[Eeglablist] Low Pass Filtering and ERSP

[Ahmad, Jumana]

Dear Gozde,
It is difficult to completely remove frequencies above your cutoff- you will likely reduce their amplitude, but most filters have a roll off. I think that due to attenuation, and the fact you will have modulated these frequencies that it is not safe to analyse them. Power (and phase) will be distorted in some frequencies more than others.
Best wishes,
Jumana 

-----Original Message-----
From: eeglablist-bounces at sccn.ucsd.edu [mailto:eeglablist-bounces at sccn.ucsd.edu] On Behalf Of Gozde BAYER
Sent: 14 December 2016 07:22
To: eeglablist at sccn.ucsd.edu
Subject: [Eeglablist] Low Pass Filtering and ERSP


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



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