a possible DRL problem

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Bo
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Joined: Mon Dec 01, 2008 10:37 pm
Location: IL

a possible DRL problem

Post by Bo »

Dear Coen

I would like to consult about a possible problem with CMS-DRL "pin-type" electrodes that may not be explained by a gel bridge.

When mounting the 64 channel cap we get in most cases very high noise levels (actually a no-signal) in one or two electrodes adjacent to the DRL (in P2 and/or PO4), while electrode offset is in normal range. The problem is not related to these 2 channels but rather to the specific location, because switching their location with other channels dislocates the noise to the new channel positioned next to the DRL.
We are well aware of a possible gel bridge between the DRL/CMS and the near-by data electrodes (I also read your correspondence with Juan at viewtopic.php?t=392), but we tried several times to carefully administer low quantities of gel under these channels, as we also do for all the other channels. In addition, when we switch the CMS and the DRL between them, the problem passes to the other side (P1 and PO3). Therefore I thought more about a DRL problem rather than a gel bridge in a specific place.

Lastly, when we use the flat DRL-CMS, data are more noisy overall. My second question would be whether this happens because we located the DRL and CMS far from each other (e.g. near the 2 ears)?

Many thanks!

bo

Coen
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Post by Coen »

In theory, it is possible that in cases of a very high Common Mode voltage, 50Hz currents via the DRL electrode cause differential 50Hz voltages across the skin around the DRL location. Please test a configuration with the CMS in the center of the other electrodes (standard position), and the DRL far away (ear, arm, etc). If this improves the situation, it might be a worthwhile recommendation to change the DRL location in the future (and we will start producing CMS/DRL sets with CMS as pin, and DRL as flat electrode).

Best regards, Coen (BioSemi)

Bo
Posts: 7
Joined: Mon Dec 01, 2008 10:37 pm
Location: IL

Post by Bo »

Thanks Coen

It may be the explanation indeed, as an FFT that I run today showed a pick at 50Hz in PO4 that was almost absent in another near-by channel. After a 30Hz lowpass filter the raw data at PO4 looked much better though still a little noisier than other channels.
I'll try to distance SMC and DRL from each other though I can't use one pin-type and one flat at the same time. But then there's the question that I raised in the former message: using a flat CMS/DRL near the ears or at other locations near the face area wouldn't introduce noise because of muscles in those regions? This is what we saw when we formerly tried this.

Thanks again

bo

Coen
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Post by Coen »

Please test with the CMS in the standard position, and the DRL-pin on the ear (improvise with some adhesive tape, it will work for the experiment). If this improves the signal compared to the standard DRL position, then we will supply a CMS/DRL set

CMS not located in the center of the measuring electrodes may increase Common Mode voltage

Best regards, Coen (BioSemi)

Bo
Posts: 7
Joined: Mon Dec 01, 2008 10:37 pm
Location: IL

Post by Bo »

Dear Coen,

After running one subject that did not show the usual noise in P2/PO4 and one that did show it in P2, and having alternated the DRL position as discussed, I can tell you the following:

A 50Hz peak was seen in Fourier transform at P2 in the subject that had noise in the raw data at this channel, only when the pin-type DRL was placed in its usual place and not when it was replaced to the right mastoid position. This is in accordance with your predictions. Furthermore, the signal to noise ratio in other example channels I looked at didn't seem to change much across DRL position in the same subject, which is a good thing (though it is a little hard to tell for sure based on so litle data).

At the same time, the second subject, who had no initial noise in P2, seemed to have a small 50Hz noise level only when DRL was placed on the right mastoid (much smaller than the P2 of the other subject with a usual DRL position, though). It looks quite marginal to me (and didn't seem to effect the ERPs that I extracted later), but maybe should be tested with more subjects

Overall, from this very small test it looks that whenever there's a problem, a relocation of the DRL solves it in a satisfactory way and with no unexpected "surprises", at least for one subject that showed noisy P2 with DRL located near CMS.

Bo

Coen
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Post by Coen »

Thanks for the tests, this is certainly valuable information !

We will consider the future production of a batch of CMS/DRL sets with pin CMS and flat DRL. However, it will take some time to change the production.

A temporary solution is to use a loose electrode holder (as normally fixed in the cap). Push the pin DRL in the loose holder, and attach this the holder+electrode directly to the skin with a 20*8 adhesive disc (as normally used for the flat electrodes).

Best regards, Coen (BioSemi)

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