[Eeglablist] ICA for ocular artifacts: split components?

[Agnese Zazio]

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.

Best,
Agnese
--
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$ 
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E-mail:
*agnese.zazio at gmail.com <agnese.zazio at gmail.com>*