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High voltage levels in EEG output
Posted: Tue May 28, 2019 8:48 pm
by prasanth
Hello. I am a student researcher using Bio semi EEG to study brain activity. I am very new to EEG analysis and have been using eeglab to open the .bdf files inside matlab. I noticed that the voltage levels for individual channels are very high, in the thousands range. Is the actual electrode output multiplied by some scaling factor while importing into matlab while using eeglab, or does it have something to do with the hardware set-up?.
Thank you!
Re: High voltage levels in EEG output
Posted: Wed May 29, 2019 5:49 pm
by Coen
The data is DC: small EEG signals superimposed on large electrode offsets. Reject the offsets with a high-pass filter or baseline correction. Reference the data to reject large Common Mode, see
https://www.biosemi.com/faq/cms&drl.htm -->
http://www.biosemi.com/pics/zero_ref1_big.gif
Best regards (BioSemi)
Re: High voltage levels in EEG output
Posted: Thu May 30, 2019 3:29 am
by prasanth
Thank you Coen, I will try that. But, I also noticed this difference between Biosemi BDFreader and eeglab. When the file is viewed in bdfreader, the voltages are in the microvolt range (as it should be normally), but when viewed in eeglab, the voltages are high (Thousands). Do you still think it might be the DC offset.
Re: High voltage levels in EEG output
Posted: Mon Jun 03, 2019 12:41 pm
by Coen
For the 24-bit data, use scaling LSB = 31.25 nV. Offsets up to 100 mV are normal.
Best regards, Coen (BioSemi)
