Ultracortex III, SSVEP, excessive noise and railing
Hi!
I am trying to build BCI using Ultracortex Mark III "Nova" & "Supernova" (REVISED) and Cyton Board based on SSVEP. This is a phenomenon that if the test subject looks at an object flashing at a certain frequency, this frequency should be readable from the visual cortex. I am using for channels: Pz, Oz, O1, O2 and my own program written in python, which uses brainflow to collect the data. The problem is I can't get consistent measurements from my channels. In one trial I get an almost perfect Fast Fourier Transform with the target frequency, and in the next one it is not visible at all. Most of the time at least one channel looks really noisy, even though I make sure the electrodes fit properly, and I feel the pressure on my scalp. To my knowledge (which is not great) impedance also looks okay. I believe the issue is with electrodes, not my program because I see the same problems with different pieces of software which were tested before. I don't have any previous experience with BCIs and EEG, so I would really appreciate any help and tips. I am attaching a folder with some screenshots from gui, my signal and fft plots. I should also probably mention that I am using reference electrode on Cz (dry one) and Ground on my forehead. I know that most people use ear clips for reference, but I don't have any. I also tried to use gold cup electrode on an ear lobe for reference, but then all channels became railed for some reason.
https://drive.google.com/drive/folders/15plJq1-lt7PUAHkLfq1Y_g12dwbHto47?usp=sharing
Comments
Hi Jano,
Consistent SSVEP detection has been problematic for some users. Here are some examples below. The first link is what you get if you use the "Google Advanced Search" button in the upper right corner.
https://www.google.com/search?as_q=ssvep&as_sitesearch=openbci.com
https://openbci.com/forum/index.php?p=/discussion/3122/ssvep-experiment-no-fft-peaks-found
https://openbci.com/forum/index.php?p=/discussion/3052/not-getting-expected-results-from-vep-visual-stimuli-eeg-data-resolved
https://www.google.com/search?as_q=ssvep+timing&as_sitesearch=openbci.com
https://openbci.com/forum/index.php?p=/discussion/2549/ssvep-fft-too-noisy
One of the reasons is that SSVEP requires very high resolution in the fidelity of the flash rate being delivered by your monitor and software. Also if any of this timing depends on CPU loading and scheduling of your processes / programs, that can also throw off the ability to get reliable SSVEP.
You can check your impedances against those mentioned in this comment / page,
https://openbci.com/forum/index.php?p=/discussion/comment/13328/#Comment_13328
re: railing, see this thread:
https://openbci.com/forum/index.php?p=/discussion/1481/how-to-become-de-railed
Regards, William
If you suspect electrode issues, such as the silver chloride coating worn off, examine the tips of the combs. The entire surface including tips should be the same dull grey. If you see different color such as white or silver at the tips, that means that electrode is worn out and needs replacement. You can do this with a screwdriver. Try swapping noisy electrodes / channels with known good ones.
You cannot mix electrode metal types. Doing that causes a galvanic / battery effect and the railing.
I had no idea, thank you for clarifying. Do you think I should stay with dry electrode reference on Cz then? Or is it necessary to buy ear clips for referencing?
Check out the previous 'railed' links.
The Cz reference and forehead Ground/Bias is fine. You do not need ear clips.
I've checked everything and still have no clue. Alternatively, it may be a skin contact issue, but I feel that all the electrodes touch the skin firmly. I will check on some friend, because on me the screws are already screwed in to the maximum depth. I have also checked calibration of the screen with MindAffect, and it seems to be working well.