[Eeglablist] Biosemi rereferencing
jeffreyj at uci.edu
Fri Aug 6 17:48:43 PDT 2004
I've been trying out the new version of EEGLAB and was curious about this
new message given when importing Biosemi BDFs:
"We highly recommend that you choose a reference channel IF these are
(e.g., a mastoid or other channel). Otherwise the data will lose 40 dB of SNR!"
Can anyone explain the nature of this message? Is it just a general
warning that BDFs should be rereferenced before data analysis (which to my
knowledge is the typical practice with all EEG), or is it a warning that
the BDFs should be rereferenced at the stage of importing into EEGLAB? Is
the loss in SNR due to the 24- vs. 32-bit difference between the BDFs and
In trying to get a grasp on this issue, I've compared these two procedures:
1) Importing the data reference-free, then rereferencing to linked
mastoids and exporting to ASCII.
2) Importing the data referenced to linked mastoids, then exporting to ASCII.
When doing this, there were occassional differences in values on the order
of .01 or .001. Are these worth worrying about? My exported values for 1
channel after doing this were in the range of about -9900 to -8800, so I
was wondering if these minute differences will eventually be transformed
into larger differences in terms of microvolts.
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