BioSemi.nl

Hi, there seems to be a largish ~17 Hz noise in all 32 channels + EXGs. When the subject is connected, I can definitely see EEG and eye movements, but superimposed there is this 17 Hz noise. When we examined the Aux channels, it looks like the continuous sinusoid seen on the Plet channel. The Box is one without the front CMS/DRL connector. I checked all electrodes with the bucket test and also tried a new set but the problem persists. We have not used this box in a few months as we typically use another one with the front CMS/DRL connector.
Does this look like a hardware problem?

Thanks,

G

Is GSR used and connected to the subject ?

Is the interference seen in the bucket test (without GSR electrodes) ?

Best regards, Coen (BioSemi)

Hi, to answer your questions:

The GSR is not used

I did a systematic bucket test yesterday, using all the 32 + CMS/DRL electrodes for connector A and 5 of the EXG electrodes (connected via the headbox to connector B, no GSR electrode). It looks like this type of noise is still there (perhaps smaller). Basically, when I have the display scale at 100uV it's mostly a flat line with 1 pixel bumps here and there, but if I go down to 10uV, the 17Hz oscillations become quite visible.
The odd thing (to me, at least), is that when I display the Aux channels I get a clean sinusoid at around 17Hz for the Plet channel (not used). So, there must be some link, as it is strange to get a sinusoidalish noise at 17Hz in all channels (when I first saw it I thought it was 50/60 Hz, but on closer examination it's 17Hz!).

We are going to give it another try tomorrow and I'll take a couple of screenshots.

Thanks for your help, I realize that it is almost impossible to diagnose things remotely.

G

Hi,

Do you know if you have a similar artifact at 48 Hz by any chance? Surprising you still see it in the bucket test. I have had artifacts visible at 16 and 48 Hz, but they disappear after cleaning the data.

Cedric

Hi,

We are working with an active two amp (64 channels + 8 EX channels) with a GSR module added recently (2018). Last week, we tested the quality of EEG signal when simultaneously recording the GSR. After re-referencing the channel using the average of all EEG channels, we computed the spectrum and noticed a sharp peak at 16 Hz. This peak is only present when GSR is recorded. Is that something normal or does it mean that our AD-box is not working properly?

Thanks

Juliette

Interference by GSR is usually Common Mode, and can be reduced substantially by referencing (which should always applied anyway before analysis). However, in a FFT over a lot of points, the 16 Hz peak will of course eventual always show up, this is normal.

Ensure a good (equal) electrode contact of both GSR electrodes. If one GSR electrode has a much higher than the other, then part of the GSR excitation current may flow via the DRL electrode, and cause Common Mode 16 Hz interference

Try experimenting with an alternative DRL location. A DRL locate between the GSR electrodes and the headcap electrodes is optimal in theory. Try locating the DRL in the neck. A pin-CMS/flat-DRL is available.

Best regards, Coen (BioSemi)

Hi Coen,

My EEG set-up is composed of 2 sets of 32 electrodes. They are plugged on ports A1-A32 and port B1-B32 of my AD-box. Set A includes pin-type CMS/DRL electrodes. Then, can I let these 2 pin-type electrodes unplugged, and use flat-type CMS/DRL instead ?

Many thanks for your help !

Juliette

No, that is not possible.
Only 1 CMSDRL cable can be inserted at the time.
If you plug -in a P32 set with CMSDRL into the A SCSI connector, than you can not also plug-in a CMSDRL cord into the front CMSDRL input.

Why would you like to use the front CMSDRL cord?

@robbie303
Because Coen suggested it in the previous email to troubleshoot the 16Hz artifact.

Try experimenting with an alternative DRL location. A DRL locate between the GSR electrodes and the headcap electrodes are optimal in theory. Try locating the DRL in the neck. A pin-CMS/flat-DRL is available.

I am wondering the same question as Juliette.

Interference by GSR is usually Common Mode, and can be reduced substantially by referencing (which should always applied anyway before analysis). However, in a FFT over a lot of points, the 16 Hz peak will of course eventual always show up, this is normal.
Ensure a good (equal) electrode contact of both GSR electrodes. If one GSR electrode has a much higher than the other, then part of the GSR excitation current may flow via the DRL electrode, and cause Common Mode 16 Hz interference.

Cohen: I am encountering the same problem as Juliette. We had the GSR input installed on our AD box about a year ago, and I have been seeing often this 16 Hz peak in my spectra when recording 64 EEG + GSR, even after filtering, average-referencing the data etc.
Are you saying it is normal to have the 16 Hz with GSR and we can only reduce it by re-referencing but never completely remove it?
Or does it mean that I need to improve my equal contact for both electrodes until it disappears? And if so, how can I check this when recording in Actiview?

Thanks,
Cedric

It is normal with the DRL on the head. It is caused by a small amount of 16 Hz excitation current flowing from the GSR electrodes to the DRL electrode. The 16 Hz leakage is minimal when the GSR electrodes have equal impedance and offset. However, this is difficult to check and control.

The 16 Hz is synchronized with the sampling rate, so can it can be rejected effectively with a sharp notch filter on exactly 16 Hz.

See above: try to relocate the DRL between the GSR electrodes (e.g. hand) and EEG electrodes (head).

Best regards, Coen (BioSemi)