[Eeglablist] FIR filter and IIR and leaks

[Baris Demiral]

Dear all,

I have two questions related to FIR (default pop_eegfilt function) and the
leaking phenomenon. Question one is: If you have two conditions which can
somehow differ in phase and amplitude showing up at the particular latency
after the stimulus onset, how much filtering based leak effect would you
observe in the erp plots? How would you get an estimate?

Here is my story:

I filtered the raw data with the pop_eegfilt with 0.1Hz highpass and 80Hz
lowpass and then epoched it, leading to two sets of conditions for
comparison. I observed that, after baselining the epochs to some
pre-stimulus interval, there is a very early statistically significant
difference between the conditions around 100-160ms where ERP amplitude was
used as a dependent measure, But, on the ERP plots the difference looks as
if it started slightly before the onset of the stimulus onset  for some
electrodes. Can I attribute this to the filfilt? If so would filtering this
data one more time with IIR help (maybe filtering with much lower values)?

Other question related to this is: I feel like using default fir filtering
is better for the continuous data at the beginning of the pre-processing
where you may like to conduct TF or ITC analysis too in later stages. I
cannot figure out how and when I should use IIR. Shall I apply it on a
continuous data at teh beginning if I am not concerned about the phase
coherence or TF? Is this a better way?

Thanks,
Baris

-- 
SB Demiral, PhD.
Department of Psychology
7 George Square
The University of Edinburgh
Edinburgh, EH8 9JZ
UK
Phone: +44 (0131) 6503063
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