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. Author manuscript; available in PMC: 2008 Sep 30.
Published in final edited form as: J Neurosci Methods. 2007 Jun 8;165(2):297–305. doi: 10.1016/j.jneumeth.2007.06.003

Fig. 5.

Fig. 5

Data from a selection of systems. For each, the power spectrum of one arbitrary channel is plotted with a logarithmic frequency axis and a dB ordinate (with arbitrary origin). MEG system 1 is a 157-channel axial gradiometer system with 3 reference channels, built by Yokogawa and installed in a high-quality MSR (60 dB at 0.01 Hz) in a quiet suburban environment. MEG system 2 is a 440-channel system also built by Yokogawa. Reference channel data were not available for this recording; instead, “synthetic reference” signals were obtained by applying ICA and selecting the three components most strongly dominated by DC and power line harmonics. MEG system 3 is a 306 channel system built by Elekta Neuromag, for which six synthetic references were derived by ICA. The green line represents the result of applying instead the SSS algorithm provided with the system. MEG system 4 is a 151-channel system built by CTF with 29 reference channels. The EEG system was a 16-channel system installed within an electromagnetically shielded booth. Four channels were devoted to ‘reference’ channels: two were attached to the subject's wrists and the other two were attached to the metallic floor and wall of the booth.