[Eeglablist] Question about Order of rereferencing and artifact removal

[Stephen Politzer-Ahles]

Hi Steven,

As for the order of operations, in our lab we always re-reference first,
but I don't in principle see why doing artifact rejection first would
affect the referencing process--given that at each datapoint you're just
subtracting a value from all channels, that process shouldn't be affected
by whether or not you removed other data points. The only way I can imagine
it would have an effect is that, when you do artifact rejection, you would
have to keep in mind that artifacts might 'look' different depending on how
the data are referenced (for instance, blinks might show a different
polarity, etc.). I've only done re-referencing to mastoids, not to average
reference, but according to my understanding the concept should be the same.

I'm not sure about the ICA issue, let's see what the others on the list
have to say...

Best,
Steve

On Thu, Mar 21, 2013 at 5:10 PM, Steven Pillen <stevendpillen at gmail.com>wrote:

> Hello, EEGLABList.
>
> There was a question we had at our lab about the order of operations when
> it comes to rereferencing and other steps of preprocessing.  Is considered
> better to remove artifactual data before rereferencing to the average, or
> before filtering the data?
>
> In addition, when we tried to rereference to the average immediately after
> removing artifactual data, when we got to the point where we tried to run
> ICA, we got an error message that read:
>
> "EEGLAB has detectat that the rank of your data matrix is lower the number
> [sic] of input data channels.  This might be because you are including a
> reference channel or because you are running a second ICA decomposition.
>  The proposed dimension for ICA is 31 (out of 32 channels). Rank
> computation may be innaccurate so you may edit this number below.  If you
> do not understand, simply press OK."
>
> It would not run with either 31, or 32 channels, and we're not sure why.
>
> Does anyone have any insight as to why?
>
> Thank you,
> Steven Pillen
>
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-- 
Stephen Politzer-Ahles
University of Kansas
Linguistics Department
http://people.ku.edu/~sjpa/
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