[Eeglablist] How to do data decomposition with ICA?

[Hannu Loimo]

Hi,

We are using ICA to remove CI-artefact from our data. ICA seems to find
pretty easily at least some of the artefact induced by CI-devise. There are
8 events in our experiment and two of those events (duration and gap
deviants) produce different form of CI-artefact in time domain. All events
are made out of syllable sequence ta-ta-ta. Gap deviant means that second
syllable starts 100ms later than in other events. Duration deviant means
that the whole ta-ta-ta sequence lasts 50ms longer. Presumably CI-artefacts
rising from different events have same kind of spatial distribution and
presumably most significant difference is how these artefact peaks are
located in time. So basically there are three differend shapes of
CI-artefact present in time domain: one for gap deviant, one for duration
deviant and one for other six stimuli. After doing ICA for whole dataset
(all events) CI-artefacts emerging from different events seem to be
contained in one component and not many. My question is:

Is it a problem to do ICA for whole dataset or shoud ICA be done for
individual events separatelly? Size of dataset is 2000 epochs and separating
events into different dataset would end up datasets as small as 150-200
epochs (too few for ICA?). So far we have been doing ICA for whole datasets,
then removed CI-artefact components and after that separated data into
different events. What I'm asking is does it affect the final outcome of our
separate event ERP composition if you remove CI-artefact components that are
calculated according to all events or is it same if you do ICA for single
events and then remove components?

thanks,

Hannu
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