What is this shift in the baseline?

DashBarkHussDashBarkHuss Chicago
in General Discussion

What is this spike in the signal that never corrects itself? It's like the whole signal's baseline moved up.

The EOG stays with its midline elevated toward 2000 micro volts.

These are the filters I have

I have a ganglion with EOG set up on channel one, with one electrode on each outside corner of the eye, being referenced to each other. I have one analog ground on my mastoid.

Comments

  • wjcroftwjcroft Mount Shasta, CA

    Where is your common reference electrode located? Is channel 1 using differential mode, or common reference? For EOG you want differential.

    https://docs.openbci.com/Ganglion/GanglionSpecs/#inverting-input-select-switches

    Your second time series looks fine. How often do you get the DC level shifts?

    Channel 3 bandpass from 10 to 100 Hz does not really work at a 200 Hz sample rate. You should drop it to 10 to 35 if that is what you want.

  • DashBarkHussDashBarkHuss Chicago

    @wjcroft said:
    Where is your common reference electrode located? Is channel 1 using differential mode, or common reference? For EOG you want differential.

    I had the switch set to differential mode. I did not have a common reference electrode in this set up. I thought you did not need one with differential mode.

    Your second time series looks fine. How often do you get the DC level shifts?

    I'm not sure I just noticed it. What can cause them?

    Channel 3 bandpass from 10 to 100 Hz does not really work at a 200 Hz sample rate. You should drop it to 10 to 35 if that is what you want.

    Ahh ok. Dr. Konkoly at northwestern told me to set it up that way for EMG a while ago. That channel was not being used in this setup though. I was only using channel 1.

  • wjcroftwjcroft Mount Shasta, CA

    re: reference

    OK, I was confused with the EOG, that is most commonly paired with EEG studies.

    re: Nyquist limits of bandwidth

    'Technically' Nyquist rule says max frequency available is half of the sample rate. So the 100 Hz can occur, but it is heavily contaminated by aliasing when sampled at 200 Hz. I guess you can leave it there if you want. When I thought you were gathering EEG, the skull blocks everything beyond 50 Hz or so.

    https://www.google.com/search?q=nyquist+aliasing

    re: base level shift

    I believe this is just happening when you turn on / start the session. The original Ganglion firmware had a 'feature' that limited the range of large voltage excursions, how much could happen in a short period of time. I think that is what you are seeing. You can just ignore start-up effects.

  • wjcroftwjcroft Mount Shasta, CA

    This page has a good image. In your case with the 100 Hz signal sampled at 200 Hz, see the figure, part C. In that graph the samples happen to occur at the peaks. Now imagine they were sampled at any other portion of the signal waveform (much more likely). You will see alias distortions.

    https://www.ni.com/docs/en-US/bundle/labwindows-cvi/page/advancedanalysisconcepts/lvac_increasing_sampling_frequency_to_avoid_aliasing.html

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