This step describes how to compute some realistic modelling of the head for EEG and MEG using the Boundary Element Method (BEM).

1. Go to BST Main window ->Tools->Head Modeling -> Advanced to open the Head Modeling Tool
2. Type "BEM" in the Head Model Name, or any descriptive field for this head model (this is optional, and for your own record)
3. In the Head Compartments section of the same main Head Modeling GUI , assign a surface envelope as a source support, just like you did for the spherical head model. This means that the BEM forward model will be computed for each elementary source of a cortical surface prior to any imaging or beamformer estimation of their amplitudes.
   1. Tutorial suggestion: Choose cortex_10000V_tess_CTF as the cortical source support in your model from the 'Cortex' pulldown menu.

Hint: You don't need to specify a Scalp compartment in this section as it will be specified in the specific BEM GUI that we shall describe hereafter (BEM Parameters).

4. In the 'Forward Modeling Methods' section, tick the MEG checkbox and select BEM. This opens the BEM Parameters window.
5. In this window select the surface layers for BEM modeling starting from the INNERMOST surface (e.g. CSF or inner skull in most cases) and finishing with the last envelope (e.g. the s
1. Tutorial suggestion: Select successively cortex_1000V_tess_CTF, outer_skull_1000V_tess_CTF and scalp_1000V_tess_CTF

Hint: Use the >> button or just click on their names in the 'Ordered Stext list of the BEM Parameters window to select the surfaces.

6. Check that the surfaces you have selected appear in the right order in the Ordered Selection listbox.

Hint: You can view the envelopes and sensors using the View Envelope and View Sensors buttons.

7. As an example, select Linear in the Basis functions section and Collocation in the Test section.

Hint: Modeling with Galerkin Test method increases the computation test time dramatically., to the benefits or increased numerical accuracy though. See specific GUI help on BEM Parameters for more information on test methods.

8. As a fast test on the computation flow of the BEM, write 150 in the Decimate envelopes down to (vertices/envelope) section.

Hint: the more nodes in your envelopes, the more accurate the BEM, the longer it takes to obtain resulting head model. A recommended value for downsizing is 1000. This choice is still computationally expensive (a few hours), but has reasonable accuracy. We also recommend the alternative analytical overlapping-sphere approach which achieves good realistic modeling, with no numerical issues and faster computation.

 

• In case you will be using a source-scanning technique such as RAP-MUSIC, leave the 'Interpolative' option checked for faster computation. In the case of an imaging approach (like Minimum-Norm), you must deselect this option
• Click Run in the Head Modeling Tool to launch the BEM computation.

These steps let us compute another head model using the Boundary Element Method. The folder C:\BSTDemoData\Study\somDemo_av.ds\BrainStorm folder, should list the following files:

Previous  Contents  Next