The adjoint method of OpenMEEG for EEG and MEG with large source space

In EEG or MEG, a lead field is the linear operator which associates unitary dipolar sources to the resulting set of sensor measurements. In practise, the source space often includes over 10 000 dipoles, which sometimes causes memory problems. The adjoint approach considers the forward problem from the viewpoint of sensors instead of sources: this drops down the number of linear systems to solve by two orders of magnitude. The adjoint approach is here proposed in the context of the Boundary Element Method, and its implementation is provided by the OpenMEEG library.