[Eeglablist] Biosemi rereferencing

[Scott Makeig]

Jeff -

I put in that warning in case someone (e.g., we at SCCN) tried to analyze 
Biosemi data without selecting a reference. Only Biosemi can record 
unreferenced data, because of their active impedance balancing approach, 
but they caution against leaving the data in this form for analysis, 
since equal noise signals are then present in all channels - noise that  
referencing removes.  Should the warning be changed to be clearer, or 
only invoked under certain conditions?

Scott Makeig

On Fri, 6 Aug 2004, Jeff Johnson 
wrote:

> 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 
> Biosemi data
> (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 
> MATLAB?
> 
> 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.
> 
> Thanks,
> Jeff
> 
> 
> 
> 
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