[Eeglablist] epoch baseline removal experience?

[Stephen Politzer-Ahles]

Hello Ida,

In general the best answer is of course to check the literature for studies
related to yours and do whatever is most accepted in your field.

I'm not familiar with any specific studies that demean the EEG using the
whole epoch (although I think it's not unheard of). But it seems to me that
if you use the whole window, between-condition differences in the ERPs at
one point in the epoch could be due to between-condition differences
somewhere else in the epoch (for instance, if two conditions elicit
identical ERPs except that one condition is h as a greater positivity late
in the epoch, then if you baseline-correct using the entire epoch then that
condition will become more negative than the other condition for the rest
of the epoch--this can be verified easily using fake data, for instance do
raw1(1:1000)=1; raw2=raw1; raw2(800:end)=5; baseline1=raw1-mean(raw1);
baseline2=mean(raw2);
, and then plot the result. ). On the other hand, if you use only the
pre-stimulus interval or part of the pre-stimulus interval in your baseline
(which is more common at least in my area), then as long as there are not
systematic differences before the stimulus event then you won't have this
problem.

Best,
Steve

On Thu, May 10, 2012 at 4:26 AM, ida miokovic <ida.miokovic at gmail.com>wrote:

> Hello everyone,
>
> Since I do not have experience in eeg signal processing, I am asking you
> for the opinion regarding epoch baseline removal (a window for this pops up
> after I do the data epoching). Epochs I am extracting are quite long: -4
> secs before and 4 secs after Marker of my interest.
>
> Why is following suggested in tutorial:
>
> *"Using the mean value in the pre-stimulus period (the pop_rmbase()
> default) is effective for many datasets, if the goal of the analysis is to
> define transformations that occur in the data following the time-locking
> events."*
>
> What are the consequences if I leave the fields in pop up window (Epoch
> Baseline Removal) empty and therefore have the whole epoch used as a
> baseline?
>
> Thank you in advance,
>
> All the best,
>
> Ida
>
>
>
>
>
>
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-- 
Stephen Politzer-Ahles
University of Kansas
Linguistics Department
http://www.linguistics.ku.edu/
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