Download The MEG Made Ridiculously Simple

The MEG Made Ridiculously Simple
David Cohen
MGH/MIT/HMS Athinoula A. Martinos Center
for Biomedical Imaging
at Massachusetts General Hospital ;
also Magnet Lab at Mass. Institute of Technology
Intro. slide for my presentation at the Cleveland Clinic on May 8, 2009.
In the following, I admit to stealing some nice slides from my office-mate Matti
Hamalainen (MSH), for which I thank him. Several other slides were blatantly lifted
from various publications and websites, here and there..…
Lets face it: parts of this talk are not really “ridiculously simple”; sorry about that. I
need to do more to simplify things, and it CAN be done But at least this is a start…
This talk takes you up to, but not including, the MEG technology of Inverse
Solutions. That is a whole other thing, and much tougher…you need more
advanced, specialty talks for that material…
1
Before explaining the MEG, I first give an early history of this technology. In the early
years, my friends, there was only one person involved with the MEG – that was me, your
speaker.. So you’re getting this straight “from the horse’s mouth”, whatever that means…
At the beginning, the time was 1967, and this picture was my first shielded room (at the U.of
Illinois). Earlier that year, in that room, I had verified the magnetic field of the heart, the
MCG (first seen by others); and was now ready to look for the first magnetic signal from the
brain, that is, to look for the MEG. It was a tough job, because there were no sensitive
magnetic detectors in those days, and the only way one could see anything from the brain
would be to use elaborate signal averaging. That is what I tried…
2
This was the type of detector I used. It is pulled apart here, for illustrative purposes. It was
basically a room-temperature copper coil, with about one half million turns. I used various
forms a ferrite rods, to enhance the small expected signal. Its’ intrinsic static or noise was much
greater than any expected brain signal… Alas, the sensitive SQUID was not yet available…
3
The signal measured was the alpha rhythm with eyes closed, and this schematic shows the
general arrangement. The trigger for signal-averaging was taken from a particular set of
EEG leads. Because the alpha rhythm signal was so small compared to the coil noise, long
averaging was necessary, to see any real alpha signal.
4
Result: This was one of the first MEG’s measured in that first experiment, hence
one the first MEGs ever recorded. It shows the magnetic field (coil output) versus
time, where the horizontal timescale was about 0.12 seconds across. The lower trace
was only a single shot, where no alpha signal is seen, because it is buried in the
noise. The upper trace is a result of about 10 minutes of averaging, and a bit more
than one cycle of alpha rhythm is seen. Measured this way, the alpha rhythm was
plotted over the head, and showed the proper distribution when triggered in this
special EEG mode. Because the equipment was so cumber-some, and the averaging
time so long, the MEG measured this way was of no practical value.
5
The distribution of the magnetic vector of the specially-selected alpha-rhythm
over the subject’s head, at one instant in time. This was the result of many
measurements over this subject’s head.
6
We now fast-forward to 1971, when I have now constructed a very good shielded
room at MIT, and we had just used the new SQUID detector. To measure the human
heart. So I was ready now to measure the MEG with the SQUID, and a subject is
here seen positioned for such a measurement, with the liquid-helium dewar at his
head. Actually, the subject was my colleague Neil Cuffin, who was very skilled at
computer modelling of magnetic signals from the human body, and enjoyed posing
as a photo subject.
7
The first actual MEG measured with a SQUID, due to the alpha rhythm, of a student
subject. The eyes-open-eyes-closed sequence produces the alpha rhythm very clearly, as
clearly as the traditional EEG. As a result, researchers at various labs became interested,
and the SQUID-measured MEG was now launched.
8
I published the first measurements in SCIENCE, and this shows one of the figures
from that paper. It is the EEG/MEG of a patient with epilepsy, who was performing
hyperventillation. Abnormal MEG signals are readily seen, indicating that epilepsy
recording was tied to the MEG from the very beginning. What a difference between
those early recordings and the advanced MEG program, here at the Cleveland
Clinic…
9
Finally, talking for a moment about Neil Cuffin (who posed in the earlier photo), I
note that we jointly published a number of papers, over a span of some years, on the
physics of the MEG. So this slide shows that we are indeed ancient! Just a little
joke, ha-ha…..
