[Eeglablist] FFT vs. PSD

Makoto Miyakoshi mmiyakoshi at ucsd.edu
Thu Oct 2 10:44:36 PDT 2014

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.


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
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Makoto Miyakoshi
Swartz Center for Computational Neuroscience
Institute for Neural Computation, University of California San Diego
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