# [Eeglablist] FFT spectrogram

Alan Velander alanvelander at gmail.com
Wed May 19 08:45:51 PDT 2021

```Thank you for your replies. I think my bigger question is about how to
average signals. If I want to look at the average signal for the left
hemisphere (combining Fp1, F3, F7, C3, P3, T7, O1) and then plot a
spectrogram of that signal over the entire EEG recording, how should I do
this? Alan

On Tue, May 18, 2021 at 12:18 PM Alan Velander <alanvelander at gmail.com>
wrote:

> Dear list members,
>
> I am a new user of EEGLAB and MATLAB, and this is my first post. I am a
> clinical neurologist who is researching quantitative EEG measures in
> critical care patients. I have used packages like Persyst in the past, but
>
> Currently, I am trying to reproduce an FFT Spectrogram of an averaged
> signal from the Left Hemisphere and an averaged signal from the Right
> Hemisphere.
>
>
> I preprocessed the 256 Hz EEG file as recommended in the tutorial. The EEG
> is now an array with 21 channels (19 EEG, 8 right, 8 left, 3 center, and 2
> ECG).
>
> I then created an average signal of each hemisphere and a spectrogram of
> each average signal.
>
> EEG.data(22,:)=(EEG.data(1,:)+EEG.data(3,:)+EEG.data(6,:)+EEG.data(8,:)+EEG.data(11,:)+EEG.data(13,:)+EEG.data(16,:)+EEG.data(18,:))/8;
> EEG.chanlocs(22).labels=‘Av_L’ %All left sided channels.
>
> EEG.data(23,:)=(EEG.data(2,:)+EEG.data(4,:)+EEG.data(7,:)+EEG.data(9,:)+EEG.data(12,:)+EEG.data(14,:)+EEG.data(17,:)+EEG.data(19,:))/8;EEG.chanlocs(23).labels=‘Av_R’
> %All right sided channels
>
> I used the spectrogram function to plot the power spectral densities of
> Av_L and Av_R. I wanted a window of 30 s, used the default noverlap,
> evaluated the entire interval of the EEG, sampled at 256 Hz, and changed
> frequency to the y axis with limits on the c and y axes.
>
> spectrogram(EEG.data(22,:),(30*256),[],313856,256,'yaxis'); caxis([0
> 20]);ylim([0 20]); colormap jet
>
> spectrogram(EEG.data(23,:),(30*256),[],313856,256,'yaxis'); caxis([0
> 20]);ylim([0 20]); colormap jet
>
> Am I on the right track? Is there some special feature or toolbox of
> EEGLAB that would make this entire process easier? Is the average of each
> channel from the right and left hemisphere just an average, or is there a
> more complicated way to combine the data? Can the power scale be changed
> from dB/Hz to uv^2/Hz or sqrt(uV)/Hz?
>
> Again, I apologize if these are newbie questions, and I appreciate any
>
> Alan
>
> --
> Alan Velander, MD
> 917 975 3832
> alanvelander at gmail.com
>

--
Alan Velander, MD
917 975 3832
alanvelander at gmail.com
```