Basis of the M/EEG signal

1. Basis of the M/EEG signal Evelyne Mercure & Bonnie Breining

2. Plan • Overview of EEG & ERP • Overview of MEG • Comparisons • EEG vs. MEG • EEG/MEG vs. Other Imaging Techniques

3. Electroencephalography • 1929: Hans Berger discovered that an electrode applied to the human scalp could record voltage variations attributed to the activity of the neurons • Amplified, plotted as a function of time => EEG signal

4. The EEG signal

5. EEG rhythms

6. Action potential • When a neuron is activated, current flows from the cell body to the axon terminal • To be registered by electrodes on the scalp many neurons would need to fire at the same time, which is unlikely given that action potentials lasts around 1msec • No dipole created • Not recorded by EEG!!!

7. Postsynaptic potentials • After an action potential neurotransmitters are released • They bind to the receptors of a postsynaptic neuron

8. Postsynaptic potential (2) • Depending on whether the neurotransmitter is excitatory or inhibitory, electrical current flows from the postsynaptic cell to the environment, or the opposite • The membrane of the postsynaptic cell becomes depolarised (more likely to generate an action potential) or hyperpolarised (less likely to generate an action potential)

9. Postsynaptic potential (3) • Electrical current begins to flow in the opposite direction within the cell body to complete the electrical circuit • A small dipole is created! • Lasts tens or even hundreds of milliseconds => more likely to happen simultaneously • To sum together, postsynaptic potentials of different neurons need to • Be simultaneous • Be spatially aligned - +

10. Pyramidal neurons of the cortex are spatially aligned and perpendicular to the cortical surface • The EEG signal results mainly from the postsynaptic activity of the pyramidal neurons

11. Volume conduction • When a dipole is in a conductive medium, electrical current spreads through this medium • The skull has a higher electrical resistance than the brain => the electrical signal spreads laterally when reaching the skull • Difficulty of source localisation

12. Recording EEG • Electrode applied to the skull or brain surface • Substance with low impedance is used to conduct electricity between the skin and electrode • Voltage is a difference in electrical potential => need a reference point!

13. Artefacts • Muscle movements • Eye movements • Blinks • Sweating • Many trials • Artefact rejection

14.      Event-related potentials • A different way of analysing the EEG signal • Time-locked to a stimulus

15. Event-related potentials (2) • Averaging • ERP components P2 => P1 => N170 =>

16. Magnetoencephalography(MEG)

17. Electricity & Magnetism • MEG measures the same postsynaptic potentials as EEG. • Basic Physics: • Electric currents have corresponding magnetic fields. • The magnetic field generated is perpendicular to the electric current. • Right Hand Rule

18. Electricity & Magnetism 2:MEG is sensitive to tangential but not radial components of signal • MEG mainly measures the activity of pyramidal neurons in the sulci that are oriented parallel to the scalp • Magnetic fields from perpendicular oriented neurons on gyri don’t project out of head

19. Magnetic Fields • Magnetic fields generated by brain activity are tiny • 100 million times smaller than the earth's magnetic field • 1 million times smaller than the magnetic fields produced in an urban environment (by cars, elevators, radiowaves, electrical equipment, etc) • MEG must be performed in shielded rooms

20. A Bit of History • In 1963 Gerhard Baule and Richard McFee of the Department of Electrical Engineering,Syracuse University, Syracuse, NY detected the biomagnetic field projected from the human heart. • They used two coils, each with 2 million turns of wire, connected to a sensitive amplifier. The magnetic flux from the heart generated a current in the wire. • They did this in a field in the middle of nowhere because of the very noisy signal.

21. More History • In the late 1960’s David Cohen, at MIT, Boston recorded a clean MCG in an urban environment. This was possible due to: • 1) Magnetically shielding the recording room. • 2) Improved recording sensitivity. (The introduction of SQUIDS)

22. Equipment SQUIDs SQUID Sensors • SQUIDs- Superconducting QUantum Interference Devices • Use principles of super-conduction to measure tiny magnetic fields • 300+ sensors in helmet shape • Cool with liquid helium

23. The sensitivity of the SQUID to magnetic fields may be enhanced by coupling it to a superconducting pickup coil (“flux transformer”) which: • has greater area and number of turns than the SQUID inductor alone. • made of superconducting wire and is sensitive to very small changes in the magnitude of the impinging magnetic flux. • The magnetic fields from the brain causes a supercurrent to flow. First Order Gradiometer Magnetometers

24. MEG data http://imaging.mrc-cbu.cam.ac.uk/meg/ brain activation film (recorded during comprehension of a spoken word)

25. EEG vs. MEG EEG MEG • Cheap • Large Signal (10 mV) • Signal distorted by skull/scalp • Spatial localization ~1cm • Sensitive to tangential and radial dipoles (neurons in sulci & on gyri) • Allows subjects to move • Sensors attached directly to head • Extracellular secondary (volume) currents • Expensive • Tiny Signal(10 fT) • Signal unaffected by skull/scalp • Spatial localization ~1 mm • Sensitive only to tangential dipoles (neurons in sulci) • Subjects must remain still • Sensors in helmet • Requires special laboratory • Intracellular primary currents’ magnetic fields • Good temporal resolution (~1 ms) • Problematic spatial resolution (forward & inverse problems) Thanks to last year’s slides & wikipedia

26. MEG/EEG vs. Other Techniques rationalist.eu/Images/introfig4.jpg

27. Advantages of EEG/ERPs/MEG • Non-invasive (records electromagnetic activity, does not modify it) • Can be used with adults, children, infants, newborns, clinical population • High temporal resolution (a few milliseconds, around 1000x better than fMRI) => ERPs study dynamic aspects of cognition • EEG relatively cheap compared to MRI • Allow quiet environments • Subjects can perform tasks sitting up- more natural than in MRI

28. Limitations of EEG/ERPs/MEG • Spatial resolution is fundamentally undetermined • Signal picked up at one place on the skull does not represent the activity directly under it • Forward problem: Knowing where the dipoles are and the distribution of the conduction in the brain, we could calculate the voltage variation recorded at one point of the surface • Inverse problem: Infinite number of solutions • Source localisation algorithms uses sets of predefined constraints to limit the number of possible solutions • Anatomical information not provided

29. References/suggested reading • Handy, T. C. (2005). Event-related potentials. A methods handbook. Cambridge, MA: The MIT Press. • Luck, S. J. (2005). An introduction to the event-related potential technique. Cambridge, Massachussets: The MIT Press • Rugg, M. D., & Coles, M. G. H. (1995). Electrophysiology of mind: Event-related brain potentials and cognition. New York, NY: Oxford University Press. • Hamalainen, M., Hari, R., Ilmoniemi, J., Knuutila, J. & Lounasmaa, O.V. (1993). MEG: Theory, Instrumentation and Applications to Noninvasive Studies of the Working Human Brain. Rev. Mod. Phys. Vol. 65, No. 2, pp 413-497. • Sylvain Baillet, John C. Mosher & Richard M. Leahy (2001). Electromagnetic Brain Mapping. IEEE Signal Processing Magazine. Vol.18, No 6, pp 14-30. • Basic MEG info: • http://www1.aston.ac.uk/lhs/research/facilities/meg/introduction/ • http://web.mit.edu/kitmitmeg/whatis.html • http://www.nmr.mgh.harvard.edu/martinos/research/technologiesMEG.php