Part II: Simulating EEG sources

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By now our head model should be fully operational so we can start placing artificial EEG sources within the brain volume. An _EEG source_ is just one or more source grid locations having a common temporal activation pattern. From a neurophysiological perspective, you can intuitively define an EEG source as a compact cluster of neurons whose activity is well represented by their average post-synaptic potentials. The time course of such average post-synaptic potentials is what we call the temporal activation of the EEG source.

Let's start with the command:

myHead = add_source(myHead);

which will create a dipole of random location and orientation. The characteristics of the dipole are stored in property Source of the myHead object. In my case, the characteristics of the random dipole are:

>> myHead.Source
ans = 

       name: []
   strength: 23.1095
orientation: [-0.6523 -0.5037 0.5664]
      angle: 47.4691
        pnt: 146
   momentum: [-15.0739 -11.6402 13.0895]
 activation: 1
      depth: 35

The information above tells us that our dipole is located at the coordinates myHead.SourceSpace.pnt(146,:). It also tells us the orientation of the dipole (a unit length vector) and its strength (the length of the dipole). The result of multiplying the strength with the orientation vector is what is called the dipole momentum. The activation of the dipole is its time course. In this tutorial we will deal only with fixed dipoles (without temporal variation of their time course). In the next tutorial we will pay full attention to such a crucial feature of an EEG source.

The method add_source that we used to generate our random dipole has several optional arguments that you can use to specify the characteristics of the dipole. For instance, the following command:

myHead = add_source(myHead, 'MinDepth', 1, 'MaxDepth', 30)

will generate a dipole that is at least 1 mm deep in the brain and at most 30 mm away from the closest point on the inner skull surface. Note that the new dipole was _added_ to our head model, i.e. the dipole that we generated previously is still in myHead.Source. You can check how many EEG sources your model has by inspecting property NbSources:

>> myHead.NbSources
ans =

	2

Each EEG source that you add to your head model is stored in property Source of your head.mri object. We can inspect the properties of the last EEG source that we just generated using the command:

myHead.Source(2)

which will display:

myHead.Source(2)

ans = 

         name: []
     strength: 1
  orientation: [-0.9549 -0.1926 -0.2261]
        angle: 47.4691
          pnt: 1
     momentum: [-0.9549 -0.1926 -0.2261]
   activation: 1
      depth: 12

To remove an EEG source use the command:

myHead = remove_source(myHead, [1 2])

where the second argument is a vector with the indices of the sources that should to be removed.

Display the help of add_source using help head.mri.add_source and read it carefully. Then try to generate dipoles with the following characteristics:

1. A radial dipole that is no more than 10 mm deep.
2. A tangential dipole no more than 10 mm away from the brain surface.
3. A random dipole that is 3 times stronger than the two dipoles above and that is located at least 40 mm deep inside the brain.
4. A compact EEG source that consists of 15 dipoles with random orientations and with strengths varying between 1 and 2.

NOTE: After creating each source, inspect the corresponding element of the Source property of myHead, in order to ensure that the source has the desired properties.

Plot the location of the dipoles that you generated above using the following method:

figure;
plot_source_dipoles(myHead, [1 2 3 4]);

where the second input argument is a vector with indices of the EEG sources that you want to plot. The result of this command should be something similar to the first figure in the gallery below (of course the dipole locations will be different in your case, as they are random). Rotate the figure and check that the third source is indeed much deeper than the first two sources.

Our first attempt at plotting the EEG sources was successful but we would like to see also the orientation and the strength of each source. To plot the strength:

figure;
plot_source_dipoles(myHead, 1:4, 'time', 1, 'sizedata', 200);

The result is shown in the second figure below. The 'sizedata' argument can be used to increase the default size of the source location markers. If we also want to plot the orientation of the dipoles we can use:

figure;
plot_source_dipoles(myHead,[1 2 3 4],'time',1, 'sizedata',200,'momentum',10)

The 'momentum' value can be used to manipulate the length of dipole of strength 1.

Each EEG source generates a characteristic distribution of scalp potentials. In the following I will loosely refer to such pattern as the _source leadfield_. The _source leadfield_ is the normalized potential distribution that we would see at the scalp if only the source of interest would be active and all other sources would be silent. You can plot the source leadfield with the command:

figure;
plot_source_leadfield(myHead,1)

A very important point is that the source leadfield is completely independent of the temporal activation of the source. This means that, if only one source is active, the normalized distribution of scalp potentials will be the same at any time, even if the underlying source has very complex temporal dynamics. This has also implications for localizing the source afterwards because the only thing that we need to (attempt to) localize an EEG source is its source leadfield.

You can also plot on the same figure the location and orientation of the source:

figure;
plot_source_leadfield(myHead,1, 'momentum', 50)

or plot that information for all EEG sources simultaneously:

figure;
plot_source_leadfield(myHead,1:4, 'momentum',50)

Below you can find some of the figures that you may generate in this section of the tutorial:

To be more realistic you can also simulate the presence of background (noisy) activity. It is difficult to define what is noisy neural activity. The easiest way is probably using an example. If you are interested in studying the generators of EEG activity in the alpha band then we would regard as noisy any neural activity that is not (closely) related to EEG-alpha.

You can generate background noise with the command:

myHead = add_source_noise(myHead, 'MaxStrength', 1);

which will create a noisy EEG source and put it in myHead.Source. You can identify the noisy source from the rest by the fact that the noisy source is called noise:

myHead.Source(5)

ans = 

           name: 'noise'
       strength: [325x1 double]
    orientation: [325x3 double]
          angle: [325x1 double]
            pnt: [1x325 double]
       momentum: [325x3 double]
     activation: [325x1 double]

You can plot the characteristics of the noisy activity using:

figure;
plot_source_leadfield(myHead,'noise','momentum',50)

You can also visualize the effect that the noisy activity has on the overall distribution of scalp potentials at the scalp:

figure;
plot_source_leadfield(myHead, 1:5)

Please familiarize yourself with methods plot_source_dipoles and plot_source_leadfield before going any further.

Now you are ready to go to Part III of this tutorial where we attempt to localize some of the EEG sources that we just simulated.