[Eeglablist] Question - EEGLAB calculating GFP vs FFT

[Tarik S Bel-Bahar]

Hi Alexander, notes below, best wishes.

   - global field power and FFT (which provides spectral or time-frequency
   metrics) are not the same. GFP is more like a global estimate across all
   channels. FFT is a method to get spectral estimates from a signal (e.g,
   from single channels, or averaged channels, usually some form of
   time-series). In the case of the article you mentioned it seems like GFP
   was computed first, and then a spectral estimate.
   - If you google "eeglablist" + your topic, you can find multiple
   instances of past responses/discussions about GFP in eeglablist. It is easy
   to compute within matlab. However, I don't think there is a GFP function in
   eeglab. Within the eeglab distribution, check out the "ft_globalmeanfield"
   function inside the fieldtrip lite toolbox, which may do the trick for you
   - See also for example GFP ERPLAB function mentioned at link below. See
   documentation for  ERPLAB functions mgfperp.m  pop_ploterps.m
   -
   https://github.com/lucklab/erplab/wiki/EEG-and-ERP-Channel-Operations#example-of-computing-global-field-power
   - You can find out more about GFP and how to compute GFP by reviewing
   articles/tools developed by Lehman, Faber, Britz, Michel, Koenig, and
   others, (e.g., CARTOOL or microstates toolboxes in matlab/eeglab, see
   also brainstorm), all easy to find via Google and/or Google Scholar.
   - Regarding graphics it is possible to get something like the graphics
   in the article you mentioned with matlab and/or eeglab, but you would
   need to compute GFP first (there is a microstate toolbox for eeglab you
   can look into)
   - If you want to replicate exactly the analysis in the article you
   mentioned, consider contacting that team, they may be able to share their
   exact methods/scripts with you.
   - Fyi, great books for EEG beginners are Mike X Cohen's books, and
   Luck's two handbooks, both of which I consider core reading. Both Cohen
   and Luck have extensive materials online that one can review. Also as a
   new user to eeglab, please check out if you haven't had a chance to the
   extensive eeglab wiki and eeglab school videos.
   - Caveat: Note that 2-minutes is a somewhat short period of time per
   participant, but it should be usable, and many groups publish on similar
   brief data.




On Fri, Jan 25, 2019 at 1:13 PM Alex Scodellaro <ajscod at gmail.com> wrote:

> Dear all,
>
> I am a junior neuromarketing researcher and am new to the EEGLAB program.
> I have 14 EEG recordings of 2-minute participant exposure to advertisement.
> I have segmented each data-set into 'epochs of interest'.
>
> I wish to ask how I can determine the total *average* GFP theta z-score
> and GFP alpha z-scores similar to the paper below. And further, can EEGLAB
> produce graphs as shown in the paper?
>
> Also, how different are GFP and FFT analyses? From what I can understand,
> they yield similar results.
>
>
> *Gordon, Ross et. al (2017). Using EEG to Examine the Role of Attention,
> Working Memory, Emotion, and Imagination in Narrative Transportation. *
>
>
>
> I would really appreciate any and all help!
>
> Thank you in advance.
> Alex
>
> *Alexander Scodellaro*
> ajscod at gmail.com / 0422211619
>
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