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Understanding M/EEG Measurements: What are we really measuring?

This article explains the basics of M/EEG measurements, including what we want to measure, how we measure it, and what we actually measure. It also explores the limitations of M/EEG and why we don't use a different measure.

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Understanding M/EEG Measurements: What are we really measuring?

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  1. M/EEG Data: What are we measuring? Jason Taylor MRC Cognition and Brain Sciences Unit (CBU) Cambridge Centre for Ageing and Neuroscience (CamCAN) 19 January 2011 | SPM M/EEG Course | Brussels [ Much stolen from James Kilner, Jérémie Mattout, Olaf Hauk ]

  2. M/EEG: What are we measuring? • What do we want to measure? • How do we measure it? • What do we really measure? • What do we not measure? • Why don’t we use a different measure?

  3. What do we want to measure? - + = Electric dipole, with magnetic field produced according to the right-hand rule Current flow in apical denrites of cortical pyramidal cells (not action potentials)

  4. What do we want to measure? ~1 million synapses must be simultaneously active to be detected - + Along with the primary current, volume currents are induced in the surrounding tissues Luckily, there are ~10 million cells per mm2 with 1000s of synapses each.

  5. How do we measure it? 1771: Luigi Galvani (<- Bologna, Italy) experiments with ‘animal electricity’. 1840s, ’50s: Du Bois-Raymond (Berlin) and Helmholtz (Berlin) describe and quantify action potential. 1875: Richard Caton (Liverpool, UK ->) measures currents between the cortical surface and the skull (ECoG) in dogs, monkeys 1924: Hans Berger (<- Jena, Germany) records first EEG in humans, describes alpha and beta waves

  6. MCG: Cohen, Science (1967) (first: Baule & McFee in 1963, Syracuse, NY) MEG: Cohen, Science (1968) -- early days of MEG (and MCG) 1960s: David Cohen (<- MIT) MCG and MEG, pre-squid

  7. MCG: Cohen, Science (1967) (first: Baule & McFee in 1963, Syracuse, NY) MEG: Cohen, Science (1968) 1979: Joy Division (Manchester, UK) release ‘Unknown Pleasures’, featuring recordings of a pulsar on the album cover (obviously inspired by Cohen’s MEG waveforms ;-).

  8. -- and now, the SQUID! 1970: James Zimmerman (Ford Co., USA) invents the Superconducting Quantum Interference Device (SQUID), an ultrasensitive detector of magnetic flux Superconductivity is zero-resistance electrical conduction that (typically) occurs at extremely cold temperatures, near absolute zero. 1973: Brian Josephson (Cambridge, UK) awarded the Nobel prize for prediction (in 1962) of ‘tunnel effect’ between two superconducting materials separated by a thin insulating layer (‘Josephson Junction’) Brian Josephson

  9. -- and now, the SQUID! This -> is not it David Cohen knew something about extreme cold temperatures, having grown up in Winnipeg, Canada (incidentally also my home town), where it’s often -30°C for weeks at a time in the winter. Cohen (Science, 1972) describes the first SQUID-based 1-sensor MEG recording David Cohen http://www.etsy.com ‘squid hat’

  10. -- and now, the SQUID! MCG MEG

  11. MEG sensor types http://meg.aalip.jp/scilab/CoilType.html

  12. Elekta Neuromag Vector View System Sensor Array 102 magnetometers 204 planar gradiometers EEG Cap (70-channel montage shown)

  13. What do we really measure?

  14. -- methods for removing/avoiding noise Independent Component Analysis (ICA) Magnetically shielded room (MSR) VEOG Signal Space Separation (SSS) - Neuromag IC1 MEG1 Plus: Filtering, averaging, robust averaging,… Before /After IC1 removed MEG2 MEG3 MEG4

  15. What do we not measure? -- dipole orientation MEG vs. EEG - MEG is insensitive to (purely) radially oriented dipoles

  16. -- source depth MEG vs. EEG - MEG is insensitive to (purely) radially oriented dipoles - MEG is less sensitive to deep sources From Olaf Hauk: http://http://www.mrc-cbu.cam.ac.uk/research/eeg/eeg_intro.html

  17. -- cancelling sources MEG vs. EEG - MEG is insensitive to radially oriented dipoles - MEG is less sensitive to deep sources - NEITHER can detect closed (cancelling) sources Ahlfors et al., HBM 2010

  18. Why don’t we use a different measure?

  19. Thanks! - The End -

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