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Realistic models of spatially extended cortical activity in MEG

Douglas Cheyne1*, Jason Lerch2, Ismail Mohamed3, Paul Ferrari4, Marc Lalancette5 and Elizabeth Pang5
  • 1 University of Toronto, Program in Clinical and Experimental Therapeutics, Department of Clinical and Administrative Pharmacy, Canada
  • 2 Hospital for Sick Children and Toronto Centre for Phenogenomics, Program in Neurosciences and Mental Health, Canada
  • 3 Alberta Children's Hospital, Department of Pediatrics, Canada
  • 4 Hospital for Sick Children, Program in Neurosciences and Mental Health, Canada
  • 5 Hospital for Sick Children, Program in Neurosciences and Mental Health and Division of Neurology, Canada

Magnetic fields detectable at the scalp surface are generated by large populations of synchronously active neurons. In particular, large amplitude epileptic discharges (spikes) may involve cortical areas up to several cm^2 [1] and it has been argued that localization methods based on point sources will result in erroneous localization for such large extended sources [2]. However, realistic models of sources in MEG must take into account attenuation of radially oriented currents and cancelling currents in sulcal walls [3]. We modeled extended cortical patches in the sensorimotor region using high-resolution cortical surface meshes derived from MRI [4] based on averaged motor fields in a healthy adult, and Rolandic interictal spikes in a 12-year old female patient, recorded with a 151-channel CTF MEG. Simulated cortical patches 14 to 2400 mm^2 in size with uniform current density (approx. 0.5 nAm/mm^2) were added to background brain activity and localized using a single dipole fit and event-related beamformer (ERB) [5]. Larger patches (> 6 cm^2) produced either very weak fields (due to cancellation), or dipolar fields with low error single dipole fits, but to regions of the patch rather than at greater depths, confirming that involvement of multiple gyri and sulci will appear as either attenuated or fragmented source patterns in MEG, rather than large uniform sources. Interestingly, ERB localization revealed both single and multiple peaks for the larger patches and was not completely suppressed due to correlated activity across the extended source. We conclude that use of physiologically realistic models is important for evaluating the effect of spatially extended brain activity on MEG localization methods.

References

1. Alarcon et al. (1994) J. Neurol Neurosurg Psychiat 57:435-449.

2. Kobayashi et al. (2005) Epilepsia 46:397-408.

3. Ahlfors et al. Hum Brain Mapp, In press.

4. Kim et al. (2005) NeuroImage 27:210-221.

5. Cheyne et al. (2007) Clin Neurophysiol 118:1691-1704.

Conference: Biomag 2010 - 17th International Conference on Biomagnetism , Dubrovnik, Croatia, 28 Mar - 1 Apr, 2010.

Presentation Type: Poster Presentation

Topic: Epilepsy

Citation: Cheyne D, Lerch J, Mohamed I, Ferrari P, Lalancette M and Pang E (2010). Realistic models of spatially extended cortical activity in MEG. Front. Neurosci. Conference Abstract: Biomag 2010 - 17th International Conference on Biomagnetism . doi: 10.3389/conf.fnins.2010.06.00390

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Received: 08 Apr 2010; Published Online: 08 Apr 2010.

* Correspondence: Douglas Cheyne, University of Toronto, Program in Clinical and Experimental Therapeutics, Department of Clinical and Administrative Pharmacy, Toronto, Canada, douglas.cheyne@utoronto.ca