[Eeglablist] re referencing to average after ICA

[Tarik S Bel-Bahar]

Greetings Raquel, some quick notes below, best wishes.



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Some drift will be attenuated with 1hz filter, but that in itself does not
require dropping of any time periods, unless you are referring to
abnormally extreme drift within a trial. However in my experience, removing
artifactual ICs does attenuate some slow artifacts.

If you haven't had a chance to yet, please google past eeglablist
discussions about filtering, ICA, and applying the weights to unfiltered
data.
In particular, i don't think there was a satisfactory conclusion on whether
it's okay to apply the weights from ICA to unfitlered data.
The usual step is apply the weights to the continuous data that is
essentially the same as the data that went into ICA.

Note that ICA won't necessarily catch slow-drift spatial maps, and that the
usual thing before ICA is to get rid of low-frequency "slow" drifts as
artifacts.

I would not leave drifts in, and/or be very clear about how I am
controlling them, or eliminating their effects (when for example, using a
baseline, or creating contrasts within-subjects).

Consider several possible strategies for drift artifact control, such as
1hz lowpass, demeaning, or detrending. These are all mentioned in past
eeglablist discussions, and often used in published papers.

If you're just starting up with EEG, just use the "basic" steps from eeglab
tutorial and makoto's pipeline. After that, use methods from high-impact
journals by respect EEG researchers focused on a protocol that is similar
to yours.

However, you can't hurt things by processing the data with and without the
drifts, and seeing for yourself the effects.

Remember that baselining your epochs/trials and similar things like that
are also filters for the data. However a 1hz lowpass filter does help ICA
get better results.

To learn more about various details and caveats.....
Google-Scholar some of the recent methods articles about baselining EEG
(for ERPs) from Luck and others.
Google-scholar the  "usual" methods in recent papers that use ICA for EEG
with eeglab. This should give you a good idea of what happens with drifts
and ICA usually.
See also Mike X. Cohen's book on time-frequency analyses for extensive
discussions of multiple points you are asking about (e.g., effects of
preprocessing on time-frequency results, etc...). I believe there is a
one-day pre-conference in Minnessota at the upcoming SPR by Mike that one
can also attned.
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