Re: [Erppcatoolkit-support] Using PARE correction for EEG data

[Joseph D. <jd...@ma...>]

Thanks for the link!  Agreed that the page is a bit confusing.  I looked around some more and found this very lucid explanation of the Biosemi system’s approach:

http://sccn.ucsd.edu/pipermail/eeglablist/2005/000933.html

Given that the CMS is essentially the equivalent of the ground electrode of a conventional system, I agree that it should not be included in the final montage.

Also, after some further thought, I changed my mind about dropping channels with no coordinate information.  The PARE computed without the missing electrodes would in some sense be inconsistent with them but practically the PARE would still provide a best estimate for them.

Thanks for the clarification!

Joe

> On Jul 6, 2015, at 06:25, Andreas Widmann <wi...@un...> wrote:
> 
> Dear Maria and Joe,
> 
>> 1) You really have another electrode so you might not want to discard it.  CMS (in the Biosemi system) is your recording reference electrode.  As I understand it from the online documentation, Biosemi terms this initial data “unreferenced” which is really misleading.  The data necessarily has a reference (you can’t have voltage data that doesn’t have a reference) and the reference site is CMS.  This site is, by definition, a flat line with zero voltage.  To save disk space Biosemi is not including this flat line with the rest of the data but it’s there.  This is relevant since it stops being flat and boring if you rereference.  You should put it back into your dataset one way or another prior to rereferencing.
> The Biosemi system does not apply common mode rejection online but expects CMR to be performed offline by re-referencing. Re-referencing is, thus, mandatory for Biosemi data and, to my understanding, the CMS channel must not be added back to the data and particularly not be included into the average reference or used for interpolation as it contains the noise.
> 
> There is some, also slightly confusing, documentation on the Biosemi website:
> http://www.biosemi.com/faq/cms&drl.htm
> 
> Best,
> Andreas

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Joseph Dien, PhD
Research Associate
Cognitive Neurology/Neuropsychology
The Johns Hopkins University School of Medicine

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