[Eeglablist] Merging files for preprocessing and ICA

[Caroline Lustenberger]

Dear EEGLAB Team

 

There is always the discussion about how many samples are needed for a good
ICA and based on the tutorial pages from your lab it seems to be quite many.
People that use high density EEG with 128 electrodes would likely need
~1000000 data points, which means more than 1 hour of EEG recording
(considering we do downsample to 250 Hz). I doubt that all of us have
recordings that are that long. At least I often have tasks that last for ~
10-20min. One idea that I thought of is to merge datasets of a person if
she/he had multiple sessions on the same day with the same task (that is
having the same net application). I would like to hear your opinion about
this idea, whether you also do this approach and what I should consider when
merging files.

 

Here is a specific example: I tried this approach for a visual memory task
during which my participants were seeing and rating pictures before a nap
and after a nap. I merged the two datasets and after preprocessing (filter,
clean_rawdata, interpolation, average referencing) I performed an ICA (tried
both AMICA and RUNICA, results are very similar). The components seemed to
be captured fine, however I clearly saw that many of the signals (e.g.
muscle, but also physiological brain components) changed in activation after
the transition of the merged files. More specific, the components were
capturing the same type of activity (e.g. muscle,  eye blinks or alpha) but
the amplitudes of the component clearly changed between the first and second
data set. That is for instance one muscle component had strong activation in
one dataset and was very low amplitude in the second part of the data.
People are likely more relaxed after sleep, are maybe more focussed and I
was wondering whether merging of datasets is not ideal because conditions
are slightly different and I even expect that oscillatory activity relative
to the stimuli will slightly differ before and after sleep for the same
task. What is your impression? Would you rather process the files separately
or merge them together to one?

 

Thanks for your valuable input and best wishes,

Caroline

 

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