BIAS pin seems to enhance noise, use alternative BIAS meas?
Hello! We are using the 8-ch Cyton, with our own 3d-printed Ultracortex, and some DIY electrodes on top of the screws, where the EEG Snap Electrodes (Comb) are attached. From almost the beginning of the measurements, the signal seemed very unstable, in a way that the SNR was very small, and thus the board was amplifying all of the frequencies, effectively getting a "white noise". After debugging with a signal generator, it seemed that the BIAS pin is very sensitive to contact. The things I tried are the following:
1. Not connecting anything to the BIAS pin.
2. Using one of the EEG channels as Bias Meas, through the OpenBCI GUI.
3. Using the proper BIAS pin with 7 channels.
The electrode placement was as follows, with the SRB connected to an ear lobe:
1. Fp1
2. Fp2
3. C3
4. Cz
5. C4
6. P3
7. P4
8. AFz, or Bias Meas, or Shorted
The rest of the settings are not modified. I am running OpenBCI GUI v6, through Processing.
I am attaching the images as reference in each case.
No Bias
Ch 8 as Bias Meas
Regular Bias with channel 8 Shorted
The question is, could there be a problem with using the regular BIAS pin? Although in the screenshots attached it seems relatively stable compared to other times, the Railing is doubled compared to the No bias and alternative Bias cases.
Edit: Forgot to mention than another thing that I use for a basic benchmarking are the eye blinks in pre-frontal channels, which is somehow "lost"-not visible in the case of the proper BIAS pin, whereas in the other cases it's visible.
Comments
Hi Elmac,
Can you describe in more detail what electrodes you are using? "DIY electrodes". You say "on top of the screws". Are you using the Print-It-Yourself electrode holders? Can you try a test with other electrode types, such as cup electrodes, paste, gel based, etc.?
With the Cyton 'Bias/Ground', this is the same as Ground in other EEG amplifiers. Your first two screen shots just show one or two microvolt noise. I do not see any legitimate EEG signals. EEG in this range is dominated by noise or local electromagnetic field induced noise.
EEG is very sensitive to the metals used in the electrodes. Generally silver chloride work ok together, but other metals mixed such as silver, steel, etc can induce a galvanic battery effect, causing large DC offsets and noise.
Regards, William
Your last screenshot shows almost identical noise patterns (around 600 uV) in most of the channels. Identical noise in many channels is usually a sign of a bad reference electrode connection. (SRB2 on Cyton).
Thank you William for the fast response!
Yes, they are the electrode holders. I had also tried with gold cup-paste electrodes, but they produce unstable signal (high railing, time series similar as the 3rd image). I will make a test again and post a picture.
Now, the DIY-electrodes are button snap-fasteners, soldered to du-pont cables which are connected to the board. The reference ear-clip, is using the same snap-fastener. On the scalp fasteners, the OpenBCI Combs are attached.
Although I suspect that this setup is not ideal, should the change in which channel is used as BIAS, produce 2 orders of magnitude in amplitude signal difference, effectively masking even the eye-blinks?
All the best,
Ilias
Look closely at both the Time Series and FFT on all three of your images above. Each channel shows exactly the same noise pattern both in time and frequency. This could be interpreted as something wonky with your SRB2 reference connection. As it is shared in common with all channels.
But... it could also mean that you have some type of environmental noise in your area that is infecting all channels. Have you tried another location / room? Has this Cyton EVER operated successfully as with the tutorial setup instructions?
https://docs.openbci.com/GettingStarted/Boards/CytonGS/
https://docs.openbci.com/GettingStarted/Biosensing-Setups/EEGSetup/#4-alpha-brain-waves-eeg
Okay, I made a test with gold cup electrodes and 10-20 paste, after applying skin prep gel. The signal couldn't be more similar.
1 - Fp1, 5 - FPz, 7 - Fp2, SRB & Bias the ear lobes
1 - Fp1, Bias board pin - FPz, 7 - Fp2, SRB on the ear lobe
1 - Fp1, Bias board pin - FPz, 7 - Fp2, SRB between FPz and the Nasion, and in another room
Btw, I had made tests with the EEG minisim, and the signals seemed ok, even in the 10 uV. Other than that, for as long as I can remember (since around Sep - Oct), I am trying to validate the recorded signal. Could there be an issue on the board reference? It would be somehow consistent with trying different electrodes/locations on the head and since the minisim produces the same signal in all of its outputs. Or am I missing anything else?
Please clarify, you wrote:
Are you saying that your Cyton worked fine at some date in the past, then around September 2024, you started getting bad data, even using the Minisim EEG simulator to send 10 uV signals?
If this is the case it sounds like your Cyton is broken. Please email Customer Support (contact at openbci.com) and ask them how to proceed, what other testing you can do there.
In September we received the board, and for recording EEG signals it seemingly never worked. With the simulator it seems to work just fine, with which I pinpointed the initial BIAS pin sensitivity for this thread. I'll contact support in any case.
Thank you very much!
Ilias