Data disrepency

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krazzed25
Posts: 1
Joined: Tue May 14, 2013 1:02 pm

Data disrepency

Post by krazzed25 » Tue May 14, 2013 3:23 pm

Hi all,

I wondered if anyone could help me. I have begun data analysis in BESA after collecting the data using the Biosemi system. The majority of the data I opened in BESA looks fine, however for two subjects there is an extreme amount of noise across all electrodes... almost as if the reference electrodes have bridged however this data looks completely different to how the data looked in real time during collection. I don't understand what has happened?! I would not have run the experiment if the electrode offsets were too high and there was a major problem... all the data looked fine but in BESA it looks like a mess. If I set the reference to original average reference the problem goes away but I don't want to do that without knowing what might have caused the problem. Any suggestions/ideas from all would be appreciated as I am a PhD student and a novice at using EEG. If anyone needs additional information for clarification let me know - I can send screen shots if required. I have spoken to others but no one seems to know what has happened.

Thanks in advance.

Confused Karima

Coen
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Re: Data disrepency

Post by Coen » Wed May 15, 2013 3:03 pm

Recheck the data in the simple BDF-reader (http://www.biosemi.com/download/ToolsFo ... er-808.zip), this should give you the same view as ActiView during recording.

I assume that with "noise" you mean hum (50 Hz interference ). If the hum level is equal on all channels in the unreferenced display, then the interference (largely) disappears after referencing. The problem is Common Mode interference (see http://www.biosemi.com/pics/zero_ref1_big.gif). The level is only a problem when a too high level remains after referencing.

High levels of Common Mode interference can be caused by a bad contact of the CMS and/or DRL electrodes (check that there is sufficient gel in the holders), or by a gel bridge between CMS and DRL (too much gel in the holders). Further guidelines to minimize interference are found in the documentation that came with the system.

Best regards, Coen (BioSemi)

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