From Neuronal activity to EEG/MEG signals
A short tale about the origins of Electroencephalography
and Magnetoencephalography
SPM Course – May 2010 – London
Jérémie Mattout
U821 INSERM
Brain Dynamics and Cognition
Lyon, France
Outline
A brief history
The EEG & MEG instrumentation
What do we measure with EEG & MEG ?
Of the importance of modelling forward
Carl Friedrich Gauss
1777 - 1855
Lionel Messi
A brief history
A brief history
From the electrical nature of brain signals …
1875: R.C. measured currents inbetween the cortical surface and the skull,
in dogs and monkeys
Richard Caton
1842 - 1926
… to the first EEG recordings
1924: H.B. first EEG in humans, description of alpha and beta waves
Alpha actiity ~ 200 μV
Hans Berger
1873 - 1941
A brief history
About 50 years later …
1962: Josephson effect
Brian-David
Josephson
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
David
Cohen
A brief history
About 40 years later… today!
Bob - 2010
The EEG & MEG instrumentation
The EEG & MEG instrumentation
EEG
Claire & JB (french scientists)
- 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, )
The EEG & MEG instrumentation
MEG
SQUIDs
Sensors
(Pick up coil)
- 269 °C
The EEG & MEG instrumentation
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!
1 femto-Tesla (fT) = 10-15 T
Alpha waves ~ 103 fT
Earth magnetic field
Urban noise
Car (50m)
Screw driver (5m)
Heart beat
Eye movements
Electronic circuit
(2m)
Brain activity
Evoked brain activity
Environmental noise
Biomagnetic fields
What do we measure with EEG & MEG ?
from a single neuron to a neuronal assembly
What do we measure with EEG & MEG ?
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
source
sink
- 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 ?
from a neuronal assembly to sensors
What do we measure with EEG & MEG ?
From a single source to the sensor: the quasi-static assumption
E: electric field
B: magnetic field
James Clerk Maxwell
(1831 - 1879)
What do we measure with EEG & MEG ?
From a single source to the sensor: EEG
Electric field lines
primary/source
currents Js
secondary/conduction
currents Jc
What do we measure with EEG & MEG ?
From a single source to the sensor: EEG
Ohm’s law
Jc = s E = - s grad(V)
s : tissue conductivities
Georg Simon Ohm
1789 - 1841
Conservation law
.Js + . Jc = 0 => . Js = .[s grad(V)]
Queen
Elisabeth II
Margaret
Thatcher
What do we measure with EEG & MEG ?
From a single source to the sensor: EEG
Simulated
example
Early auditory
evoked repsonse
- 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
What do we measure with EEG & MEG ?
From a single source to the sensor: MEG
>
Right hand rule
Barak Obama
What do we measure with EEG & MEG ?
From a single source to the sensor: MEG
Radial dipole
Tangential dipole
What do we measure with EEG & MEG ?
From a single source to the sensor: MEG
source amplitude
Biot & Savart’s law
source orientation & size
Jean-Baptiste Biot (1791-1841)
Félix Savart (1791-1841)
sensor location
source location
- 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
What do we measure with EEG & MEG ?
From a single source to the sensor: MEG
MEG
EEG
What do we measure with EEG & MEG ?
spatial resolution (mm)
Summary
EEG
20
invasivity
MEG
weak
strong
SPECT
15
OI
ECoG
PET
10
5
fMRI
sEEG
MRI(a,d)
1
10
102
103
104
105
temporal resolution (ms)
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
From EEG/MEG data to neuronal sources ?
MEG
inference
EEG
Of the importance of modelling forward
Forward model
MEG
Generative models
EEG
Head tissues
(conductivity & geometry)
Dipolar sources
Of the importance of modelling forward
Gain vectors & Lead-field matrix
Simulating data
Y = g()
scalp
data
forward
model
-1 layer vs. 3 layers
- spheres vs. realistic surfaces or volumes
- analytical vs. numerical solutions
source
parameters
1 source 1 gain vector
All sources 1 gain operator or
lead-field matrix
Of the importance of modelling forward
Inverse problem
Modelling empirical data
Y = g(1) + g(2) + 
scalp
data
forward
Model
(lead-fields)
Unknown
source
Parameters ?
Karl Friston
Will Penny
Marta Garrido
Stefan Kiebel
Jean Daunizeau
James Kilner
Vladimir Litvak
Guillaume Flandin
Rik Henson
Rosalyn Moran
Gareth Barnes
JM Schoffelen
Christophe Phillips