Improvement of S/N in evoked neuromagnetic fields by eliminating a spontaneous field component

In MEG studies, it is important to obtain evoked fields with good signal-to-noise ratios (S/N) and with a small number of epochs in averaging. The noises are considered to be mainly spontaneous neuromagnetic fields. In the present study, the authors propose a method to improve the S/N. The basic principle of this method is the elimination of a principal component (PC) of multichannel-recorded neuromagnetic fields, utilizing the synchronized characteristics of spontaneous rhythmic activities dominating the fields. The authors applied the method to neuromagnetic fields measured by a 37-channel MEG system, on which computer-generated evoked fields were superposed, in order to examine possible improvement in S/N. It was found that elimination of the first PC could improve the S/N of the evoked fields. The improvement rate of SIN, compared to conventional simple averaging, reached more than 50% after averaging over 128 epochs