Module forward 

Spherical forward model (EEG gain matrix computation).

Computes the lead-field (gain) matrix that maps dipole source activations to EEG electrode potentials using a multi-shell spherical head model.

Model

The default 3-shell model (Berg & Scherg, 1994) uses:

Shell	Radius (m)	Conductivity (S/m)
Brain	0.067	0.33
Skull	0.070	0.0042
Scalp	0.075	0.33

The Berg & Scherg approximation replaces the exact series expansion with a small number of fitted dipoles (typically 3), making the computation fast while retaining good accuracy.

Example

use exg_source::forward::{make_sphere_forward, SphereModel};
use exg_source::source_space::ico_source_space;
use ndarray::Array2;

// Electrode positions (simplified, 4 electrodes on the scalp)
let elec = Array2::from_shape_vec((4, 3), vec![
    0.07, 0.0, 0.04,
   -0.07, 0.0, 0.04,
    0.0, 0.07, 0.04,
    0.0,-0.07, 0.04,
]).unwrap();

// Source space
let (src_pos, src_nn) = ico_source_space(2, 0.06, [0.0, 0.0, 0.04]);

// Build forward model
let sphere = SphereModel::default();
let fwd = make_sphere_forward(&elec, &src_pos, &src_nn, &sphere);
assert_eq!(fwd.gain.nrows(), 4);
assert_eq!(fwd.n_sources, src_pos.nrows());

References

• Berg, P., & Scherg, M. (1994). A fast method for forward computation of multiple-shell spherical head models. Electroencephalography and Clinical Neurophysiology, 90(1), 58-64.
• de Munck, J. C. (1988). The potential distribution in a layered anisotropic spheroidal volume conductor. Journal of Applied Physics, 64(2).

Structs

SphereModel

Parameters of a multi-shell spherical head model.

Functions

make_sphere_forward

Compute a fixed-orientation EEG forward model using a spherical head.

make_sphere_forward_free

Compute a free-orientation EEG forward model using a spherical head.