Tutorial: Compute a head model

This tutorial is still based on Sabine Meunier's somatotopy experiment, called TutorialCTF in your Brainstorm database. The recordings have already been imported and analyzed at the sensors level, and they are now ready for source estimation.

Forward problem

The first step consists in computing a model that explains how an electric current flowing in the brain can influence what is recorded out of the head, by the EEG or MEG sensors.

• This problem is called forward problem.

• Its result is called head model in Brainstorm interface, but can also be referred as forward model or leadfield matrix.

• In Brainstorm software, we consider that the electric or magnetic activity which is recorded by the sensors is produced mainly by a set of electric dipoles located at the surface of the cortex.
  • Contrary to a dipole localization method, we fix the dipoles locations (cortex surface) and orientations (perpendicular to the cortex), and try to estimate the activity of each dipole at each time sample.
  • The grid of sources (dipoles) that is used is defined by the cortex surface we have imported in one of the previous tutorials; each vertex of this surface is considered as dipole.
  • The default surface distributed with Brainstorm have around 15.000 vertices. So we will have 15.000 dipole amplitudes to estimate. Using less vertices would just lower the resolution of the results; using more produces too much data and might lead to memory issues.
• What we expect to get at the end of this process is a matrix which size is [Number of sensors x Number of sources]
• For computing this matrix, two methods are available for MEG recordings in Brainstorm, and will be computed in this tutorial:
  • Single sphere model: the head is considered a homogeneous sphere
  • Overlapping spheres method:
  • BEM was implemented but still need to be validated. Coming soon.

To understand properly what is a forward model, why this approach was chosen, what is the best method, etc. please visit the ?Theory section. For now we will only focus on how you compute a head model with the Brainstorm interface.

Single sphere model

Select the TutorialCTF protocol, close all the figures, and follow these steps:

1. Right-click on the StimRightThumb condition and select Compute head model. The Head modeler window will appear.
   
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2. The only thing you can choose here is the forward model you will use: Single sphere or overlapping sphere.
   • Choose Single sphere
   • You can also edit the Name of the file that will be created. The default name may seem awkward at beginning, but it really makes some sense: