[Eeglablist] Re-referencing drastically changes my power spectrum

[Ulrich Pomper]

Dear EEGlab list,

I have a question regarding to order of two common preprocessing steps: 
filtering and re-referencing to common average.

In my current EEG study, I initially filter the raw, continuous data 
into low (2-12 Hz) and high frequency components (13 - 80 Hz), and 
afterwards re-reference the data to common average. However, this 
re-referencing step really messes up the spectrum of the data in a way 
that makes me suspect an error (please see panels 7 and 8 in this figure 
[1]).

When I re-reference first and filter second, on the other hand, the 
spectrum stays intact and the data seem OK (see panels 5 and 6).

I was hoping that someone might be able to explain to me what is going 
on here, and potentially confirm that re-referencing first and filtering 
second is actually the correct way to go.

I'd highly appreciate any input!

Many thanks,

Ulrich

-- 
Dr. Ulrich Pomper

Senior Scientist
Department of Cognition, Emotion, and Methods in Psychology
  University of Vienna
  Liebiggasse 5
  1010 Vienna
  Austria

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ulrich.pomper at univie.ac.at
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