[Eeglablist] FFT vs. PSD

Sebastian Grissmann sebastian.grissmann at lead.uni-tuebingen.de
Mon Oct 20 05:00:25 PDT 2014


Dear Arno,

No, I was using 'specmode', 'fft', 'nfft', 1024 (to get a finer  
grained picture) 'logtrials', 'off' (the default)

Thanks for the info! I was assuming (based on the plots) that the log  
is always computed.

Best,
Sebastian

Quoting Arnaud Delorme <arno at ucsd.edu>:

> Dear Sebastian,
>
> Did you try 'specmode', 'fft', 'logtrials', 'on'?
> In one case (logtrials off), the function computes the power for  
> each trials, average power across trials, then compute the log for  
> display.
> In the second case (logtrials on), the function computes the log  
> power for each trial then averages these trials.
> So it is a matter of when you take the log.
>
> It is a matter of taste. Log transformation tend to reduce the  
> noise, so maybe the second option is better.
>
> Arno
>
> On Oct 2, 2014, at 10:44 AM, Makoto Miyakoshi <mmiyakoshi at ucsd.edu> wrote:
>
>> Dear Sebastian,
>>
>> It seems you are asking 'which one is correct' or 'what the correct  
>> method', but there is no such a thing as 'the only solution' here.  
>> If you change the computational method, the result also changes. As  
>> long as your result can be replicated by other researchers by using  
>> the methods and parameters you describe in the paper, and also as  
>> long as your reviewer says no to your methods and parameters (or  
>> don't say anything about them), you are fine. You can, and should,  
>> choose your methods and parameters and you owe full responsibility.
>>
>> To conclude, you can choose whichever methods/results you like.  
>> Just be confident; instead of saying 'which one should I use',  
>> which is so nice of you, but once you learn both are valid you  
>> should say 'what's wrong with using this'. I know how you feel  
>> since my degree is psychology too.
>>
>> Makoto
>>
>> On Thu, Oct 2, 2014 at 12:37 AM, Sebastian Grissmann  
>> <sebastian.grissmann at lead.uni-tuebingen.de> wrote:
>> Hi there,
>>
>> I´m a PhD student (who studied Psychology) and currently trying to  
>> analyze my first EEG dataset. I first started to compute my channel  
>> spectra ( pop_precomp() ) via a FFT (using the default spectopo  
>> parameters: ‘specmode’, ‘fft’, ‘logtrials’, ‘off’), but when I was  
>> later looking at specfreqs (returned from std_specplot) I found  
>> that my frequency bins were quite broad (>1Hz). Since I need a  
>> higher frequency resolution for my analysis I tried PSD instead of  
>> FFT to compute my spectra (spectopo parameters: ‘specmode’, ‘psd’,  
>> ‘logtrials’, ‘off’). Now I had a very good spectral resolution, BUT  
>> the spectra looked quite different. For example, the alpha peaks  
>> were gone (or strongly diminished) in the PSD spectra and the  
>> statistics also returned very different results. I later found out  
>> that I can also increase the resolution of the FFT via zero-padding  
>> (spectopo parameters: ‘specmode’, ‘fft’, ‘nfft’, 1024, ‘logtrials’,  
>> ‘off’), but the spectra still look quite different.
>>
>> Here is the link to some figures. The p-value for the statistics  
>> was always 0.05.
>>
>> https://www.dropbox.com/sh/8isx5pwms6ifxuj/AADJO6bmqG0kVPlvTeWCbzALa?dl=0
>>
>> I´m using EEGLAB v13.2.1; Sample rate = 250Hz; epoch length =  
>> 700ms; trials = 59-306; Bandpassfilter = 1-30Hz
>>
>> Can anyone help me?… PLEASE…
>>
>> Best,
>> Sebastian
>>
>>
>> Sebastian Grissmann (Mag. rer. nat)
>> Neuroengineer / PhD student
>>
>> LEAD Graduate School
>> University of Tübingen
>> Europastrasse 6
>> 72072 Tübingen
>> Germany
>> Phone  +49 7071 29-73604
>> _______________________________________________
>> Eeglablist page: http://sccn.ucsd.edu/eeglab/eeglabmail.html
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>>
>>
>> --
>> Makoto Miyakoshi
>> Swartz Center for Computational Neuroscience
>> Institute for Neural Computation, University of California San Diego
>> _______________________________________________
>> Eeglablist page: http://sccn.ucsd.edu/eeglab/eeglabmail.html
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>
>



Sebastian Grissmann (Mag. rer. nat)
Neuroengineer / PhD student

LEAD Graduate School
University of Tübingen
Europastrasse 6
72072 Tübingen
Germany
Phone  +49 7071 29-73604


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