[Eeglablist] Signal/noise loss, re-referencing Biosemi data

[Jeff Eriksen]

I concur with Tom Ferree's recent response to this. I will add that I
believe the Biosemi amp can record using a real-time electronically derived
average reference, but you can simply ask them directly or look at their web
site. I am aware of one clinical system, the H2O ambulatory EEG recorder
from Grass-Telefactor that also records against a physical average
reference. This may have some clinical usefulness, but I think it is
dangerous because a single bad electrode will then contaminate the
recording. 

-Jeff Eriksen

-----Original Message-----
From: eeglablist-bounces at sccn.ucsd.edu
[mailto:eeglablist-bounces at sccn.ucsd.edu] On Behalf Of Jim Kroger
Sent: Monday, May 09, 2005 11:34 AM
To: jkroger-crl.NMSU.Edu :
Cc: eeglablist
Subject: [Eeglablist] Signal/noise loss, re-referencing Biosemi data

Recently, the following was posted by Arno, and I'm looking for more 
details. I've searched the archives thinking there was an earlier, longer 
post on this, but can't find it. I have 2 questions about it below:

--------
Finally, do not forget to re-reference your data (otherwise there is a
40dB loss in signal/noise).
--------

1. Biosemi is collected and stored raw, as I understand it, and referencing 
is done in software; when I use EMSE I select "common average 
reference."  Is a similar operation what is meant by re-referencing? I 
gather from vague recollection of the previous post that Arno meant the 
data must be referenced with a single electrode as the reference.....that 
is, the common average reference scheme should NOT be used, or the S/N loss 
will occur. Is this correct? If so, since we did not use a reference 
electrode for some collected data, which electrode is recommended as the 
reference electrode? We are looking at visual attention and working memory, 
so occipital, frontal, and parietal are important.

2. Then, should we be collecting data with a reference electrode?  Any 
reason to do with nose, earlobes, mastoids?  Linked?

Thanks
Jim


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