[Eeglablist] epoch baseline removal experience?

[Stephen Politzer-Ahles]

Hello Ida,

The purpose of baseline correction is not to know when the event happened
(hopefully you already know that) but to ensure that when you compare data
from different epochs or conditions you are controlling for differences
that happened prior to the event (and thus are not relevant to the
processing of that event). For example, if you have two events that
elicited identical ERPs, but one event happened to occur right after a
large positive drift whereas the other event happened to occur right after
a large negative drift, you would incorrectly conclude that the events
elicited different ERPs; by doing baseline-correction you subtract out
those irrelevant pre-stimulus differences and avoid making an incorrect
conclusion like that.

As for your second question: it is not necessary to use the entire
pre-stimulus interval for baseline correction. So it should be fine for you
to use for example a [-1000 0] ms baseline even if your epoch is actually
bigger than that.

Best,
Steve Politzer-Ahles

On Fri, May 11, 2012 at 1:47 AM, ida miokovic <ida.miokovic at gmail.com>wrote:

> Dear Makoto,
>
> thank you for your answer, it cleared the doubts in my head regarding this
> =). As I understood, the purpose of Baseline Removal is for me/us to have
> better insight when event in observed epoch happened, so the value around
> corresponding marker is expected to be zero. Right?
>
> I have one more question regarding this - does it matter if I Remove
> Baseline for example (-1000ms to 0ms) if I have epoch that is longer (-4
> secs to 4 secs)? I read in Q&A list Arno's answer regarding similar
> question where he said that ICA can be unstable if the epochs baseline is
> too short, so he suggests longer baselines (i.e 1 sec).
>
> Thanks,
>
> Ida
>
> On Thu, May 10, 2012 at 9:45 PM, Makoto Miyakoshi <mmiyakoshi at ucsd.edu>wrote:
>
>> Dear Ida,
>>
>> The consequence would be that you may not have near-zero potential
>> at/around time zero (and this time zero which should be an onset of
>> whatever event). Usually people want to reset their data to zero
>> microvolt at/around time zero, so they subtract mean of short time
>> period immediately before it (for example, -200 ms to 0 ms as a
>> baseline period). Am I answering to your question? If not, let me
>> know.
>>
>> Makoto
>>
>> 2012/5/10 ida miokovic <ida.miokovic at gmail.com>:
>>
>> > 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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>>
>>
>> --
>> Makoto Miyakoshi
>> JSPS Postdoctral Fellow for Research Abroad
>> Swartz Center for Computational Neuroscience
>> Institute for Neural Computation, University of California San Diego
>>
>
>
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-- 
Stephen Politzer-Ahles
University of Kansas
Linguistics Department
http://www.linguistics.ku.edu/
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