The major challenge of MEG, the inverse problem, is to estimate the very weak primary neuronal currents from the measurements of extracranial magnetic fields. The non-uniqueness of this inverse solution is compounded by the fact that MEG signals contain large environmental and physiological noise that further complicates the problem. In this paper, we evaluate the effectiveness of magnetic noise cancellation by synthetic gradiometers and the beamformer analysis method of synthetic aperture magnetometry (SAM) for source localisation in the presence of large stimulus-generated noise. We demonstrate that activation of primary somatosensory cortex can be accurately identified using SAM despite the presence of significant stimulus-related magnetic interference. This interference was generated by a contact heat evoked potential stimulator (CHEPS), recently developed for thermal pain research, but which to date has not been used in a MEG environment. We also show that in a reduced shielding environment the use of higher order synthetic gradiometry is sufficient to obtain signal-to-noise ratios (SNRs) that allow for accurate localisation of cortical sensory function.
Bibliographical noteNOTICE: this is the author’s version of a work that was accepted for publication in Journal of Neuroscience Methods. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Adjamian, Peyman; Worthen, Sian F.; Hillebrand, Arjan; Furlong, Paul L; Chizh, B.A.; Hobson, Anthony R.; Aziz, Qasim and Barnes, Gareth R. (2009). Effective electromagnetic noise cancellation with beamformers and synthetic gradiometry in shielded and partly shielded environments. Journal of Neuroscience Methods, 178 (1), pp. 120-127. DOI 10.1016/j.jneumeth.2008.12.006
- magnetic shielding
- synthetic gradiometers
- regularised and unregularised