I hope I've placed this question under the right section.
Is there a way to measure GSR (Galvanic Skin Response) with OpenBCI? I understand there is a need to inject some current to the electrode for measuring conductance.
How can this be done using OpenBCI?
Thank you,
Mike
Comments
http://eeghacker.blogspot.com/2014/04/openbci-measuring-electrode-impedance.html
http://eeghacker.blogspot.com/2014/04/impedance-of-electrodes-on-my-head.html
It's probably unlikely that typical GSR setups work in this way. For one thing, I would guess they use a DC voltage instead of AC like the ADS1299. So using the ADS1299 like this, your results may be similar to conventional GSR. Might be easier just using ideas from other GSR hackers,
https://www.google.com/search?q=gsr+arduino
William
http://openbci.com/forum/index.php?p=/discussion/84/ads1299-electrode-impedance-measurement-algorithm
Chip has checked in his code to do this to the github. Note there is one downside to turning the injection on: it changes the input impedance of the ADS1299 from 1 gigaohm to 500 megohms. See this paper for implications of that decision,
http://wwe.eeginfo.com/research/ElectrodeImpedance.pdf
This is a consideration if you are using saline solution based passive sensors for the EEG portion of your combined setup.
As I mentioned, this injected current is AC, not DC as is used in most traditional GSR devices. So may give different results. You would certainly NOT want to use any conductive solution or paste on the sensors involved in the simulated GSR measurement, as these would null out any contribution from the sweat on the subject's skin.
Regards,
William
If enough metallic ions are transported off the material then the material will become polarized. when this happens the resistance of the electrode climbs higher and higher until it becomes an open circuit. Stainless steel, and in fact most metals are easily polarized and thus unsuitable for the purpose of sensing bio potentials. There are only four metals suitable for consideration as bioelectrode. They are, Mercury, Silver, Platinum/hydrogen, and carbon. The Mercury calomel is almost perfect. However, for obvious reasons not practical, the Platinum/hydrogen electrode is great for standards but because of the hydrogen gas, impractical. Carbon electrodes are noisy. That leaves, Silver. Which turns out to have many desirable features. In particular it has excellent noise properties and a low and easily balanced offset potential.
In any event if you run a DC current between two Silver electrodes one will behave as an anode and the other a cathode. Eventually, one surface will be depleted of surface ions from the AgCl2 silver chloride layer and the impedance will climb, or worse the DC current will start driving silver off the surface to which it is usually plated onto, into solution. Do it long enough and you will loose your electrode and be left with worthless, noisy, junk.
Using AC drive puts back what it taken off during the previous half of the AC waveform preserving the integrity of your electrode.
What about Gold? The best electrolyte for cold is cyanide. I don't think you want that on your skin do you? In any event gold is just as bad a choice as stainless.
As far as electrodes used for EEG, typical metals used are gold, silver, tin. These all work fine. The most stable however is silver - silver chloride, Ag-AgCl.
https://en.wikipedia.org/wiki/Silver_chloride_electrode#Biological_electrode_systems
(But I do understand your comments on electrode metals are regarding the chemistry of DC current injection.) As an aside, EAV (Electroacupuncture According to Voll), has long used DC for their resistance measurement. With the same downsides of polarization that you mention. Motoyama developed his AMI (Automatic Meridian Instrumentation), to remedy that, using very short pulses.
http://www.cihs.edu/ami/ami_info.asp
William
William
I see it's been a while since the possibilty of obtaining Galvanic Skin Respose (GSR, or, I think more commonly: electrodermal activity, EDA) with OpenBCI was discussed. From what I've seen, it's still not possible to get EDA from OpenBCI. Or is there a way now, or are there plans for this?
Best,
Marius
This seems like it's possible with the Pic32 according to @biomurph
If that's true, i don't see why we can't add this as a board mode, like we did for analog and and digital input reading. @wjcroft am i following this thread correctly?
Would be great to make a shield or something that could measure this, or like a little plug in to the GPIO. If all we need is an analog input, we could make a little plug for the top of the open bci that just clips in, send an over the air command to the board, and boom, you are measuring and doing the math and such of the GSR. We could send these back to the computer in every sample, much of this work is already done in 3.1.0, with the board mode changing and such. This could even maybe work on the ganglion if the pins lined up the same.
1.) as far as I know there is no study that did evaluate
whether it can be measures at the ears. Ears only have 30-40 sweat
glands per cm² comapred to 350 per cm² in the palms. Here is a study
that compared 16 different sites. Besides fingers and feet, the forehead
(175 / cm²) had a nearly similiar pattern:
https://pdfs.semanticscholar.org/b467/fcaf02206d187a6e6471bc7521bb099ce50b.pdf
however,
GSR measurements on the forehead will probably interfere with eeg
measurement as you are inducing electric current? not sure. probably can
be filtered out easily.
2.) It might also interests you that the
MIT guys founded a company and sell their GSR wrist sensors at a high
price: https://www.empatica.com/research/e4/
3.) An earlier post
mentioned that the g.tec USB amplifier can be used to measure GSR. We do
this in our lab. You can purchase the gUSBamp and an additional GSR
sensor from g.tec that is plugged into the amplifier via touchproof
connections. Do you guys think that it will be safe to plug such a
sensor into the openBCI cyton board or will it cause damage to the board
or participant? I am not an electrical engeneer - so may you help me
out? Here are some technical specifications (scroll down):
https://shop.neurospec.com/gsr-sensor-kit
Many thanks,
Johannes