[FieldTrip] Decreased baseline level after using ICA in ERP data

[Lozano Soldevilla, D. (Diego)]

Dear Beom Jun, I see multiple scenarios why this baseline activity decrease could happen. First of all, how the component you're rejecting look like (i.e. "blink component")? Do you see this activity decrease after the baseline period? The "quality" of the ICA decomposition, how well your artifact/component of interest has been isolated by algorithm in time (i.e. blink time courses) and space (marked frontal topography), will determine the activity that later on you'll reject/select. If your decomposition is not well suited, the rejection of a particular IC activity might have "extra" activity you don't want to reject (effect of interest), might be the algorithm is not able to isolate the components of interests (i.e. artifacts) or a combination of both. To evaluate the quality of your ICA decomposition you might have a look here ( http://www.ncbi.nlm.nih.gov/pubmed/19162199 ). Basically, the authors find that the ICA decomposition improves significantly " increased by removing the mean EEG at each channel for each epoch of data rather than the mean EEG in a prestimulus baseline" . In addition (see here: http://sccn.ucsd.edu/pipermail/eeglablist/2012/004925.html ), high-pass filtering above ~1hz improve the results. It's very important to feed ICA as much relevant data as you can use. The more the data, the better the decomposition. There's a rule of thumb that says that for a reliable IC decomposition 20 time points per channel 2 is needed (see here for a reference http://www.ncbi.nlm.nih.gov/pubmed/16904745 ) I hope that helps, Diego ----- Original Message -----
> From: "Beom Jun Min" <mbj0310 at gmail.com>
> To: "FieldTrip discussion list" <fieldtrip at science.ru.nl>
> Sent: Monday, 24 June, 2013 6:27:47 AM
> Subject: [FieldTrip] Decreased baseline level after using ICA in ERP
> data
> Dear all,
> I have ERP data and now I am dealing with ICA to remove muscle and eye
> artifacts.
> However, I found that after ft_rejectcomponent, the baseline level of
> the segmented epoch decreased. (The baselinewindow is [-0.2 0].)
> The baseline level decreased even though I rejected only one
> component.
> My script is shown below.
> %% Removing the Artifacts
> cfg = [];
> cfg.component = [ ]; % to be removed component(s)
> post_ICA_temp6 = ft_rejectcomponent(cfg, comp_detrand, data_raw);
> %% timelocking
> cfg = [];
> timelock_temp6 = ft_timelockanalysis(cfg, post_ICA_temp6);
> %% Plot
> figure;
> cfg = [];
> cfg.layout = lay;
> cfg.interactive = 'yes';
> cfg.channel = ['all', {'-EKG', '-EMG'}];
> ft_multiplotER(cfg, timelock_temp6)
> Is there something that I missed?
> Thanks.
> BJ
> --
> BeomJun Min, M.D.
> Department of Medical System Engineering (DMSE)
> Gwangju Institute of Science and Technology (GIST)
> 261 Cheomdan-gwagiro(Oryong-dong), Buk-gu, Gwangju
> 500-712, Republic of Korea (South)
> Phone: +82-62-715-3266 / Fax: +82-62-715-3244
> E-mail: mbj0310 at gmail.com , http://bmssa.gist.ac.kr
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-- PhD Student Neuronal Oscillations Group Donders Institute for Brain, Cognition and Behaviour Centre for Cognitive Neuroimaging Radboud University Nijmegen NL-6525 EN Nijmegen The Netherlands http://www.ru.nl/people/donders/lozano-soldevilla-d/
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