[Eeglablist] Low Pass Filtering and ERSP

[Gozde BAYER]

Dear EEGLAB List,

During my raw EEG data recording from different subjects, I observed 
(online from the FFT plot of related software) some kind of 'noise' 
standing still at around 20-25 Hz and I could not get rid of that for 
some reason. So I decided to let EEG stream under
this condition. Therefore I applied low pass filtering to the raw data 
at 20 Hz, and analyzed offline the remaining frequencies with EEGLAB. I 
created STUDY design for the purpose of the experiment. But when I plot 
ERSP of a specific channel, I could still distinguish frequencies 
surviving above 25 Hz. I checked from the workspace of Matlab as well. 
What could be the reason of that? Since I am sometimes getting this 20 
Hz peak during raw EEG data recoding (by the way, I am trying to find 
out why this is the case), it is important for me to really understand 
the phenomenon behind that..Indeed, those frequencies just above 20 Hz 
are also of great importance for my future analysis..If they are able to 
survive -somehow- then would it be reliable to analyze beta frequency 
band under this condition?

I appreciate any help,
Best wishes.

Gözde BAYER