[Eeglablist] About band-pass filtering

[Arnaud Delorme]

Dear Marco,

sorry for the late feedback.

> I have a question about filtering. I would like to band-pass filter 
> some EEG data consisting of epochs of about 1.5 sec, sampled at 250 
> Hz, between 0.5 Hz and 20 Hz. Since I have around 370 data points per 
> epoch, I'm forced to use a 120-points order high-pass filter. As 
> recomended many times in this mailing list, I first high-pass filter, 
> then low-pass filter. But when I look at ERPs, I still see slow 
> oscillations which seem suspicious to me.

It seems hard to remove oscillations at 0.5 Hz (2-second period) in 
1.5-second epochs. You should apply the filter before epoching the data. 
Another reason for filtering the continuous data is that filters may 
introduce artifacts at the beginning and at the end of each data epoch.

> The problem gets worse if I filter the concatenated data before epoching. 

Do you mean continuous data (or did you artificially concatenated your 
data epochs)?

> Could this be due to the shortness of the highpass filter? I tried 
> also with 1 Hz and 2 Hz, but the result is similar. Or anyone sees 
> another explanation?

You can visualize filtered data using the spectopo() function (menu Plot 
 > Channel spectra and maps). If you low pass data below 20Hz you should 
see ripples above 20 Hz with about 20-40dB decrease in signal amplitude. 
The same is also true for highpass. This way, you can actually visualize 
which portion of the data was filtered out.

Though non-linear filters do not preserve phase information across 
frequencies, Max Pozdin from the Georgia Institute of Technology wrote a 
plugin for EEGLAB for filtering data using nonlinear IIR filters 
(available soon). This plugin may help achieving better stopband 
attenuation.

Arno