Performance of an MEG adaptive-beamformer technique in the presence of correlated neural activities: correlation-coefficient estimation and time-course retrieval

The influence of temporarily correlated source activities on neuromagnetic reconstruction by adaptive beamformer techniques is investigated. It is known that the spatial filter weight of an adaptive beamformer cannot perfectly block correlated signals. This causes two major influences on the reconstruction results: time course distortions and reductions in reconstructed signal intensifies. The reduction in signal intensity for sources with a medium degree of correlation is found to be insignificant. The time course distortion for such sources, however, may not be negligible. Our analysis shows that the magnitude correlation coefficient between two correlated sources can be accurately estimated by using the beamformer outputs. A method of retrieving the original time courses using estimated correlation coefficients is developed. Our numerical experiments demonstrate that reasonably accurate time courses can be retrieved from considerably distorted time courses even when the signal-to-noise ratio is low.