[Eeglablist] Spectopo Function Negative Power Values

Tarik S Bel-Bahar tarikbelbahar at gmail.com
Fri Nov 10 09:47:40 PST 2017

Note that you can also look in the spectopo output to see if the values are
negative or not when initially computed by the function.

Note also that the graphical output for spectopo may transform the spectral
data, so you also want to try plotting the output data directly.

Thanks for letting the list know when you have a solution so that other
users can benefit from your experience.

Last, see Chronux toolbox and also Mike X. Cohen's time frequency book/code.

On Nov 9, 2017 10:24 AM, "Tarik S Bel-Bahar" <tarikbelbahar at gmail.com>

​Hello Courtney,

*If you haven't yet, it would be best to do all steps with eeglab tutorial
data, and see if you get normal values following the recommended steps in
the eeglab tutorial. Use the GUI only, and examine the eegh output after
successful steps. Do you see similar large negative values ? Is your code
different from the eegh output ?

*Also, do you see similar negative values across all your single-subject
files or just some? If just some, then this might be an issue with some of
your single subjects.

*Make sure you are doing preprocessing steps correctly and in right order.
How you filter and clean the data is important.

*If you reach a solution please share with the list so that other users can
benefit from your experience.

*You may also benefit from reading Mike X. Cohen's excellent
book/excercises/videos about time-frequency for eeg.

​*In the future, it would be beneficial to send pictures and more info
about your previous steps. that will make it easier to better understand
your issues and possibly get more responses.

On Tue, Nov 7, 2017 at 7:11 AM, Courtney Jimenez <cojimenez at ucdavis.edu>

> Hi all,
> We are using eeglab's spectopo function to calculate power spectral
> density estimates for our data. However, we're getting large negative
> values in many channels across many frequencies.
> How can we correct this?
> We use the following code from the matlab command line:
> spectopo(EEG.data(:,:),0,EEG.srate,'freqrange',[4 50],'title',EEG.setname)
> Thanks,
> Courtney
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