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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! 49 Thank you for listening (I hope). Now, polite applause….. That’s all, folks…. Please don’t ask me horrible questions…. 50