[Eeglablist] FFT spectrogram

Scott Makeig smakeig at gmail.com
Wed May 19 16:05:42 PDT 2021


Alan -

When you say that you want to average signals for the left hemisphere -
should you not say, more accurately, that you want to average signals
arriving at scalp electrodes mounted over the left hemisphere. These are
quite different statements, as each scalp channel records the ever-changing
difference between two quantities, the summed voltages arriving at one of
the electrodes, minus summed voltages arriving at the other (typically
common reference) electrode. Because of this, and the widespread pattern of
volume conduction from brain to scalp, each scalp channel sums activity
over a good portion of the cortex, including some (appropriately oriented)
cortical territories locates close to *either* electrode. Therefore,
summing scalp channel signals involving scalp electrodes placed over the
left hemisphere sums considerable activity originating in the right
hemisphere as well.

If sums of scalp channels do not make precise spatial filters, is there a
way to make spatial filters are more precise?  Yes -- you can design a
better spatial filter (i.e., one focused on a particular brain territory of
interest) using various methods (ICA, beamforming, ...).

This is fact-of-life in EEG research.

Scott Makeig

On Wed, May 19, 2021 at 6:50 PM Alan Velander via eeglablist <
eeglablist at sccn.ucsd.edu> wrote:

> 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
> > I am trying to learn more about how Persyst calculates different
> measures.
> >
> > 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
> > help you can provide.
> >
> > Alan
> >
> > --
> > Alan Velander, MD
> > 917 975 3832
> > alanvelander at gmail.com
> >
>
>
> --
> Alan Velander, MD
> 917 975 3832
> alanvelander at gmail.com
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-- 
Scott Makeig, Research Scientist and Director, Swartz Center for
Computational Neuroscience, Institute for Neural Computation, University of
California San Diego, La Jolla CA 92093-0559, http://sccn.ucsd.edu/~scott


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