# [Eeglablist] Question - EEGLAB calculating GFP vs FFT

Tarik S Bel-Bahar tarikbelbahar at gmail.com
Fri Jan 25 12:53:19 PST 2019

```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
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
- 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!
>
> Alex
>
> *Alexander Scodellaro*
> ajscod at gmail.com / 0422211619
>
> This email (and any attachments) is confidential and may be subject to
> legal or other professional privileges. It is intended solely for the
> receipt of the named addressee(s). It is subject to copyright and may only
> be copied or distributed with the consent of the copyright owner. If you
> are not a named addressee, you must not use, disclose, retain or reproduce
> all or any part of the information contained in this email or any
> attachments. Please notify the sender immediately by return email and
> destroy all copies of the email. Any confidentiality, privilege or
> copyright is not waived or lost because this email has been sent to you by
> mistake.
> _______________________________________________
> Eeglablist page: http://sccn.ucsd.edu/eeglab/eeglabmail.html
> To unsubscribe, send an empty email to
> eeglablist-unsubscribe at sccn.ucsd.edu
> For digest mode, send an email with the subject "set digest mime" to
> eeglablist-request at sccn.ucsd.edu
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://sccn.ucsd.edu/pipermail/eeglablist/attachments/20190125/42f8c84c/attachment.html>
```