Note that this reference documentation is identical to the help that is displayed in MATLAB when you type “help ft_datatype_headmodel”.

```  FT_DATATYPE_HEADMODEL describes the FieldTrip MATLAB structure for a volume
conduction model of the head that can be used for forward computations of
the EEG potentials or the MEG fields. The volume conduction model represents
the geometrical and the conductive properties of the head. These determine
how the secondary (or impressed) currents flow and how these contribute to
the model potential or field.

A large number of forward solutions for the EEG and MEG are supported
in FieldTrip, each with its own specification of the MATLAB structure that
describes the volume conduction model of th ehead. It would be difficult to
list all the possibilities here. One common feature is that the volume
conduction model should specify its type, and that preferably it should
specify the geometrical units in which it is expressed (e.g. mm, cm or m).

An example of an EEG volume conduction model with 4 concentric spheres is:

r: [86 88 94 100]
c: [0.33 1.00 0.042 0.33]
o: [0 0 0]
type: 'concentricspheres'
unit: 'mm'

An example of an MEG volume conduction model with a single sphere fitted to
the scalp with its center 4 cm above the line connecting the ears is:

r: [12]
o: [0 0 4]
type: 'singlesphere'
unit: 'cm'

For each of the methods XXX for the volume conduction model, a corresponding
function FT_HEADMODEL_XXX exists that contains all specific details and
references to literature that describes the implementation.

- type

Optional fields:
- unit

Deprecated fields:
- inner_skull_surface, source_surface, skin_surface, source, skin

Obsoleted fields:
- <none specified>

Revision history:

(2015/latest) Use the field name "pos" instead of "pnt" for vertex positions.

(2014) All numeric values are represented in double precision.

(2013) Always use the field "cond" for conductivity.

(2012) Use consistent names for the volume conductor type in the structure, the
documentation and for the actual implementation, e.g. bem_openmeeg -> openmeeg,
fem_simbio -> simbio, concentric -> concentricspheres. Deprecated the fields
that indicate the index of the innermost and outermost surfaces.