Basis of the MEEG signal Evelyne Mercure Bonnie

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

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

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

The EEG signal

EEG rhythms

Action potential n 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 1 msec No dipole created n Not recorded by EEG!!! n n

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

Postsynaptic potential (2) n n 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)

Postsynaptic potential (3) n n 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 n Be simultaneous n Be spatially aligned - +

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

Volume conduction n 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

Recording EEG n n n 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!

Artefacts n n Muscle movements Eye movements Blinks Sweating Many trials Artefact rejection

Event-related potentials A different way of analysing the EEG signal n Time-locked to a stimulus n

Event-related potentials (2) n n Averaging ERP components P 2 => P 1 => N 170 =>

Magnetoencephalography (MEG)

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

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

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

A Bit of History n n n In 1963 Gerhard Baule and Richard Mc. Fee 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.

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

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

n The sensitivity of the SQUID to magnetic fields may be enhanced by coupling it to a superconducting pickup coil (“flux transformer”) which: n n Magnetometers n 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

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

EEG vs. MEG EEG • Cheap • Large Signal (10 m. V) • Signal distorted by skull/scalp • Spatial localization ~1 cm • Sensitive to tangential and radial dipoles (neurons in sulci & on gyri) • Allows subjects to move • Sensors attached directly to head • Extracellular secondary (volume) currents MEG • Good temporal resolution (~1 ms) • Problematic spatial resolution (forward & inverse problems) • Expensive • Tiny Signal(10 f. T) • 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 Thanks to last year’s slides & wikipedia

MEG/EEG vs. Other Techniques rationalist. eu/Images/introfig 4. jpg

Advantages of EEG/ERPs/MEG n n n 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 1000 x better than f. MRI) => 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

Limitations of EEG/ERPs/MEG n Spatial resolution is fundamentally undetermined n n n 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

References/suggested reading n n n 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: nhttp: //www 1. aston. ac. uk/lhs/research/facilities/meg/introduction/ nhttp: //web. mit. edu/kitmitmeg/whatis. html nhttp: //www. nmr. mgh. harvard. edu/martinos/research/technologies. MEG. php