[FieldTrip] ICA in TMS-EEG

[Bingshuo Li]

Dear FieldTrip Community,

I recently started to analyze some TMS-EEG datasets and I encountered some
questions regarding to using ICA to remove eye movement/muscle artifacts in
our EEG data. As I am quite new to the analysis of TMS-EEG, I would like to
inquire the FT community for some hints or suggestions. Below are the
details of my questions:

//Description of Data Processing//
- EEG with 64 channel, sampling frequency 2500 Hz, electrode impedance less
than 5 kOhm
- Every epoch consists of 1s prior to and 1s after TMS (130-150 trials per
subject)
- TMS contaminated data points were cut out symmetrically -18ms to +18ms
relative to TMS onset. Cubic spline interpolation is used to fill in the
cut.
- Bandpass 0.5 - 80 Hz, with BUT and filter order 3.
- Discrete Fourier transform filter (cfg.dftfilter) to remove 50 Hz line
noise
- Visual inspection and rejection of trials with obvious unstable signal or
channels.

 //ICA//
- ICA algorithm: runica
- Demeaned data for ICA training (baseline is defined as the entire epoch
-1 to +1s)
- Unmixing matrix applied to non-demeaned data for component removal

/////QUESTIONS/////
Please see the image below for a typical result of ICA from a subject with
TMS applied at M1 (32 epochs for ICA training):
https://www.dropbox.com/s/chwo2jnwi72saba/ica1.png

Q1: It seems obvious to me that component 1 and 2 are of eyeblink origin.
However, what about component 5, 12, 20, 28? Topology-wise, they seem to
have a very anterior origin, but data in the time domain does not seem to
correlate with component 1 and 2 very well (judging visually..)

Q2: What can you say about components 7, 9, 13 and 18? Are these cranial
muscle artifacts?

Q3: Also, for components 42 and 54, given their high focality, are these
more or less a indication of bad/unstable electrodes?

- I guess maybe I am asking too many questions. I think my main problem
here is that I do not know what can be a good procedure / rules in manually
selecting ICA components for rejection? (I tried to look in the literature
but I couldn't find any that can answer my questions). And sometime I have
the feeling that my ICA results look like a mess and maybe there were
something wrong with my pre-processing or even data collection?

Thank you guys in advance for any input! I look forward to hearing from you!

Regards,

Bingshuo Li
MSc. Student, Neuroprosthetic Group, CIN, Uni Tübingen
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.science.ru.nl/pipermail/fieldtrip/attachments/20131126/a3d0ec60/attachment-0001.html>