noisy waveform in EEGLAB? [resolved]

jctaylorjctaylor Canada
edited August 24 in General Discussion

Hi folks,
I'm hoping someone can help me with this. I'm using a cyton+daisy to try to record through LSL and Lab Recorder. I'll describe my typical procedure, but I've monkeyed around with variations without success so far.

Normally, I start the OpenBCI GUI and then plug in the dongle and turn on the headset to PC. I then start the GUI and start streaming in order to insure that no channels are railed. I have tried using the LSL stream from the GUI, but EEGLAB threw a bit of a fit when I tried to open that file. So I run openbci_lsl from an Anaconda (Python) window. I run LabRecorder.exe from another Anaconda window and save an .xdf file.

The way things look when I open the .xdf file using either the xdfimport or the Mobilab matlab extensions is as shown below:

The way the channels look in the GUI is more normal (to my untrained eye)

Does anyone have any clue what I'm doing wrong or how I might go about problem-solving this? At first I thought it was related to my marker stream from a psychopy program, but it happens even without that running (which is what I described above). I've messed around without success. Any help would be greatly appreciated.

Comments

  • wjcroftwjcroft Mount Shasta, CA

    JC, hi.

    Your EEGLAB graph shows huge amounts of mains noise (60 or 50 Hz). This is normally filtered out by the OpenBCI_GUI, but you need to tell EEGLAB to do similar. The GUI uses a 'notch' filter at (60 Hz in your screenshot). The data recordings or stream from the GUI is NOT filtered, it is raw.

    William

  • William,
    Thank you very much for your comment! I wondered about wall current, but the blobs only appeared about 5-6 per second so thought it must be something else. I also thought that with the headset working off battery, wall current wouldn't be an issue. But I'm not surprised I am wrong about that. I will be back in the lab later today and will try out your suggestion. Thanks very much for taking your time on this!
    Jamie

  • wjcroftwjcroft Mount Shasta, CA

    If you turn off the filtering in the GUI, you'll see the same large amounts of mains noise. Check the FFT window, it will show a large spike at the mains freq.

  • Hi William,
    You are right - I see a prominent spike around 60 Hz in the FFT of the data. Filtering to the 0.1Hz - 30 Hz range yielded a much more reasonable set of channel plots. Now I can start sampling a longer series and try creating averaged ERPs. Many thanks for your help. I knew about mains noise, but I didn't realize it would be such a prominent issue in a battery-powered EEG headset, so I didn't imagine that could be the problem.
    best
    Jamie

  • wjcroftwjcroft Mount Shasta, CA

    Mains noise is ubiquitous in the environment. There are inexpensive EMF meters that can demonstrate this. Even some phone apps if your phone has a magnetometer.

    https://www.google.com/search?q=emf+meter+app

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