EEG Newb Question: EEG snap electrodes vs Gold Cup electrodes
idiomancy
USA - PA
I purchased the Low Cost Biosensing Bundle , which contains so called "snap EMG/ECG cables" but which have specific "EEG snap electrodes". This is kind of mixed messaging. It seems to be claiming that the comb shaped snap electrodes are superior for use in areas with hair. However, elsewhere I have seen it claimed that Gold Cup electrodes are the standard for EEG.
So which is it? Which is better? Snap EEG Electrodes or Gold Cup? What about for areas with hair? Can you mix the two?
Bonus question:
What kind of tape is supposed to be used with gold cup electrodes that I see in everyone's pics?
References:
bundle -
https://shop.openbci.com/products/low-cost-biosensing-starter-bundle
eeg snap electrodes -
https://shop.openbci.com/collections/frontpage/products/emg-ecg-snap-electrode-cables?variant=32372786958
https://shop.openbci.com/products/eeg-snap-electrodes
gold cup electrodes -
https://shop.openbci.com/collections/frontpage/products/openbci-gold-cup-electrodes
https://shop.openbci.com/products/ten20-conductive-paste-8oz-jar
Comments
Hi Idioman,
The bundle page you linked to, has this comment:
So your bundle assumed you wanted to start with the dry electrodes, held in place by the velcro band. It is your choice whether you want to use dry or wet electrodes. Dry is easier to setup and no mess. Wet requires using paste or gel to make contact with the scalp. The EMG electrodes included in your bundle are held in place with a little circle of adhesive, but in the center you can see a blob of gel.
Wet electrode systems give a slight bit more signal amplitude than the passive dry electrodes. Many neurofeedback practitioners use the cup / paste electrodes, because there is a long history of that. Usually EEG is either done with all dry or all wet, not a mix. But you could mix if you want. The cup electrodes are not held to the scalp with 'tape', but with the Ten20 paste, also sold in the Shop.
BCI users use a wider variety of electrodes, so less attached to the cups / paste. For example the headsets sold in the shop.
William
Thank you very much, William!
I did see that suggestion, but I hadn't seen any guidance about why to prefer one over the other. Is this related to the notion of "active" vs "passive" electrodes at all? Are the comb electrodes active electrodes?
In the Shop, the Thinkpulse electrodes are actives. They contain a tiny pre-amplifier behind each dry conductive rubber comb.
https://shop.openbci.com/products/thinkpulse-16-channel-starter-kit-1
All other types of electrodes in the Shop are 'passive', no pre-amp, all the microvolt sensing is done in the chip on the Cyton or Ganglion board.
I explained above that the wet electrodes, gel (used with an elastic cap) or paste, are common in certain fields like neurofeedback or neuroscience research. You can choose which works best for you. There is no 'industry standard'.
Ahh, thank you that's the first simple explanation of active vs passive electrodes I have seen. That's very helpful
Hi, @wjcroft
I just read from EEGinfo protocol guide and EEGer that it is important for neurofeedback to have all electrodes made of the same material:
"Do not mix electrode types. Electrode connections using mixed combinations of metals can lead to incorrect reporting of EEG data." - https://www.eeger.com/eeger-answers/eeger4-neurofeedback-software-general-information-and-cautions
Are you familiar with this concept or know of any research on it? I could not find any studies that actually presented experimental results. It appears to me that some experienced clinicians, like the Othmers, simply say not to mix materials and give theoretical reasoning.
I have been using the openbci sintered Ag/AgCl ear clips for ground and reference, paired with the gold cup electrodes on the scalp. I recently got some openbci snap Ag/AgCl flat electrodes. I will try switching to these for scalp placements and see if there is noticeable difference, though they are harder to paste on the head without falling off.
I believe that silver chloride electrodes are the 'exception' to the widely known advise to avoid mixing electrode metals. That is because the chloride coating is not technically a 'metal'. Those electrodes are generally a plastic base, plated with silver, then chlorided.
For all other solid or plated metals, including tin, silver, gold -- mixing metals can cause galvanic battery effects with a strong DC offset potential from the created battery.
But even with the silver chloride electrodes, if the chloride wears off, exposing the base silver, you have the same issue with battery effects.
re: OpenBCI earclips. I don't believe these are sintered. They are just plain silver chloride. Sintered type generally cost substantially more than plain. And also come with increased cleaning requirements if using with the Othmer style infra-low frequency (ILF) protocols.
https://www.eeginfo.com/knowledgebases/215/silver/silver-chloride-electrode-tips
William, thank you.
You are correct; the ear clips are not sintered, and neither are the snap electrodes.
With this galvanic battery effect, how would you spot this? Is it only noticeable in lower frequencies below 1Hz? My ear clips have some visible wear on them, so I should notice this galvanic effect when using gold electrodes, right?
A second question: I am struggling to hold a good signal with the openbci Ag/AgCl snap electrodes. In the openbci GUI, they show a low impedance, always in the green, but then the time series will show railed. Brainbay also does not show any signal. I am using nuprep and 10-20 paste. Yesterday, I was able to hold good signal with one electrode for over an hour, while a second placement dropped out in a few minutes (first time getting these cables/electrodes out of the bag). Today, I could not get either wire/electrode to hold good signal for more than a minute. When I switch over to gold-cup electrodes, everything appears fine. I am using the Ag/AgCl ear clips. I am shooting in the dark here - could this be from letting 1020 paste get under the snap electrode and make contact with the snap cable?
https://www.google.com/search?q=what+is+the+galvanic+voltage+between+silver+and+gold
.150 volts is 150 millivolts or 150,000 microvolts. That is a substantial DC offset. The phrase "...when these two metals come into contact..." is not quite the case with EEG. However you do have electrolyte on both electrodes, so they are somewhat connected through the body/skin impedance.
re: 'dry' silver chloride snap electrodes. Generally these are intended for dry applications, using the velcro headband. It is possible that if paste got into the female snap receptacle, it could cause corrosion. If you are going to use paste anyway, then the cup electrodes are probably easier.
re: your ear clips. You did not mention if you were using these dry or with paste. My guess is that using with paste would be awkward because they would be hard to clean after sessions. There are double-cup earclips on the market, that are more suited to paste applications.
re: railing is generally associated with signals out of the range of the Time Series channel range you have set. Could be related to high offset.