[Eeglablist] Decay in frequency contents when ICA is applied

[Haj Morez]

I am using AMICA ICA for decomposing EEG signals and study behavior of the
EEG signals before and after applying ICA.
At the moment I am trying  to compare spectral content of raw EEG signals,
with spectral content of the ICA decomposition.
I am using the "spectopo()" function from EEGLAB, and calculate the peak
amplitude at 10 Hz averaged over all channels. Then trying to compare it
with peak amplitude at 10 Hz averaged over all components (ICs).
I noticed, there is a dramatic decay in the spectral content of the signal
when ICA is applied. The mean peak amplitude of the ICs is dramatically
lower than channels! (there was no IC rejection).
The 10 Hz peak amplitude averaged over channels is ranging between 20-50
dB, however it ranges between -20 to -5 dB when it is calculated over ICs.
I really appreciate it if you could help me to explain the reason why it
happens.

P.S. I had same results when used EX-infomax ICA.

Kindest regards,
Haj
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