[Eeglablist] FFT vs. PSD

Sebastian Grissmann sebastian.grissmann at lead.uni-tuebingen.de
Thu Oct 2 00:37:31 PDT 2014

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.


I´m using EEGLAB v13.2.1; Sample rate = 250Hz; epoch length = 700ms;  
trials = 59-306; Bandpassfilter = 1-30Hz

Can anyone help me?… PLEASE…


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

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

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