The BESA beamformer is a modified version of the linearly constrained minimum variance vector beamformer in the time-frequency
domain as described in Gross et al., "Dynamic imaging of coherent sources: Studying neural interactions in the human brain",
PNAS 98, 696-699, 2001. It allows to image evoked and induced oscillatory activity in a user-defined time-frequency range,
where time is taken relative to a triggered event.
The computation is based on a transformation of each channel's single trial data from the time domain into time-frequency space.
This transformation is performed by the BESA source coherence module and results in the complex spectral density Si(f,t),
where i is the channel index.