High voltage levels in EEG output

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prasanth
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Joined: Thu May 23, 2019 9:12 pm

High voltage levels in EEG output

Post by prasanth » Tue May 28, 2019 8:48 pm

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!

Coen
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Re: High voltage levels in EEG output

Post by Coen » Wed May 29, 2019 5:49 pm

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)

prasanth
Posts: 2
Joined: Thu May 23, 2019 9:12 pm

Re: High voltage levels in EEG output

Post by prasanth » Thu May 30, 2019 3:29 am

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.
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EEGLAB ind channel results.JPG
EEGLAB ind channel results.JPG (64.98 KiB) Viewed 214 times

Coen
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Re: High voltage levels in EEG output

Post by Coen » Mon Jun 03, 2019 12:41 pm

For the 24-bit data, use scaling LSB = 31.25 nV. Offsets up to 100 mV are normal.

Best regards, Coen (BioSemi)

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