FieldTrip

The volume conduction model is an important ingredient for source analysis, since it determines how a source within the brain is visible on the EEG or MEG sensors. All forward and inverse computations that are done within Fieldtrip share the same low-level code for the computation of the forward model potential or field. Since the volume conduction model depends on the geometry, you have to provide information on that geometry for your subjects (i.e. position and radius of sphere).

Fieldtrip supports the following volume conduction models for EEG:

• single sphere
• multiple concentric spheres with up to 4 shells
• boundary element model (the “system matrix” should be computed outside of FieldTrip)
• all forward models supported by the Neuromag meg-calc toolbox

and the following volume conduction models for MEG:

• single sphere
• multiple spheres, one for each sensor
• realistic single-shell model based on leadfield expansion (Nolte method)
• all forward models supported by the Neuromag meg-calc toolbox

If you wish, you can also use a forward model for which you have computed the leadfield matrices outside of fieldtrip in combination with the inverse source reconstruction algorithms that are implemented within FieldTrip. Some matlab code that demonstrates how to do it can be found in this example.