[Eeglablist] ICA component

[Dorian Grelli]

Hi guys,
I am very new with eeg data analysis and it would be great to have some
support from you!

I found the quotation below from this tutorial:
http://sccn.ucsd.edu/wiki/Chapter_09:_Decomposing_Data_Using_ICA

*"Very important note: We usually run ICA using many more trials that the
sample decomposition presented here. As a general rule, finding Nstable
components (from N-channel data) typically requires more than kN^2 data
sample points (at each channel), where N^2 is the number of weights in the
unmixing matrix that ICA is trying to learn and k is a multiplier. In our
experience, the value of k increases as the number of channels increases.
In our example using 32 channels, we have 30800 data points, giving
30800/32^2 = 30 pts/weight points. However, to find 256 components, it
appears that even 30 points per weight is not enough data. In general, it
is important to give ICA as much data as possible for successful training.
Can you use too much data? This would only occur when data from radically
different EEG states, from different electrode placements, or containing
non-stereotypic noise were concatenated, increasing the number of scalp
maps associated with independent time courses and forcing ICA to mixture
together dissimilar activations into the N output components. The bottom
line is: ICA works best when given a large amount of basically similar and
mostly clean data. When the number of channels (N) is large (>>32) then a
very large amount of data may be required to find N components. When
insufficient data are available, then using the 'pca' option to jader.m
<http://sccn.ucsd.edu/eeglab/locatefile.php?file=jader.m>** to find fewer
than N components may be the only good option.*"

I don't know if each of my datasets has enough datapoints for performing an
ICA. Each dataset has 20 channels, last 2 minutes and is 4 seconds epoched,
baseline corrected and pass band filtered. I also reject bad epochs.

Which is the meaning of "trials" in the quotation above? Would be better to
have longer registrations?
When I run ICA I got 20 components. Why are there some examples with 256
components?

Dorian

Dorian
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