Basis of the M/EEG signal

Basis of the M/EEG signalBasis of the M/EEG signal

Evelyne Mercure Evelyne Mercure

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Bonnie BreiningBonnie Breining

PlanPlan

Overview of EEG & ERPOverview of EEG & ERP Overview of MEGOverview of MEG ComparisonsComparisons

EEG vs. MEGEEG vs. MEG EEG/MEG vs. Other Imaging Techniques EEG/MEG vs. Other Imaging Techniques

ElectroencephalographyElectroencephalography

1929: Hans Berger discovered that an 1929: Hans Berger discovered that an electrode applied to the human scalp could electrode applied to the human scalp could record voltage variations attributed to the record voltage variations attributed to the activity of the neuronsactivity of the neurons

Amplified, plotted as a function of time => Amplified, plotted as a function of time => EEG signalEEG signal

The EEG signalThe EEG signal

EEG rhythmsEEG rhythms

Action potentialAction potential When a neuron is activated, current flows from the cell body When a neuron is activated, current flows from the cell body

to the axon terminalto the axon terminal To be registered by electrodes on the scalp many neurons To be registered by electrodes on the scalp many neurons

would need to fire at the same time, which is unlikely given would need to fire at the same time, which is unlikely given that action potentials lasts around 1msecthat action potentials lasts around 1msec

No dipole createdNo dipole created

Not recorded by EEG!!!Not recorded by EEG!!!

Postsynaptic potentialsPostsynaptic potentials

After an action potential After an action potential neurotransmitters are released neurotransmitters are released

They bind to the receptors of They bind to the receptors of a postsynaptic neurona postsynaptic neuron

Postsynaptic potential (2)Postsynaptic potential (2) Depending on whether the Depending on whether the

neurotransmitter is excitatory or neurotransmitter is excitatory or inhibitory, inhibitory, electrical current electrical current flowsflows from the postsynaptic cell from the postsynaptic cell to the environment, or the to the environment, or the oppositeopposite

The membrane of the The membrane of the postsynaptic cell becomes postsynaptic cell becomes depolarised (more likely to depolarised (more likely to generate an action potential) or generate an action potential) or hyperpolarised (less likely to hyperpolarised (less likely to generate an action potential)generate an action potential)

Postsynaptic potential (3)Postsynaptic potential (3) Electrical current begins to flow in the Electrical current begins to flow in the

opposite direction within the cell body to opposite direction within the cell body to complete the electrical circuitcomplete the electrical circuit

A small dipole is created!A small dipole is created! Lasts tens or even hundreds of Lasts tens or even hundreds of

milliseconds => more likely to happen milliseconds => more likely to happen simultaneouslysimultaneously

To sum together, postsynaptic potentials To sum together, postsynaptic potentials of different neurons need toof different neurons need to Be simultaneous Be simultaneous Be spatially alignedBe spatially aligned

+

-

Pyramidal neurons of the Pyramidal neurons of the cortex are spatially cortex are spatially aligned and perpendicular aligned and perpendicular to the cortical surfaceto the cortical surface

The EEG signal results The EEG signal results mainly from the mainly from the postsynaptic activity of postsynaptic activity of the pyramidal neuronsthe pyramidal neurons

Volume conductionVolume conduction When a dipole is in a When a dipole is in a

conductive medium, conductive medium, electrical current spreads electrical current spreads through this mediumthrough this medium

The skull has a higher The skull has a higher electrical resistance than electrical resistance than the brain => the electrical the brain => the electrical signal spreads laterally signal spreads laterally when reaching the skull when reaching the skull

Difficulty of source Difficulty of source localisationlocalisation

Recording EEGRecording EEG Electrode applied to the skull or brain surfaceElectrode applied to the skull or brain surface Substance with low impedance is used to conduct electricity Substance with low impedance is used to conduct electricity

between the skin and electrode between the skin and electrode Voltage is a difference in electrical potential => need a reference Voltage is a difference in electrical potential => need a reference

point!point!

ArtefactsArtefacts

Muscle movementsMuscle movements Eye movementsEye movements BlinksBlinks SweatingSweating

Many trialsMany trials Artefact rejectionArtefact rejection

Event-related potentialsEvent-related potentials

A different way of analysing the EEG signalA different way of analysing the EEG signal Time-locked to a stimulusTime-locked to a stimulus

Event-related potentials (2)Event-related potentials (2)

AveragingAveraging ERP ERP

componentscomponents

P1 =>

N170 =>

P2 =>

MagnetoencephalographyMagnetoencephalography(MEG)(MEG)

Electricity & MagnetismElectricity & 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 FieldsMagnetic 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 HistoryA 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 HistoryMore 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)

EquipmentEquipment

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 dataMEG data

http://imaging.mrc-cbu.cam.ac.uk/meg/

brain activation film (recorded during

comprehension of a spoken word)

EEG vs. MEGEEG 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

EEG MEG

Thanks to last year’s slides & wikipedia

MEG/EEG vs. Other TechniquesMEG/EEG vs. Other Techniques

rationalist.eu/Images/introfig4.jpg

Advantages of EEG/ERPs/MEGAdvantages of EEG/ERPs/MEG

Non-invasive (records electromagnetic activity, does not Non-invasive (records electromagnetic activity, does not modify it)modify it)

Can be used with adults, children, infants, newborns, clinical Can be used with adults, children, infants, newborns, clinical populationpopulation

High temporal resolution (a few milliseconds, around 1000x High temporal resolution (a few milliseconds, around 1000x better than fMRI) => ERPs study dynamic aspects of better than fMRI) => ERPs study dynamic aspects of cognitioncognition

EEG relatively cheap compared to MRIEEG relatively cheap compared to MRI Allow quiet environmentsAllow quiet environments Subjects can perform tasks sitting up- more natural than in Subjects can perform tasks sitting up- more natural than in

MRIMRI

Limitations of EEG/ERPs/MEGLimitations of EEG/ERPs/MEG Spatial resolution is fundamentally undeterminedSpatial resolution is fundamentally undetermined

Signal picked up at one place on the skull does not represent the Signal picked up at one place on the skull does not represent the activity directly under itactivity directly under it

Forward problem: Knowing where the dipoles are and the Forward problem: Knowing where the dipoles are and the distribution of the conduction in the brain, we could calculate the distribution of the conduction in the brain, we could calculate the voltage variation recorded at one point of the surface voltage variation recorded at one point of the surface

Inverse problem: Infinite number of solutionsInverse problem: Infinite number of solutions Source localisation algorithms uses sets of predefined constraints Source localisation algorithms uses sets of predefined constraints

to limit the number of possible solutionsto limit the number of possible solutions Anatomical information not providedAnatomical information not provided

References/suggested readingReferences/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.htmlhttp://www.nmr.mgh.harvard.edu/martinos/research/technologiesMEG.php