10
A simple but useful approach:
Think of the MEG as a special form of
EEG, where the MEG usually sees less,
but usually sees it better.
Now we return to our main goal, explaining the MEG. I will use the above
statement as a guide, and our job here is to show how we arrive at this conclusion...
11
Some of the basic things we will talk about here. A neural electrical source,
simplified here as the red arrow called a current dipole, generates current (dark
lines) in the conducting medium of the head. The dipole induces voltages V on the
scalp surface, which are measured by the EEG. In addition, all the currents,
including the source current in the dipole itself, produce a magnetic vector (blue) in
and around the head, measured by the MEG. The MEG detector is shown here only
as a simple loop, which measures the magnetic vector passing through it. It is
actually far more complicated, as we shall see…
12
Our plan:
1.
To show the connection between MEG and EEG when the electrical
source is only a battery in a wire circuit or, next, a current dipole in
a salt-water blob.
2.
To show how actual neural sources of MEG/EEG can be
approximated as dipoles; how, in a salt-water blob, MEG and EEG
see different aspects of these dipole sources.
3.
To explain that the magnetic fields generated by the neural sources
are very weak, and we here take a side trip, to show how the MEG
equipment works to measure these weak fields.
4.
Returning to our main plan, we show what happens to the MEG and
EEG when the blob is replaced by the spherical head, with its layers
of different conductivities. This head geometry filters differently
what the MEG and EEG see, so they become complementary.
This is the organization for the remainder of my talk. I now proceed to the first step.
13
Our starting point is this simple circuit: a battery, some wire, and two resistors. The
arrows in the wire show that an electrical current is flowing in the circuit.
14
Here we add several things:
A voltmeter, shown as measuring the voltage V across R1;
A drawing of an ordinary battery, such as a simple AA, meant to be interchangeable
with the battery symbol in the circuit;
A short, heavy arrow, called a current dipole, which is a very short thin battery; this
is also interchangeable, although in this “wire circuit” it doesn’t matter if the battery
is long or short. It will be used a bit later.
This measurement of V is essentially the EEG, but in a simple line wire circuit
instead of the extended human head. The sum of V’s across both resistors tells us
the voltage of the battery, which is directly related to the amount of + and - charge
at its terminals. In a real EEG of the human head, we learn about the voltages and
charges of the neural sources in the brain, located in a conductor which is much
more complicated than this wire circuit. But basically it is the same idea. In a while,
we will relate the voltages and charges to the internal current generator, which is a
quantity more in vogue than the charges.
But the EEG is a fluctuating voltage in time, isn’t it? And we are here measuring
the steady dc voltage. Well, it turns out, if there is no inductance and capacity in the
circuit (true for the human head), that a fluctuating voltage curve can be viewed as a
sequence of dc situations, say every millisecond . That is, the EEG can be frozen in
time whenever you like, and a dc analysis made. We are here taking a snapshot in
time.
15
Now we add one more thing, which is the magnetic field produced by the circuit,
called B with an arrow over it indicating that it is a vector quantity. Every piece of
current , as shown in the lower horizontal leg of the circuit, produces a magnetic
field around it (according to the right-hand rule), hence the entire space around the
circuit is pervaded by a magnetic field. We note that the previous quantity V is a
scalar quantity, that is, just a number without any direction. A measurement of one
component of the magnetic field vector is actually what we call the MEG, and tells
us about the machine inside the battery, called the current generator.
In review, a measurement of V tells us about the charges and voltage of the battery,
while a measurement of the B-vector tells us about the current generator inside the
battery; but this generator gives rise to the charges hence the voltage. Stated
otherwise, the current generator inside the battery produces the charges at the
battery poles, and it is these charges which make the battery do things to the outside
world. The voltmeter (EEG) also can get the same information about the internal
generator of the battery, but one has to fold in the value called the total resistance of
the circuit. The magnetic measurement gives that information with one less step;
one need not know the value of the resistors. Basically, this is how the MEG and
EEG are related. Now we will extend the situation to real life, the humam head,
instead of a simple circuit of wires.
