slow drifting on EEG

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kyungho
Posts: 2
Joined: Tue Oct 03, 2017 6:13 am

slow drifting on EEG

Post by kyungho » Sun Oct 22, 2017 6:30 am

Dear all,

Hello, I am posting my issue for any help :)

Now I am conducting P300 BCI speller paradigm, and some parameters are set as below:

bandwidth: [0.1 70] Hz,
Notch: 60 Hz
Fs: 512Hz
Channel: 32 (MK2 box, pin type electrodes) + 5 EXG(ECG, EOG, EMG)
Reference: CMS electrode to left ear mastoid, DRL electrode to right ear mastoid (flat type electrodes)
Environment: laboratory
Data collection: BCI2000 software

Problems:
Some slow drifts frequently occur during recording EEG, and it makes P300 classifier failed to generate feature weight.
With the same parameters, someone did not show slow drift, and someone showed slow drift over recording. There was no severe body movement during experiment (due to ear reference, all channels contain ECG component though).

Since I do not have rigorous knowledge for inspecting EEG yet, I attached picture containing drift for your help.
re.png
re.png (108.92 KiB) Viewed 187 times
[relatively good]
bad-b.png
bad-b.png (136.6 KiB) Viewed 187 times
bad-g.png
bad-g.png (132.02 KiB) Viewed 187 times
[bad]


My concern is whether using ear reference is good choice for P3 ERPs. We have other CMS/DRL electrode (pin type) around parietal area.
What do you think of ear mastoid reference or head reference?

Thank you.

Regards,
Kyungho Won.

Coen
Site Admin
Posts: 873
Joined: Fri Mar 26, 2004 7:00 pm
Location: Amsterdam, Netherlands
Contact:

Re: slow drifting on EEG

Post by Coen » Mon Oct 23, 2017 2:25 pm

Always reference the data before further processing, see https://www.biosemi.com/faq/cms&drl.htm

Raw data (with respect to CMS) is NOT valid data, do not regard or use CMS or DRL as reference. With proper referencing, location of CMS and DRL does not affect the data (but may affect interference level), also see:

viewtopic.php?f=1&t=33
viewtopic.php?f=1&t=40
viewtopic.php?f=1&t=177

Best regards, Coen (BioSemi)

kyungho
Posts: 2
Joined: Tue Oct 03, 2017 6:13 am

Re: slow drifting on EEG

Post by kyungho » Mon Oct 23, 2017 3:16 pm

Dear Coen,

Sorry for my ignorance. I think I made a big mistake.

Can I ask you very very basic things?

According to my understanding,

The signal appearing without referencing is voltage-difference between electrodes and CMS, and it's not supposed to be used

In other words, if I want to use left/right ear mastoid reference, do I have to attach two extra channels (maybe EXG?) to left and right ear mastoid,

locate CMS/DRL to any other place (e.g., head cap) , and finally subtract the average of left/right ear mastoid from EEG?

Am I understanding right?

Thank you.

Regards,

Kyungho Won

Coen
Site Admin
Posts: 873
Joined: Fri Mar 26, 2004 7:00 pm
Location: Amsterdam, Netherlands
Contact:

Re: slow drifting on EEG

Post by Coen » Tue Oct 24, 2017 12:01 pm

The above is fully correct.

Best regards, Coen (BioSemi)

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