Correction of disturbances caused by temporary and continuous head movements

This study was performed to assess the effectiveness of the spatiotemporal Signal Space Separation (tSSS) and head movement compensation (MC). Continuous MEG data were acquired from 20 adult and 10 child subjects in four conditions: (1) A reference with a static head position, (2) the subject changed the head position twice, (3) the subject moved the head constantly, and (4) two magnetized pieces were attached to the scalp and the subject moved the head. The recordings were repeated with two whole-head MEG systems. Auditory and tactile stimuli were applied. The tSSS and MC were performed on the recordings, and evoked fields were selectively averaged. Neuronal activity was modeled with equivalent current dipoles (ECDs) whose locations and strengths were fitted to the auditory and somatosensory evoked fields (AEF and SEF). ECD parameters were compared to those obtained from the reference responses. The average differences of the dipole localizations from the reference recordings with the two systems were below 10 mm. In condition 1, the responses were unaffected by tSSS. In conditions 2-4, the head position could be tracked reliably, and tSSS suppressed the interference caused by magnetized impurities. In condition 2, the average dipole localization difference from the reference location was below 10 mm both for SEF and AEF. Dipole waveforms and amplitudes were similar to the reference results. In conditions 3 and 4, continuous and regular movements influenced the AEFs and triggered bursting rhythmic activity from the motor cortex and muscles moving the head. Consequently, the average AEF dipole localization differences were between 10 and 15 mm and the amplitudes were about 40% smaller than the references. The SEF results were, however, similar to the reference results. In summary, tSSS and MC effectively suppressed disturbances caused by head movements and magnetized impurities.