16
INFINITE SALT-WATER CONDUCTOR
We do this by first imagining an infinite bath of saltwater, and we put a battery in
this bath, except now we want the battery to be very short and thin, because here the
size makes a difference. So we draw the short, heavy arrow called a current dipole.
This is the simplest electrical situation we can imagine in a saltwater bath. The
current dipole makes the current loops in the saltwater, and all the current pieces
(including the internal dipole generator) produce a magnetic field everywhere,
shown as the two heavy circles, as examples. But we need at least one more thing to
approach a living situation, which is a boundary containing the saltwater.
17
SALT-WATER BLOB
We have now put an arbitrary boundary around the salt water, and call the whole
thing “a blob”. The internal generator is again the dipole. But the current loops are
no longer symmetrically arranged around the dipole, because of the arbitrary
boundary shape. All the currents, including the internal dipole current generator,
again produce a magnetic field vector over all space, now iside and outside, shown
as the heavy loop or closed loop. Not shown here are the surface potentials,
produced by the internal generator, although they are important.
Using our previous ideas and terminology, potential measurements over the blob
surface (EEG data) will tell us about the voltage or charges on the ends of the
current dipole. Magnetic measurements around the blob (MEG data) will tell us
about the current generator within the dipole. However, we believe these days that
knowledge of the current generator is more useful than knowledge of the dipole’s
charges, so efforts are made to convert the EEG’s charge data of the dipole to
current-generator information. This is done by folding in the resistance of the salt
water, as seen by the dipole. Stated otherwise, the MEG directly gives source
current information, while the EEG can give the same information if the resistivity
of the blob is used.
An important point is this: If the blob shape is arbitrary (not special like a sphere)
and the resistivity of the salt-water is accurately known, the MEG and EEG will
usually give the same info about the current-generator information. One method
will usually be no better than the other.
18
Our plan:
1.
To show the connection between MEG and EEG when the electrical source is only a
battery in a wire circuit or, next, a current dipole in a salt-water blob.
2.
To show how actual neural sources of MEG/EEG can be approximated as dipoles;
how, in a salt-water blob, MEG and EEG see different aspects of these dipole
sources.
Now we go to step 2 of our plan, where we say something about actual neural
sources in the brain.
19
This picture shows pyramidal cells and their distribution in the six layers of the human brain
cortex. The source of both MEG and EEG are pyramidal cells in some of layers 3, 4, 5 and
6. In particular, the sources are the slow post-synaptic signals which are generated in these
cells.
20
Left: A typical pyramidal cell, consisting of dendrites, cell body, and axon. Right: A
magnified view of this type of cell. Three little squares show synapses on dendrites,
and on the cell body. The synapses on the dendrites are usually excitatory, while
those on the cell body are usually inhibitory. The fast action potentials travelling in
axons end up at the synapses, where they produce slow post-synaptic signals in the
target cells. These are summed in a complex way by the machinery in the cell body,
which “decides” if it will send out a resulting action potential down the axon of that
cell. It is the slow post-synaptic signals which are the sources of MEG/EEG.
21
This is yet another view of a pyramidal cell, rotated horizontally for ease of
illustation. The idea here is that, at any instant in time, all the complex slow
excitatory signals sum and look like a single dipole, when viewed from relatively
far away, say from several mm. Further, because all the pyramidal cells are
oriented the same way over a small area of cortex, these single dipoles sum and
look like just one dipole, when viewed from yet further away. There is a directionalspatial summing here, which make these small-area post-synaptic signals visible far
away, on the MEG/EEG. Of course if the source extends around a curve of the
cortex, say both sides of a sulcus, then there can now be a spatial cancellation.
