[Eeglablist] re Shielding paint: unlikely to work

[Emmanuelle TOGNOLI]

Dear Balaji,

We record most EEG in my lab with absolutely no environmental noise (see
e.g. absence of 60Hz in the typical spectrum attached). There are three
important components for this:
1-the recording environment: we established a careful setup with a
shielded room (with ground draining the stray currents down to earth) and
with a well-designed electrical arrangement
2-the amplifier performance, and especially its CMR (common mode
rejection): it tells how much attenuation is provided to the common
signal/environmental noise (a CMRR of more than 100dB is recommended, ours
is 108dB)
3-the subject preparation: the CMR works well as long as the noise is
identical between the reference and the other electrodes (or the two poles
of a bipolar recording). It requires low and matched impedances, and
steady attachments. We do not bypass dermabrasion (e.g. nuprep...) and we
spend a lot of time to secure the montages (you would diagnose this
problem if the noise differs across channels in a random -not regional-
fashion).

In my opinion, both aspects (1- not having environmental noise reaching
the subject and 2-3- removing the noise with the CMR) are important
contributors to clear recording.

May I suggest the excellent: Cutmore TRH., James DA., (1999). Identifying
and reducing noise in psychophysiological recordings. International
Journal of Psychophysiology, 32 : 129-150.

I hope it helps, kind regards,
______________________
Dr. Emmanuelle Tognoli
http://www.ccs.fau.edu/eeg
Center for Complex Systems and Brain Sciences
Florida Atlantic University

> Hello all,
>
> Just to follow up on this shielding issue e-mails..
>
> I know different people have different opinions on shielding. In our
> unshielded EEG  lab we see hell lot of 60 Hz artifact and few other
> artificats whose nature is unknown. There are two obvious reasons I could
> see. One, there is Chiller placed right below our floor for cooling our 3T
> magnet.. Second, there is a electrical junction box that runs through our
> wall outside our room.
>
> I just wanted to know if ANY kind of shielding can prevent the electrical
> interference from these sources? I would really appreciate if anybody can
> chip in their thoughts from their experience regarding this.
>
> Thanks a lot in advance,
> Balaji
>
>
>>>> "Fouquet N.C." <N.C.Fouquet at swansea.ac.uk> 1/21/2008 10:00 AM >>>
> In the lab where I work, it's 50 Hz (UK).
> I had contacted a collaborator who is now working in a company doing EMF
> testing, for his advice on that sort of paint shielding etc stuff for my
> lab and basically what he said was that the problem was more likely to
> be an eathing or earthing loop issue ("50 Hz noise is very often from
> injected or stray currents rather than radiated fields"). And in his
> opinion and experience, it's very hard to shield against 50 Hz fields,
> and metal wallpaper etc won't work too well.
>
> I personally didn't pursue the idea any further.
>
> Sorry guys,
> Nathalie

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