A distributed spatio-temporal EEG/MEG inverse solver

We propose a novel ℓ1ℓ2-norm inverse solver for estimating the sources of EEG/MEG signals. Based on the standard ℓ1-norm inverse solvers, this sparse distributed inverse solver integrates the ℓ1-norm spatial model with a temporal model of the source signals in order to avoid unstable activation patterns and “spiky” reconstructed signals often produced by the currently used sparse solvers. The joint spatio-temporal model leads to a cost function with an ℓ1ℓ2-norm regularizer whose minimization can be reduced to a convex second-order cone programming (SOCP) problem and efficiently solved using the interior-point method. The efficient computation of the SOCP problem allows us to implement permutation tests for estimating statistical significance of the inverse solution. Validation with simulated and human MEG data shows that the proposed solver yields source time course estimates qualitatively similar to those obtained through dipole fitting, but without the need to specify the number of dipole sources in advance. Furthermore, the ℓ1ℓ2-norm solver achieves fewer false positives and a better representation of the source locations than the conventional ℓ2 minimum-norm estimates.