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What are we measuring with EEG and MEG ?. Isabel Zlobinski & Xavier De Tiège. Introduction. Neurophysiological background. EEG. MEG. Introduction. In both methods, the measured signals are generated by the same synchronized neuronal activity in the brain. TEMPORAL RESOLUTION.

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Presentation Transcript
slide1

What are we measuring with

EEG and MEG ?

Isabel Zlobinski & Xavier De Tiège

slide2

Introduction

Neurophysiological background

EEG

MEG

slide3

Introduction

In both methods, the measured signals are generated by

the same synchronized neuronal activity in the brain

TEMPORAL RESOLUTION

EEG and MEG are 2 functional cerebral imaging techniques

that are closely related

The main interest of M-EEG compared to other techniques

slide5

Follow the rapid changes in cortical activity

Reflect ongoing signal processing in the brain

The temporal resolution of M-EEG

slide6

Neurophysiological background

Glial cells

Structural support

Metabolism

Ions & NTT transport

Myelin

Neurons

Information-processing units

slide7

Grey matter

cell bodies & dendrites

White matter

axones (myeline)

Cortex & basal ganglia

slide8

Like other cells, the neurons are

surrounded by a membrane

The membrane divides the tissue into

intra- & extracellular compartments

with different ions [ ]

The difference in ions [ ] is maintained against

their [ ] gradientby

special proteins that pump selected ions

Na+-K+ pump (3 Na+ out, 2 K+ in)

slide9

The differences in ions [ ] & the permeability

of the membrane for each ion

The resting state of the neurons can be modified by

Action Potentials

Postsynaptic Potentials

Axones

Synaptic junctions

+ + + + + + + + + + + + + +

- - - - - - - - - - - - - - - - - - -

Resting potential

- - - - - - - - - - - - - - - - - - -

+ + + + + + + + + + + + + +

Goldman ’s equation

slide10

Action Potentials

Generated at the cell body/axone junction

Depolarization

Hyperpolarization

Repolarization

slide11

Action potentials

- Generate 2 current dipoles = quadrupole

parallel, equal intensity, opposite directions => 0

- Quadrupolar field decreases with distance as 1/r³

(compared to 1/r² for dipolar field)

- Duration = 1 ms temporal summation between neighbouring fibers difficult

Not observable with M-EEG

slide12

Postsynaptic Potentials

Synaptic junctions

mainly on cell body &

dendrites

Action potential at the synaptic junction

of the presynaptic neuron

Action Potentials

Liberation of neurotransmitters

Receptors

Ion channels activated

De- or hyperpolarization

slide13

Acetylcholine or glutamate

Activate Na+ and Ca++ channels

Depolarization

Excitatory PSP

Summation of EPSP

Action potential at the cell body/axon junction

GABA

Activate Cl- channels

Hyperpolarization

Prevents action potential generation

Inhibitory PSP

slide14

EPSP

Are measured with M-EEG

- Generate intracellular currents and

extracellular currents

- Generate (approximately) one current dipole

- Dipolar fields decrease with

distance as 1/r²

- Duration = 10 ms

temporal summation between neighbouring fibers

more effective

A single EPSP produces a current dipole

along the dendrite with a stenght of +/- 20 fA m

Too small to be measured with M-EEG

slide15

M-EEG see sources with strenght

on the order of 10 nA m

Cummulative summation of one million of synaptic

junctions in a small region is required

As apical dendrites of pyramidal

neurons of the cortex tend

to be perpendicular to the cortical surface

Cummulative summation of EPSP

in the same direction is more easily

obtained with apical dendrites of

pyramidal cells

M-EEG signals are mainly produced by

PSP generated at apical dendrites

of pyramidal cells in the cortex

slide17

M E G

-graphy

-encephalo-

Magneto-

Record magnetic fields

generated by brain activity

slide18

PSP induced intracellular currents (primary currents) and

extracellular currents (secondary currents)

Secondary currents yield potential differences on the

scalp of the head that can be measured by EEG

MEG measures magnetic fields induced mainly by

primary currents

Cummulative summation of

PS primary currents

of millions apical dendrites

of pyramidal cells in one cortical area

Generates a magnetic field

measurable by MEG

slide19

Primary currents

Induced magnetic

field

"Right Hand Law"

Volume currents

slide20

Tangential currents will produce magnetic fields

that are observable outside the head

Radial currents will not produce magnetic fields outside the head

MEG only detects tangential currents

slide22

MEG measures the fluctuations of frequency (Hz) and

amplitude (T) of the brain magnetic signal

10 fT (10-15) to about several pT (10-12)

BUT

Earth ’s magnetic field is about 0.5 mT

Urban magnetic noise is about 1 nT to 1 µT

Moving vehicules, moving elevators, radio, TV, powerlines, etc.

The electrical activity of the heart, eye blinks also generate a field 2 to 3 order

of magnitude larger than the signal from the brain

Noise is about a factor of 10³ to 106 larger than the MEG signal

slide23

We need very sensitive MEG sensors

to pick up the brain magnetic fields

SQUIDs

MEG measurements need noise

cancellation with extraordinary accuracy

Design of the SQUID

Magnetic shielded room

Hardware and software

Averaging

slide24

Superconducting QUantum Interference Device

SQUIDs are sensitive to very low magnetic fields

The SQUIDs   "translate" the magnetic field into an electrical current

which is proportional to this field

To have their superconductive properties,

the SQUIDs need to be maintained at-269 °C

They are cooled in

liquid He

slide25

The different types of pick-up coils

CTF system

Axial and planar

gradiometer

Magnetometers

slide26

1980

1995-2000

Whole-head sensors arrays which use 100 to 300 sensors

at different locations

slide27

Noise cancellation

Compensation coil

compensates for variations in the

background field

SQUID Design

1st order axial gradiometer

This SQUID will only be

sensitive to inhomogeneous

changes of magnetic fields

between the 2 coil

Pick-up coil

picks up the signal from the brain

Background fields will be

spatially uniform

Shielded room

Reduce the effect

of external magnetic

disturbances

slide28

Hardware and softwares

Use of reference

A linear combination of the reference

output is subtracted from the

MEG primary sensor output

Low-pass filter, high pass filter

50-Hz filter, etc...

Use of filters

Use of specific softwares

Averaging of brain signals

slide29

With MEG, you can make (as in EEG) :

- Continuous acquisition of brain signals and study some

events that appear « randomly » (Epileptic abnormalities, etc.)

- Evoked response: averaged MEG signals that are synchronous with

an external stimulus or voluntary motor event

slide33

References :

- Hämäläinen et al., Reviews of Modern Physics, 1993

- Baillet et al., IEEE Signal Processing Magazine, 2001

- Jeremie Mattout PhD thesis

- Murakami & Okada, J Physiol, in press