meg magnetoencephalography [compatibility mode]
DESCRIPTION
.magnetic encephelon graphy,ic analysis with the mmastry netron the high energy electro magTRANSCRIPT
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 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
For better source reconstruction …
…high spatial sampling is crucial .
not soideal
expensive-but closer toideal
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
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.