How large are the neural signals? We know that EEG signals are of the order of
microvolts, but what about the MEG?
22
For magnetic field strength, we here use the physics unit of “gauss”, although in
brain-scanning labs the unit of “tesla” and its subdivisions (for example
picotesla) are usually used. One tesla equals ten thousand gauss. Two main
points are seen:
1. The SQUID, at the far left, is the detection device which is sensitive enough to
measure the weak fields from the human brain.
2. The human brain signal is about one-millionth of the urban noise. How to
measure this small fluctuating MEG in the presence of the relatively enormous
fluctuating background?
We first say something about the SQUID (Superconducting Quantum
Interference Device), which makes the MEG possible.
23
Our plan:
1.
To show the connection between MEG and EEG when the electrical
source is only a battery in a wire circuit or, next, a current dipole in
a salt-water blob.
2.
To show how actual neural sources of MEG/EEG can be
approximated as dipoles; how, in a salt-water blob, MEG and EEG
see different aspects of these dipole sources.
3.
To explain that the magnetic fields generated by the neural sources
are very weak, and we here take a side trip, to show how the MEG
equipment works to measure these weak fields.
To talk about the SQUID, we now go the second half of point 3, and take this side
trip, to talk about equipment.
24
PREAMP
LIQUID HELIUM
SQUID
PICK-UP COIL
HEAD
Simple, basic setup in using the SQUID detector, in this case measuring the
magnetic field over the head, using only a single SQUID channel. The items of
interest are the pickup coil (green) and the SQUID (red); these are superconducting
devices, hence must be located in a liquid-helium dewar. The magnetic vector from
the head (not shown here) passes through the superconducting pickup coil; the
resulting signal is fed upward into the extremely sensitive transducer called the
SQUID, and its signal is then fed upward again into the preamp (pink), which gives
an external signal to be recorded.
25
PICKUP COIL CONFIGURATIONS
The basic idea of three common types of pick-up coils used in MEG systems. The
curved lines here are meant to be the magnetic field vector B. The magnetometer is
the simplest pickup coil, and measures that component of the magnetic vector which
is normal to its plane, called BZ. In the two types of gradiometers shown here, one
measures the difference of BZ in the axial direction z, and the other in the tangential
direction y. In each of the gradiometers, an unwanted distant background signal is
largely cancelled out, although not completely. Because of this reduced sensitivity
to distant sources, the two gradiometers do not see deeper sources in the brain as
well as the magnetometer sees them, and are used for quite superficial cortical
sources.
26
Several methods are used to cope with the unwanted background fluctuations.
Gradiometers (previous slide) are one method. Another method, used by all MEG
systems, is to perform the measurements inside a magnetically-shielded room, such
as this old well-known MEG room at MIT, shown here for the second time. The
walls here have five layers: three of moly-permalloy, and two of pure aluminum.
Both materials perform different types of “passive” shielding. In addition, this MIT
room used “active” shielding, explained a bit further on,
27
We interrupt with another little joke (ha-ha)… This is supposed to be me and
Cuffin, a long time ago…Sorry about that….
28
This is yet another way of reducing background fluctuations, called “active
shielding”. This type of shielding is commonly used with a passive shielded room,
and is shown here acting on an external unwanted signal (big black arrow)
impinging on a passive room. It works this way: A magnetic detector (small blue
rod), usually a fluxgate magnetometer, senses the black arrow, and sends a (red)
current around the room which creates a counter magnetic field (red arrow). The
black arrow has thus been reduced.
Active systems come in various forms; the most common type is external sensor
and external cancellation, as shown here. Recently one MEG supplier has been
using internal sensor and internal current loops..
29
Finally, we arrive at a state-of-the-art commercial MEG system, called VectorView. It is
made in Finland, and sold by the Elekta Co. at a cost of about $3 million US, including the
large shielded room in which it housed, not readily seen here. Unlike the previous singlechannel SQUID slide, it is a helmet or whole-head system, with 306 SQUID detectors
surrounding the head. They are contained in a large liquid-helium dewar within the white
gantry. The inset shows the arrangement of the 306 pickup coils over the head; these are the
little squares at 102 locations, with three different coils on each square.
