[Eeglablist] ICA for ocular artifacts: split components?

Agnese Zazio agnese.zazio at cognitiveneuroscience.it
Tue May 4 01:12:04 PDT 2021

Dear all,

I am using ICA to correct for ocular movements in a TMS-EEG dataset with 74
EEG channels, recorded with a sampling rate of 9600 Hz with
online reference on FPz.
Before ICA, I applied the following preprocessing steps: time interpolation
around the TMS pulse (from 1 ms before to 2 ms after the TMS pulse), high
pass filter at 1 Hz, downsampling at 4800 Hz, epoching from -200 to 500 ms
after the TMS pulse (n=180 epochs), baseline correction (-100 -2 ms), SOUND
algorithm for noise removal (Mutanen et al., NeuroImage 2018), artifact
rejection. For ICA, I am using this code:

EEG = pop_runica(EEG, 'chanind', [1: size(EEG.data,1)], 'interupt','on');

When exploring the ICA components, In a few cases I found a 'strange'
pattern: two components with a similar topography (but reversed in
polarity) seem to reflect the same component but in distinct time-windows.
As can be seen in the plot trials-by-time, the first component shows an
ocular pattern up to about 250 ms, and the second component displays a
similar pattern but from 250 ms on. What seems strange to me is that the
temporal interruption at about 250 ms is very sharp. At this link (
https://urldefense.com/v3/__https://drive.google.com/file/d/1YiRjx4BPEPC6-NFySPbN-sNHwN6MmbLC/view?usp=sharing__;!!Mih3wA!TSRqGkavVV4G_JOcKWIJFYOeQf68i2IF5NhBGlcT8W-_EeR9wq5KX0qMgBMeeuXsM_AxCw$ )
you can see a figure which makes it much easier to understand the problem.

Is anyone familiar with this issue?
I'm using Matlab 2020b and eeglab v.2020.0.

Thank you in advance.

Agnese Zazio, PhD
Neurophysiology Lab
IRCCS Centro San Giovanni di Dio Fatebenefratelli
Via Pilastroni 4, 25125 Brescia, Italy
Phone number: (+39) 0303501-596
Site: *https://urldefense.com/v3/__http://www.cognitiveneuroscience.it__;!!Mih3wA!TSRqGkavVV4G_JOcKWIJFYOeQf68i2IF5NhBGlcT8W-_EeR9wq5KX0qMgBMeeuVh3da2rQ$ 
<https://urldefense.com/v3/__http://www.cognitiveneuroscience.it/__;!!Mih3wA!TSRqGkavVV4G_JOcKWIJFYOeQf68i2IF5NhBGlcT8W-_EeR9wq5KX0qMgBMeeuVKp4TURw$ >*
*agnese.zazio at gmail.com <agnese.zazio at gmail.com>*

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