The impact of the choice of magnetometers and 2 gradiometers in source reconstruction after signal 3 space separation 4 5

Background: Modern MEG devices include 102 sensor triplets containing one 14 magnetometer and two planar gradiometers. The first processing step is often a signal space 15 separation (SSS), which provides a powerful noise reduction. A question commonly raised by 16 researchers and reviewers is which data should be employed in source reconstruction: (1) 17 magnetometers only, (2) gradiometers only, (3) magnetometers and gradiometers together. The 18 MEG community is currently divided about the proper answer and strong arguments in favor and 19 against these three approaches often expressed. Methods: First, we provide theoretical evidence 20 that both gradiometers and magnetometers contain the same information after SSS, and argue that 21 they both result from the backprojection of the same SSS components. Then, we compare 22 beamforming source reconstructions from magnetometers and gradiometers in real MEG 23 recordings before and after SSS. Results: Without SSS, the correlation between source time series 24 extracted from magnetometers and gradiometers was high, with Pearson correlation coefficient 25 r=0.5-0.8. After SSS, these correlation values increased dramatically, reaching over 0.90 across all 26 cortical areas. Conclusions: After SSS, almost identical source reconstructions (r>0.9) can be 27 obtained with magnetometers and gradiometers, as long as regularization is selected appropriately 28 to account for the different properties in magnetometers and gradiometers covariance matrices. 29 30