Post on 15-Mar-2016
description
From Neuronal activity to EEG/MEG signals
Jérémie MattoutU821 INSERMBrain Dynamics and CognitionLyon, FranceSPM Course – May 2010 – London
A short tale about the origins of Electroencephalographyand Magnetoencephalography
A brief history
The EEG & MEG instrumentation
What do we measure with EEG & MEG ?
Of the importance of modelling forward
Outline
Carl Friedrich Gauss1777 - 1855
Lionel Messi
A brief history
A brief history
From the electrical nature of brain signals …
… to the first EEG recordings
Richard Caton1842 - 1926
Hans Berger1873 - 1941
1875: R.C. measured currents inbetween the cortical surface and the skull, in dogs and monkeys
1924: H.B. first EEG in humans, description of alpha and beta waves
Alpha actiity ~ 200 μV
A brief history
About 50 years later …
DavidCohen
1962: Josephson effect
1968: first (noisy) measure of a magnetic brain signal [Cohen, Science 68]
1970: James Zimmerman invents the ‘Superconducting quantum interference device’ (SQUID)
1972: first (1 sensor) MEG recording based on SQUID [Cohen, Science 1972]
1973: Josephson wins the Nobel Prize in Physics
Brian-DavidJosephson
A brief history
About 40 years later… today!
Bob - 2010
The EEG & MEG instrumentation
The EEG & MEG instrumentation
EEG
- The EEG cap sticks to the subject’s head- EEG measures are not much sensitive to environmental noise (except for 50Hz)
- EEG data depend upon a choice of reference- EEG data might be corrupted by artefacts (blinks, saccades, heart beat, sweat,muscle activity, breathing, swallowing, yawning, sweat, 50Hz, )
Claire & JB (french scientists)
The EEG & MEG instrumentation
Sensors(Pick up coil)
SQUIDs
MEG - 269 °C
There are different types of sensors
Magnetometers: measure the magnetic flux through a single coil
Gradiometers: measure the difference in magnetic flux between two points in space (axial/planar ; order 1, 2 or 3)
The EEG & MEG instrumentation
MEG essentially measures… noise!
The EEG & MEG instrumentation
Heart beat
Eye movements
Brain activity
Evoked brain activity
Biomagnetic fields
Earth magnetic field
Environmental noise
Urban noise
Car (50m)
Screw driver (5m)
Electronic circuit(2m)
1 femto-Tesla (fT) = 10-15 TAlpha waves ~ 103 fT
What do we measure with EEG & MEG ?from a single neuron to a neuronal assembly
From a single neuron to a neuronal assembly/column
- A single active neuron is not sufficient. ~100.000 simultaneously active neurons are needed to generate scalp measures.
- Pyramidal cells are the main direct neuronal sources of EEG & MEG signals.
- Synaptic currents but not action potentials generate EEG/MEG signals
What do we measure with EEG & MEG ?
The dipolar model
- A current source in the brain corresponds to a neuronal column and is modelled by a current dipole
- A current dipole is fully defined by 6 parameters: 3 for its position & 3 for its moment (includes orientation and amplitude)
- A dipolar moment Q = I x d ~ 10 to 100 nAm
What do we measure with EEG & MEG ?
source
sink
What do we measure with EEG & MEG ?from a neuronal assembly to sensors
From a single source to the sensor: the quasi-static assumption
What do we measure with EEG & MEG ?
James Clerk Maxwell(1831 - 1879)
E: electric fieldB: magnetic field
From a single source to the sensor: EEG
What do we measure with EEG & MEG ?
primary/sourcecurrents
secondary/conductioncurrents
Electric field lines
JcJs
From a single source to the sensor: EEG
What do we measure with EEG & MEG ?
Georg Simon Ohm1789 - 1841
Ohm’s law
Jc = E = - grad(V) tissue conductivities
Margaret Thatcher
QueenElisabeth II
Conservation law
.Js + . Jc = 0 => . Js = .[grad(V)]
From a single source to the sensor: EEG
What do we measure with EEG & MEG ?
- EEG is sensitive to both radial and tangential sources- EEG is sensitive to conductivities which explains the low resolution scalp topographies- To model EEG data, it matters to account for real tissue conductivity and geometry
Simulatedexample
Early auditoryevoked repsonse
>
From a single source to the sensor: MEG
What do we measure with EEG & MEG ?
Right hand rule
Barak Obama
Tangential dipoleRadial dipole
What do we measure with EEG & MEG ?
From a single source to the sensor: MEG
source locationsensor location
source orientation & sizesource amplitude
- The magnetic field amplitude decreases with the square of the distance between the source and the sensor => MEG is less sensitive to deep sources
- Pure radial sources will remain silent
Félix Savart (1791-1841) Jean-Baptiste Biot (1791-1841)
What do we measure with EEG & MEG ?
From a single source to the sensor: MEG Biot & Savart’s law
MEG
EEG
What do we measure with EEG & MEG ?
From a single source to the sensor: MEG
Summary
spati
al re
solu
tion
(mm
)
invasivity
weak strong
5
10
15
20
temporal resolution (ms)1 10 102 103 104 105
sEEG
MEG
EEG
fMRI
MRI(a,d)
PET
SPECT
OIECoG
What do we measure with EEG & MEG ?
Of the importance of modelling forward
« Will it ever happen that mathematicians will know enough about the physiology of the brain, and neurophysiologists enough of mathematical discovery, for efficient cooperation to be possible ? »
Jacques Hadamard (1865-1963)
Of the importance of modelling forward
inference
MEG
EEG
From EEG/MEG data to neuronal sources ?
Forward model
Generative models
MEG
EEGDipolar sources
Head tissues(conductivity & geometry)
Of the importance of modelling forward
Gain vectors & Lead-field matrix
Y = g() Simulating data
sourceparameters
forwardmodel
scalpdata
-1 layer vs. 3 layers- spheres vs. realistic surfaces or volumes- analytical vs. numerical solutions
1 source 1 gain vector
All sources 1 gain operator orlead-field matrix
Of the importance of modelling forward
Inverse problem
Y = g(1) + g(2) + Modelling empirical data
Unknownsource
Parameters ?
forwardModel
(lead-fields)
scalpdata
Of the importance of modelling forward
Jean Daunizeau
Karl Friston
James Kilner
Stefan Kiebel
Guillaume Flandin
Vladimir Litvak
Christophe Phillips
Rik Henson
Marta GarridoWill Penny
Rosalyn Moran
Gareth Barnes JM Schoffelen