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

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

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

The EEG signal

EEG rhythms

Action potential When a neuron is activated, current flows from the cell body to the axon terminal 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 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 No dipole created Not recorded by EEG!!! Not recorded by EEG!!!

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

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

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

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

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

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

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

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

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: 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 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.

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)

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

Magnetometers First Order Gradiometer 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.

MEG data brain activation film (recorded during comprehension of a spoken word)

EEG vs. MEG Good temporal resolution ( ~1 ms) Problematic spatial resolution (forward & inverse problems) 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 EEGMEG Thanks to last year’s slides & wikipedia

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

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

Limitations of EEG/ERPs/MEG Spatial resolution is fundamentally undetermined Spatial resolution is fundamentally undetermined Signal picked up at one place on the skull does not represent the activity directly under it 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 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 Inverse problem: Infinite number of solutions Source localisation algorithms uses sets of predefined constraints to limit the number of possible solutions Source localisation algorithms uses sets of predefined constraints to limit the number of possible solutions Anatomical information not provided Anatomical information not provided

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 Sylvain Baillet, John C. Mosher & Richard M. Leahy (2001). Electromagnetic Brain Mapping. IEEE Signal Processing Magazine. Vol.18, No 6, pp Basic MEG info: