion in fact these days is being called into question, since you want to avoid any possibility of breaking through the skin barrier. A number of other EEG headsets are using simple saline pads, such as the Emotiv EPOC, so impedances in this range are quite normal and functional.
I do see the value of the ability to check our impedances so we can see when they have gotten significantly out of range. Or when the ADS1299 is being used in say a medical / clinical setting where they might have strict rules about acceptable ranges for their applications or research.
Thanks again to Chip and everyone who is making this measurement possible, and adding to our knowledge of ADS1299 EEG practice.
Another well known clinical Ag-Ag/Cl saline system is that from EGI http://www.egi.com/ , their Geodesic Sensor Nets. Here's a paper where they discuss their comparison of 5K vs 40K impedance differences in the EEG signal. No significant differences were found.
ReplyDeletehttp://www.eeginfo.com/research/ElectrodeImpedance.pdf
For those wanting to experiment with the 5K / 10K impedance standards, here is a common prep gel used,
http://www.amazon.com/D-O-Weaver-NuPrep-Skin-Prep/dp/B002C2UQIA
The pdf link by the EGI authors above has moved to the author's site at U Oregon,
Deletehttp://www.csi.uoregon.edu/members/ferree/pubs/EEG2001.pdf
Hi Chip :D
ReplyDeleteSo, I have a question, which might not be really related to your post but on the topic of electrodes: Most high-end eeg electrodes are Ag/AgCl coated. And I've always wondered why is that? What is the advantage of coating EEG electrodes with Ag/AgCl give?
My understanding (which is poor) is that the different metals used in electrodes are chosen based on their patient safety (non-reactive metals like silver and gold are good) and for the way that they interact with the electrolytic gel/paste used to join the electrode to the skin. You're looking for an electrode construction who's interaction with the gel is very stable so as to NOT introduce spurious signals or low-frequency drift.
DeleteSpecific to the silver electrodes, the introduction of the AgCl helps them to have an even smoother (electrically speaking) transition to the electrolytic gel/paste...that's why they're so good.
For medium and high-frequency EEG (say, above 2 Hz), I've not really seen much difference between the different electrode types. If you care about low frequency EEG, however, you should definitely try different electrode types. It may prove to be very important.
Most importantly, be sure to use all the same electrode type on your head...do not mix-and-match different types electrodes for the same experiment. Second, be sure your electrodes are all clean...with no dried electrode gel/paste from previous experiments. These are the two most important things to remember with your electrodes!
Chip
Chip, Ahmed, hi. Great question and advise. Here's a paper that I've found very eye opening.
Deletehttp://www.eeginfo-neurofeedback.fr/fileadmin/images/was_ist_neurofeedback/wie_funktionierts/SCP_recording_electrodes_and_paste.pdf
The paper is addressing the measurement of very low frequency EEG (here called SCP, slow cortical potentials). But is applicable to all EEG. What stands out is the stability and low drift of the Ag-AgCl. I used to use Tin in the past, and just as the graphs show, it's probably the MOST unstable of them all(!)
William
Very well written and informative article, thank you :-)
ReplyDeleteALOKA UST-9124
Hey, thanks Mr. Robot Frank for that spam link! Your blogger.com profile is full of spam. What motivates you?
DeleteHi Chip, this whole blog is really amazing and I'm very grateful for all the information you share. I had a question, I was looking for these gold cup electrodes, here in Peru, Latin America; but i don´t find a place where get them. Where did you get them and how about the prices of these electrodes? Also, I heard that for cables I could use audio cables, that means those cables of the earphones?
ReplyDeleteThanks a lot! Greetings from Peru!!!
Hi Ronlando, I bought them from here:
Deletehttp://www.mfimedical.com/eeg-cup-disc-electrodes.html
There have the re-usable electrodes, which are nice but expensive.
I've also used the "disposable" electrodes, which are much cheaper. The disposable electrodes can be used a few times, but they do shed some of their metal coating when you wash them. They probably can't be cleaned and sterilized as easily as the gold ones, so once you use one, I wouldn't share it with someone else.
Good luck!
Chip
Hello mr chip, i'm septian. I have some question, can u reply to my email? This my email salbhut@gmail.com
ReplyDeleteThanks