Before revision:  This is a source (state) GGM hidden by the MEEG observation equation , i. e. equivalent to a frequency domain Linear State Space Model (LSSM) . It is unhidden due to the Type II Likelihood approximated representation: Expected Log-Likelihood (ELL).
After revision:  This is a source (state) GGM hidden by the observation equation of Magneto-encephalographic (MEEG) signals. HIGGS is equivalent to a frequency domain Linear State Space Model (LSSM) . It is unhidden due to the Type II Likelihood approximated representation: Expected Log-Likelihood (ELL).