The EEG inverse problem is an ill-posed problem because for all admissible output voltages, the solution is non-unique (since p >> N) and unstable (the solution is highly sensitive to small changes in the noisy data). There are various methods to remedy the situation (see e.g. [7-9]). As regards the EEG inverse problem, there are six parameters that specify a dipole: three spatial coordinates (x, y, z) and three dipole moment components (orientation angles (θ, φ) and strength d), but these may be reduced if some constraints are placed on the source, as described below.
