[Eeglablist] filters, ICA and erp

[Jason Palmer]

Hi Sara,

In my experience, using a sharp 1Hz high pass filter is best for ICA, and
doesn't significantly reduce ERP amplitude--the ERPs I know of are at least
2 Hz, so the 1Hz high pass shouldn't be a problem. The main issue is to
eliminate slow drifts in the data which make the mean non-stationary.

If you want to look at low frequencies specifically, you might do low pass
filtering, or band pass between 0.1Hz and say 30 Hz, to try to remove high
frequency sources, leaving only the low frequency sources, but I doubt this
would improve ERP results over a ! Hz high-pass filter.

Average reference is also fine if you are doing ICA after. Spreading muscle
artifacts etc. to other channels is not a problem since ICA will remove the
muscle activity etc. and put it in a single source (usually).

After you do average reference, the data rank goes down by 1, so if you have
94 channels avg referenced, ICA should give you back 93 components/sources.

Hope this is helpful.

Jason

-----Original Message-----
From: eeglablist-bounces at sccn.ucsd.edu
[mailto:eeglablist-bounces at sccn.ucsd.edu] On Behalf Of Sara Graziadio
Sent: Wednesday, October 05, 2011 7:46 AM
To: eeglablist at sccn.ucsd.edu
Subject: [Eeglablist] filters, ICA and erp

Hello,
I would like just a suggestion about some data cleaning/analysis I am doing.
I am doing an ERP analysis and I want to clean my data first with the ICA.
In theory, though, I should not use an high-pass cutoff higher than 0.1 Hz
to not reduce the erp amplitude. On the other side the ICA does not work
well if the high-pass cutoff is lower than 0.5 Hz...what is then the best
method to apply? Has anybody tested how robust the ica is with a 0.1Hz
filter? 
I have also another question: I am doing the analysis on 94 electrodes
referenced to Fz. I planned to average reference the data but actually there
is quite a large spread of noise on all the electrodes with this method
(muscular artefacts for example from the temporal electrodes). But actually
almost all the papers are using the average reference so I was surprised, am
I the only one having this problem of noise? Would not be better just to
keep the Fz reference and then perhaps to average the erps for every
different cortical area and do the analysis on these averaged erps?

Thank you very much

Best wishes 

Sara Graziadio
Research Associate
Newcastle University



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