Magnetoencephalography (MEG) and its role in studying human neurophysiology

David Poeppel

Cognitive Neuroscience of Language LabCognitive Neuroscience of Language LabDepartment of Linguistics and Department of Biology

Neuroscience and Cognitive Science ProgramUniversity of Maryland College Park

Additional slides courtesy of:• Kanazawa Institute of Technology/Eagle Technology• Prof. Dr. Kensuke Sekihara, Tokyo• Prof. Dr. Timothy Roberts, Toronto• Prof. Dr. Riitta Salmelin, Helsinki

Non-invasiverecording fromhuman brain

Positron emissiontomography (PET)

Functional magneticresonance imaging (fMRI)

Excellent spatialresolution (~1-2mm)Limited temporalresolution (~1sec)

Hemodynamictechniques

(Functionalbrain imaging)

Electro-encephalography(EEG) Limited spatial

resolution (~1cm)Excellent temporalresolution (<1msec)

Electro-magnetictechniques

Magneto-encephalography(MEG)

D. Poeppel , A. Braun et al.

V

skull

CSF

MEG

EEGBscalp

recordingsurface

orientationof magnetic field

Origin of the signal

CSF

tissue

- noninvasive measurement- direct measurement.

currentflow

How small is the signal?1 0 - 4

1 0- 5

1 0- 6

1 0 - 7

1 0- 8

Earth field

Urban noise

Inte

nsity

of m

agne

tic s

igna

l(T)

EYE (retina)Steady activityEvoked activity

LUNGSMagnetic contaminants

LIVER Iron stores

FETUSCardiogram

BRAIN (neurons)Spontaneous activityEvoked by sensory stimulation

SPINAL COLUMN (neurons)Evoked by sensory stimulation

HEARTCardiogram (muscle)Timing signals (His Purkinje system)

GI TRACKStimulus responseMagnetic contaminations

1 0- 9

1 0 - 1 0

1 0- 1 1

1 0- 1 2

1 0 - 1 3

1 0- 1 4

1 0 - 1 5

Contamination at lung

Heart QRS

MuscleFetal heart

Spontaneous signal (α-wave)

Signal from retina

Intrinsic noise of SQUID

Inte

nsity

of m

agne

tic s

igna

l(T)

Evoked signal

Biomagnetism

Cardiogram

LIMBSSteady ionic current

Magnetic contaminations

MUSCLEUnder tension

requires sensitive detectors(low noise-high gain amplification)

Superconducting Quantum Interference Devices (SQUIDS) with differential measurement

Magnetometer GradiometerKIT SystemCTF SystemBTi-4D

Planar type Axial type

50 mm base line

NeuroMag VectorViewBTi-4D Magnes

NeuroMag VectorView

Superconductivity

- Magnetic flux quantization

To construct a highly sensitive detector

- Josephson effect

- Linearization

axial gradiometer

recording surface

Capturing the signal

For a gradiometer of thistype, a signal from cortexlooks different between the twocoils because of the distancebetween the two coils.

A signal from far away, however,will look similar in size to the

recording surface

neuronal source

will look similar in size to the two coils.

This gradiometer principle canhelp further with the challengingproblem of measuring small source that exist in an electro-magnetic environment with manylarge source (subways, elevators,computers, etc.).

In addition to using gradiometers:Noise reduction using reference channels

　

約250mm

In addition to using gradiometers and reference channels for noise reduction:

Magnetically shielded room (MSR)

Sensor layout: recording from 160 channelsResponse peak at 98ms after onset of an auditory st imulus, in the left and right temporal lobes.

Butterfly plot: overlay of the channels over right temporal lobeResponse peak at 98ms after onset of an auditory st imulus

Contour plot: distribution of magnetic field at peak response

For better source reconstruction …

…high spatial sampling is crucial .

not soideal

expensive-but closer toideal

High density sensor array

front viewbottom view

Magnetic source imaging (MSI): MEG + MRIDipole fit at response peak, 98ms after onset of st imulus

Somatosensory evoked field (SEF)

SI 40ms

SII 160 ms and >300ms

AI 100ms

Dipole locations subsequent to somatosensory and au ditory stimulation(primary and secondary somatosensory as well as pri mary auditory areasand the time of response peak).

Disbrow et al. (2001) J. Neurophysiol.

What is the benefit of using MEG?

MEG

EEG

-As high temporal resolution as EEG …… but much easier and quicker to set up (kids, patients)

-Magnetic fields are not differentially attenuated ….…. easier to get a reasonable estimate of source over time

- Sensitivity to within-subjects effects

fMRI (yellow blobs) and MEG (red dots) show remarkably consistent co-localizationRoberts & Poeppel, forthcoming

Sanders, Sekihara, Poeppel 2003

MEG is a technique that allows you to

(i) record brain activity directly, withexcellent temporal resolution (ms)

(ii) design within-subjects experiments and evaluate single -subject dataevaluate single -subject data

(iii) test models of cognitive processes and evaluate how these models map on to the brain.