30
Simple approximation to the three pickup coils at each location, in the previous
slide. There are two planar gradiometers and a magnetometer, pulled apart only for
illustration. Actually, this slide goes back to 1975, when I had designed and used
this 3-channel system. Later in about 1988 when manufacturers began to make
whole-head system, the Finland company apparently liked the idea and redesigned
these for their own use, making them square instead of circular, as shown in the
next slide.
31
Blow-up of one of the squares in VectorView. It is the actual 3-coil arrangement at
one location, where the coils are called flux-transformers. It is a system of chips at
the center comprising three SQUIDs, all on the square about 2 cm on a side.
32
Considering the entire large VectorView gantry, it can be tipped to allow two
different positions of the subject being measured. Light blue is the liquid helium,
which slowly boils away and is replenished with about 80 fresh liters every week.
33
Example of somatosensory data
This shows the raw data coming out of the VectorView system, from an MEG
measurement. Most MEG measurements involve repetitive stimuli and signal
averaging. We here see the averaged traces (about 80 repetitions) from the 306
channels due to a somatosensory stimulus -- a small shock to the wrist every second
or so. The stimulus is given at 0 msec on a timescale of -100 to 300 msec. The
signals are seen to be largest over the somatosensory cortices. The next step is make
a sequence of maps over the head, at various instants in time. It will be these maps
which show the spatial distribution and allow an inverse solution, showing the
sources in the head..
34
MSH
This is the kind of map I was talking about. This is now a different measurement,
due to averaging the auditory evoked response at the time of 97.3 msec after
stimulation. The map is of the component of magnetic field normal to the head, in
units of femtotesla (fT). From maps such as these, an inverse solution can be
performed, to find the neural source current at that particular instant, so that the goal
of the MEG measurement has been attained. An example of an inverse solution is
seen in the next slide.
35
Two presentations of an inverse solution. At left is an MEG map over the head, at
165 msec after a picture of a face was presented to the subject. The pattern is
roughly dipolar (as we will later see). Hence it was generated by the (green) dipole
in the head, under the null line of the map; this is one way to illustrate a simple
dipole solution. On the right side, this same dipole is laid onto an MRI image of
that subject, hence part of the face-recognition area of the brain seems to be situated
at that location; this is a somewhst different simple presentation, for that instant in
time. Most presentations are more elaborate.
36
But for extended or complex sources:
Alas, there is no unique solution to
the inverse problem. We have to make
educated mathematical guesses….
This applies to both MEG and EEG,
separately and together.
The previous slide has shown us how to find the neural source with the MEG when
it is a dipole. But most sources are not dipolar, and are extended or complex. In that
case the MEG’s job becomes much tougher. An entire mathematical technology
has developed for MEG/EEG to make educated and elaborate estimations in
“solving the inverse problem”. But this technology is extensive, and we don’t
review these efforts here… They give both localized and extended source pictures,
where examples areshown in the next slide.
But lets note that one the experts in such inverse solution is my host here, Dr. John
Mosher….Another expert is the source of some of these slides, Prof. Matti
Hamalainen, at Massachusetts General Hospital.
37
These are examples of extended sources, on an inflated view of the brain, due to
performing an inverse solution on a particular MEG map (not shown here). This
map was made when the subjects read words and made a judgment (pressing a key
if the word referred to an object or animal greater than one foot in length). MEG
maps were produced by subtracting the signals evoked by novel words from those
evoked by words which had been read previously in the same task. At left, the
sources of the map viewed from the subject’s left, at 540 msec post stimulus-onset.
At right is a variation of the inverse solution, from the same contour map, but
sources are now constrained to be on the cortical surface. Hence the sources found
depend on assumptions used in the inverse solution..
38
So, using the MEG is very expensive !
Why do it ?
We are now at the main thrust of this talk… In other words, what is different about
the MEG, compared with EEG, that makes the MEG expense worthwhile?
