Strange looking drifts through all channels

[juhint]

My collegue and I just started recording eegs for our thesis and have run into some problems with Biosemi. The first few recordings went very well, but in all but one of the last five sessions we had some major problems with odd-looking drifts. We have come to our wits end.
We have low off-sets for all electrodes, CM is in range, eeg looks beautifully (visually) – but every so often (and unfortunately quite often) we have a drift that through all electrodes and the whole screen – it is like one big solid line – this is usually followed by another drift right after. Our computer technician advised us that it probably has something to do with the cms- or drl-electrode because the drift goes through all channels - like I said, it is a solid line. We tried taping the cms- and drl-cable to the headcap – so we can avoid unnecessary pulling on the cable when our subject moves. The Cm- in range-light does not come off or flicker during the drifts at all.
We don’t know what else to do. We had one good 4-hour-recording this morning and ran into the same problems again this afternoon.
Any suggestions? Problem-solutions? Has this ever happened to anyone?

Thanks a lot!
Julia

[juhint]

I have uploaded a small data-file showing these drifts to the media center of my e-mail provider.
The link is:
http://www.gmx.de/mc/wwFBVLvSE9gu12eTl6XCufrsLhbr0W

Instructions in English:
click on gmx media center starten
a folder opens where you can see one data file named probe.bdf
put a check mark in the box next to the name of the data file
open the “Datei”-menu and click on “download”
if automatic download does not start use the blue link that is provided in the small download window

Julia

[Coen]

The artefacts in your file ("bump" on most channels from t=76 to t=80) look like slow waves due to sweating, as discussed in thread viewtopic.php?t=28.

Best regards, Coen (BioSemi)

[juhint]

Dear Coen,
thanks for your answer - I do have a few more questions though:

If it was sweating, wouldn't those artefacts happen at a more regular basis? We sometimes have artefacts at the beginning of a recording and then nothing again for more than 10 minutes - wouldn't our subject keep on sweating and we would keep on seeing those artefacts on a more continous basis? And why do we see them in all channels?

This just started happening last Wednesday when we started our recordings on a twice-daily basis - we had no problems before that - we had five recordings since Wednesday and had those artefacts with all but one subject. We feel that the problem is getting worse - the file that I put online is an example from a very bad patch yesterday.

Can it be faulty electrodes as well? Our thesis advisor thinks they may have gotten wet inside - this would explain why we didn't see those artefacts on Monday morning - our last recording before that was Saturday morning - maybe they had time to dry out? We usually only have one hour time difference between the recordings of two subjects - we always clean the electrodes with water between two recordings - maybe they are still wet and that causes interference? Can this cause interference that seems to simulate sweating of a subject?

If it is indeed sweating - what can we do to prevent it from happening?

Thanks a lot,
Julia

[Coen]

When water or gel enters the electrode, they will stop working, and they won't work again (blue light keeps blinking). Problems with leaking electrode have never been reported to us.

A possible cause is pollution of the Ag-AgCl tips with traces of non-silver metal. However, the low frequency noise will then also be visible by testing the electrodes in salt water.

Please follow the recommendations of Dr Praamstra and colleagues: try to keep the subject comfortable in a relatively cool environment.

Best regards, Coen (BioSemi)

[juhint]

thanks