[Eeglablist] Low Pass Filtering and ERSP

[Gozde BAYER]

Dear Makoto, 

Thank you very much for your reply. 

I have attached the ERSP analysis of channel C3 in jpg format. There are
two conditions (hs,ss) from 7 subjects and the data were lowpass
filtered at 20 Hz in the preprocessing step. As you may realize, there
are still ersp data values just above 20 Hz (extending to about 30 Hz).
I think it is due to the lowpass FIR filter that lowers the frequency
amplitudes (above the filter value) slowly..But as you mentioned, the
frequencies just below it (20 Hz) still seem intact. In this case Notch
filtering may not be necessary (as some people in my lab have
recommended). Because in either case I will be analyzing the lower
frequencies..Have I understood correctly?.. 

Thank you very much again,
Best wishes.
Gözde BAYER 

On 30-12-2016 02:17, Makoto Miyakoshi wrote:

> Dear Gozde, 
> 
> If you can upload screenshot of the data in question, I can help you better. Let's identify what was wrong. 
> 
> Generally speaking, it would be still ok to apply 20-Hz low pass filter if you say only < 20 Hz of your data seems intact, just in the worse case. 
> 
> Makoto 
> 
> On Tue, Dec 13, 2016 at 11:21 PM, Gozde BAYER <gbayer at hacettepe.edu.tr> wrote:
> 
>> 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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> 
> -- 
> 
> Makoto Miyakoshi
> Swartz Center for Computational Neuroscience
> Institute for Neural Computation, University of California San Diego

  

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