39
Our plan:
1.
To show the connection between MEG and EEG when the electrical
source is only a battery in a wire circuit or, next, a current dipole in
a salt-water blob.
2.
To show how actual neural sources of MEG/EEG can be
approximated as dipoles; how, in a salt-water blob, MEG and EEG
see different aspects of these dipole sources.
3.
To explain that the magnetic fields generated by the neural sources
are very weak, and we here take a side trip, to show how the MEG
equipment works to measure these weak fields.
4.
Returning to our main plan, we show what happens to the MEG and
EEG when the blob is replaced by the spherical head, with its layers
of different conductivities. This head geometry filters differently
what the MEG and EEG see, so they become complementary.
To answer this question, we now move to step 4 of our plan.
40
Specifically, when we replace the salt-water
blob by a spherical model of the human head,
simple physics shows that:
Two new important differences appear
between EEG and MEG
I had earlier said or implied, when we had an arbitrary (non-symmetrical) boundary to
the blob and homogeneous salt-water inside, that there were no important differences
between MEG and EEG. But now there appear two important differences. As we shall
see, it is these two differences that make the MEG worthwhile.
41
AIR
AIR
SALT WATER
We have now put a perfect spherical boundary around the salt water, so that it is no
longer an arbitrary blob. But the salt water inside is still uniform, continuous and
homogeneous (we’ll soon change that). And, for a source, lets consider a current
dipole with either of two orientations: radial, or tangential to the surface. Now: it
can be shown, most importantly, that the radial dipole on the left produces no
external magnetic field whatever. Nothing outside! But the tangential dipole does
produce an external magnetic field, shown as the large blue circular vector. In
contrast, both orientations do give a surface potential (not shown here). Thus, the
EEG sees both a tangential and radial dipole, while the MEG sees only a tangential
dipole. That is our first important new difference between MEG and EEG.
But this would also be true if we had spherical layers of different conductivities,
such as skull and scalp, as long as they are spherical. Stated otherwise, making
spherical, ideal boundaries has filtered out the radial sources from the MEG. In the
actual human head, this first MEG-EEG difference has two major consequences.
But before talking about these two consequences, we will briefly argue why there is
no external field of the radial dipole.
42
We use a cute little physics argument which I like, instead of a complicated
mathematical argument. Consider the toroid, as shown in this slide. Actually a true
toroid has many, many turns, unlike this rough drawing. In mathematical physics it
can be shown that, when current flows in the wire, there is zero external magnetic
field around the toroid (not true inside the toroid). Now consider the doughnut
shapes of the current loops around the radial dipole, in the previous slide. It can be
argued that this doughnut shape is the sum of an infinite number of toroids of
different sizes and strengths, centered on the dipole. Because each toroid gives zero
external magnetic field, the net result is that outside all the toroids, in the air, there
is no magnetic field. None whatever.
For those who like physics puzzles, you can also prove this point by using
symmetry and the right-hand rule, but it’s a bit tricky.
Now back to our program…
43
TANGENTIAL DIPOLE:
SEEN ON MEG
SEEN ON EEG
RADIAL DIPOLE:
NOT SEEN ON MEG
SEEN ON EEG
The two consequences of the MEG not seeing the radial dipole: The first is seen in
this slide. We had mentioned that the pyramidal cells are oriented vertically to the
cortex. This means that they are radial to the spherical skull in the gyri, and
tangential to the skull in the sulci. Therefore the MEG does not see sources on the
gyri which are truly radial, but sees them clearly in the sulci. The EEG sees both,
but sees the radial somewhat better because it is closer to the surface.
The second consequence is not seen on the slide, but I will simply explain it in
words. This involves deep sources. A dipole which is at the center of a spherical
system is always radial to the surface, therefore gives zero magnetic external field.
Thus,in a real head, the closer a dipole is to the center, the more suppressed is the
external field. In contrast, a dipole at the center does give measurable potentials on
the surface, and the deep EEG is not highly suppressed.
