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GUI Help
BEM Parameters
This window is used to set the parameters for Boundary Element Method (BEM) head model.
See the reference [1] for more information. (See m-file help, m-file help)
CHECK BOXES
BEM COMPUTATION
| Interpolative |
Select to use interpolative approach to compute
the forward solution using BEM, only if you are running a source scannign technique (e.g. RAP-MUSIC).
You should deselect this option when using an imaging approach where sources are distributed on a brain interface.
If selected, exact computation is run on a set of points distributed within the inner head volume and any subsequent request for a forward gain vector (e.g. during a volumic source scan using RAP-MUSIC) is computed using an interpolation of the forward gain vectors of the closest volumic grid points. This allows faster computation of the BEM solution during source search. If not selected, exact computation is required at every source location. Hint: When sources have fixed location, e.g. constrained on the cortical surface, non-interpolative approach is recommended. |
BASIS
| Constant |
Select to use the planar triangles with constant potential potential across the surface of each triangle. |
| Linear |
Select to use planar triangles with linearly varying potential across the surface of each triangle. Hint: The system of linear equations is dense so that a
model with a set of surfaces with a total of 10,000 nodes would
require about 1 GB of memory and take on the order of several
hours to solve. |
TEST
| Collocation |
Select to control approximation error at the same discrete locations as the control
points. |
| Galerkin |
Select to control approximation error as either a constant or linear function
across the entire triangle. |
INSULATED SKULL APPROACH
| Select to use Insulated Skull approach to overcome numerical problems that arise due to the relatively small conductivity of the skull compared to the brain. Hint: Leave it on to get more stable results. |
RE-COMPUTE TRANSFER MATRICES
Transfer matrix should be computed at least once for a given electrode/sensor configuration for a given set of parameters. This transfer matrix could be re-used for these parameters even if the source space is changed. Hint: Uncheck this box only if you change the source space for a fixed parameter set.
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TEXT BOXES
DECIMATE ENVELOPES DOWN TO (vertices/envelope)
| Decimation index for nodes (if Linear basis function is selected) or faces(if Constant basis function is selected). |
LIST BOXES
AVAILABLE ENVELOPES
| Shows the available envelopes for BEM modeling. Select the layers starting from innermost layer towards the outermost layer. Hint: Just click on the envelope or use the >> and << buttons to select the layers. |
ORDERED SELECTION
| Select the envelopes for BEM modeling starting from innermost. Hint: Typically 3 layers are used: Inner Skull, Outer Skull and Skin. |
BUTTONS
VIEW ENVELOPES
| Click to see the selected envelopes. |
VIEW SENSORS
| Click to view the sensors. |
| [1] |
Ermer JJ, Mosher JC, Baillet S, Leahy, R.M., "Rapidly recomputable EEG forward models for realistic head shapes", Phys Med Biol 2001, Apr;46(4), pp. 1265-1281 pdf |
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