[Eeglablist] About band-pass filtering

[Marco Buiatti]

Hello everybody,

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

Thank you in advance,

Marco

-- 
Marco Buiatti

Neurophysique et Physiologie du Systeme Moteur - CNRS UMR 8119
UFR biomedicale Les Saints-Peres
45 rue des Saints-Peres 75270 Paris cedex 06
Tel: +33(0)142862146
Fax: +33(0)149279062
http://www.neurophys.biomedicale.univ-paris5.fr/~buiatti/