[Eeglablist] phase shift and ICA

[Makoto Miyakoshi]

Dear Dr. Gonzales,

I suppose you mean you compare phases between channels and ICs. It's not
shift that makes difference between scalp channel ICs and EEGs. Let me
explain what it is. Let's say we have only 2 channels and 2 ICs. The
relation of scalp channel EEG and ICs are like this:

Channel 1 = 0.3 X IC1 + 0.7 X IC2
Channel 2 = 0.7 x  IC1 + 0.3 x IC2

Here I determined 0.3 and 0.7 arbitrarily but actually these are determined
by ICA. Anyway, when you compare Channel 1 with IC1 in the above example,
they are dissimilar to each other NOT because of phase shift (unless IC1
and IC2 are similar, which is hard to imagine since they are supposed to be
temporally independent).

If the decomposition is like

Channel 1 = 0.99999 X IC1 + 0.00001 X IC2
Channel 2 = 0.00001 x  IC1 + 0.99999 x IC2

Then your Channel 1 and IC1 should be very similar.

Makoto



2013/4/3 karlo gonzales <thats_karlo at yahoo.com>

> Dear Dr. *Miyakoshi,*
> *
> *
> *I am using  EEGLAB and ICA tools to clean EEG data. i am recording  EEG
> from a 64 channel cap (NEUROSCAN), with  a fixed reference electrode, which
> is  located between CZ and CPZ. *
> *i do not re-reference my data for further analysis.** So, i just wonder,
> if applying ICA, would shift the phase of signals (specially low
> frequency)? is there anyway to check it? *
> *
> *
> *many many thanks for all your help and support,*
> *Karlo*
> *
> *
>
>
>
>
>
>


-- 
Makoto Miyakoshi
Swartz Center for Computational Neuroscience
Institute for Neural Computation, University of California San Diego
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