So two groups of dipoles are suppressed on the MEG compared with EEG: those
located on the sulci, and those located deeply.
44
Thus, on the whole, the MEG sees
less than the EEG sees.
As a result of this first difference between MEG and EEG in a conducting sphere
with concentric shells, that the MEG sees no radial dipole, we can make the above
statement. Again, this is because the MEG suppresses both the sources in the gyri,
and very deep sources,. Therefore, averaged over the entire head and many
situations, the MEG sees less than the EEG sees. But in particular situations (say
sulci sources) the MEG may sometimes see more, when signal/noise is taken into
account.
45
Now we return to the second important differences between MEG and EEG, when
there are spherical concentric boundaries. We do this by calculating MEG and EEG
maps over the head, for a tangential dipole which, as we know, is seen by both
MEG and EEG. The model chosen for the calculation is a standard four-layer
spherical model; the dotted section is the skull.. The tangential dipole is placed 2.7
cm below the skull surface, and MEG/EEG maps are computed. The two resulting
maps are shown on the upper spherical caps These are seen to differ in several
ways. First. they are at 90° to each other, which is not an important difference from
our point of view. The second difference is that the EEG map is about 30% larger;
this is most important, and is due to the smearing out of the EEG by the highresistivity skull (the MEG is not smeared.). It means, other things being equal, that
MEG localizes about 30% better than does the EEG, in those directions. However
others things are not equal, and the job of localizing the EEG is tougher because the
data of the conductivities of the different layers are necessary; (not so for MEG).
The conclusion: the MEG localizes a dipole better than the EEG, by about 30-50%.
The EEG localization accuracy has been well-measured to be an average of about
+/-9 mm; we can therefore expect an MEG localization accuracy to be +/- 4 or 5
mm. This is all for tangential dipoles, with specific noise conditions, and specific
modelling of the head. With more perfect modelling, we would expect better
localization accuracy for both MEG and EEG, but the MEG should always be a bit
better, because of the skull smearing.
46
So simple physics says:
• The MEG usually sees less than does the
EEG
• The MEG localizes a dipole somewhat
better than does the EEG
This above statement is the basic thrust of this talk.
But we should note that in several ways the MEG is complementary to the EEG:
For example, sulci vs. gyri, and superficial versus all depths.
The fact that the MEG sees less is important and most useful, as shown in the
following two examples.
47
Here we go back to the late 1980’s, and we talk about the somatosensory evoked
potential, due a small repetitive shock to the wrist. In particular, the EEG potential
maps at N20 and P30. These produced dipolar-looking EEG maps on the scalp
surfaces, but there was a controversy at that time, whether the sources were the two
radial dipoles on the left, or the single tangential dipole on the right. These would
give about the same EEG maps, with the + and – peaks as shown. But the MEG
settled the controversy, because it also showed a strong dipolar pattern, impossible
with radial dipoles. Thus the MEG clearly selected the source on the right. This nice
finding by the MEG was due to the MEG not seeing the radial dipole.
48
Auditory N100 in EEG and MEG
EEG
EEG
rh
MEG
lh
MEG
rh
lh
MSH
Here is a second example, going back perhaps to the time of mid-1990’s. We talk
about the auditory evoked potential N100 on the EEG, well-known as the vertex
potential. The EEG maps are shown at the upper left and right, at about the latency
97 msec. This is what EEGers saw, long before MEG maps were measured, just a
negative blanketing of the top of the scalp. Thus the source appeared to be a single
radial dipole centered left and right. However, when MEG maps were measured in
the mid-1990’s, at bottom left and right, it was apparent that there were two
tangential sources, symmetrically left and right. Thus there were actually two tilted
sources. This was not only made possible by the MEG not seeing the radial
component of each side, also that the smaller extent of the dipole maps minimized
the spillover left and right.
Sometimes, less is more!
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Thank you for listening (I hope).
Now, polite applause…..
That’s all, folks…. Please don’t ask me horrible questions….
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