[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.
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
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