Bruce Luber: TMS Parameters and Protocols
Bruce Luber: Hello, my name is Bruce Luber. I'm a staff scientist at the NIMH, National Institute of Mental Health, and I'd like to talk about some of the basics behind TMS, transcranial magnetic stimulation, by talking about the basic parameters – the physical parameters – that we use to change TMS as we apply it to the brain, and then give you some idea of some of the protocols we use to test its effects.
So, what is TMS? Basically, we're looking at — by B field, I mean magnetic field — we're looking at magnetic field, the changing magnetic field, which produces a changing electric field inside the brain, and that leads to depolarization of neurons in the brain locally, right near the coil that we use to produce it. So, there's, for instance, an example of a coil. The magnetic field surrounds the coil and sort of a toroid shape in this case. As you see, this yellow line moving around, that's the electric current moving around the coil. Inside of it is a very large current. It creates a magnetic field that's on the order of an MRI machine, for instance. It can go up to two Tesla. And that — and the thing is, that that electric field only lasts for less than a millisecond. One pulse is less than a millisecond long. It's a very fast rising and falling, changing current that creates a magnetic field, also very fast rising and falling, which induces electric current in the head near the coil, and that creates changes — this is a little section of an axon — changes in a neural membrane such that you get neuronal depolarization. And that can lead to subthreshold changes even before an action potential gets created. So, if the field's not big enough to actually create an action potential, it still can create effects in the membranes of the cells and the synapses of the cells. That can create longer-term effects in terms of intercortical inhibition or intercortical facilitation or excitation. But it also does create — when it's a strong enough field — it will also create action potentials that set the neurons off. And that, of course, leads to neurotransmitter release in the synapse of the cell, and that further leads to synaptic action there and then sets off the neurons that it's connected to and so on in the brain leading to changes in behavior.
So, I'm very excited. I'm an experimental psychologist. I'm very excited about TMS myself. I've been doing it for 24 years now because you can non-invasively affect behavior, change the brain, and test what's going on where locally in the brain for various functions like memory or sensation. And — but it's also very useful in terms of therapeutic applications. So, TMS is already FDA approved for depression and OCD. And it's already being looked at in terms of long-term changes it might be able to make in schizophrenia and anxiety. It's been used for a long time looking at pain and looking at recovery from stroke as well as movement disorders. And it's also looked at and studied for, in terms of tinnitus and epilepsy. And it's basically being used in any kind of thing that neurologically or in terms of psychiatry can- is substantiated in the brain and can we do things about it. There are many targets of TMS in the brain, the dorsal lateral prefrontal cortex, the temporal parietal areas in the brain, motor areas, and motor systems in the brain, temporal memory systems, and sensory systems as well. So, this is a tool that can be used in a variety of ways, and hopefully, in many therapeutic ways. For instance, TMS, like I said, has already been approved by the FDA for use in depression. And this is an example of a — one of the large studies that lead to that approval. And you can see the blue columns — the blue columns that you see here, show that the rate of — a response rate in terms of depression increasing as you go from week two to week four to week six, okay. And we actually give TMS — the way that they give TMS in the world right now is to give it for six weeks. You can see that six weeks — after six weeks- there was a notable increase of response to the TMS and a decrease in depression relative to placebo. So, that's why we're using TMS.
But let me just start talking about the various parameters that are involved in TMS. There are a great number of them. So, there's a single pulse that's given. And so, each pulse has certain characteristics to it. And you can have a certain shape to it. The shape that you actually use determines how the neuron is locally — the membrane- is locally affected by it. But you can also do more than one TMS pulse. You can do trains of TMS pulses, and that provides a number of other parameters. So, you certainly can talk about the intensity of the pulse, all right, how strong we make the pulse. But also then, in a train, the frequency of the train, how many pulses per second you apply, how long the train is, how much time between trains you allow, how many trains you do, and then how many sessions of all these trains that you do. All these things can be explored. And it even gets more complicated because you don't have to have one frequency. You can have one frequency inside of another frequency.
Another FDA approved form of TMS now is called theta burst. And in theta burst, you actually do small bursts of pulses that are high frequency, say 50 times a second or 50 hertz. And then do those bursts every 200 milliseconds. And that means you're doing them five times a second or at five hertz. All right, so you're doing five hertz and 50 hertz at once. It's called theta burst because theta is like — the theta rhythm in the brain is about five hertz. Fifty hertz is actually the intrinsic gamma rhythm in the brain. And so, you're using some of the rhythms in the brain and nesting frequencies inside of other frequencies in a biological way that's actually interesting and probably very potent in the brain. This is called patterned stimulation. And it's just another way to combine intertrain intervals and frequencies in the brain. And these are just the temporal parameters. There's also spatial parameters. So, the shape of the coil that you're using. The target, the actual spatial target — where in the brain you stimulate are also very, very important to what happens with the TMS.
So, I'd like to go over some of these parameters, and I'll start with the spatial parameters themselves and basically looking at it in terms of targeting our coils. So, and I — there are a number of different ways to target your coil outside the head to aim at different places in your brain. Now, you can do it — the FDA approved a method that originally — I'm sorry, the originally approved method was to use a probabilistic location. In other words, to find the place in the brain and over the motor cortex that produced a stimulation of a muscle that you could see, for instance, a hand muscle that you could see and then just move five centimeters in front of that. And that placed you approximately over the dorsal lateral prefrontal cortex, which is a target for alleviating depression.
Now, another way of doing scalp positioning of the coil is to use the 10-20 — the International 10-20 EEG system that sets up exactly where, in a relative way depending on the size of the person's head, you put electrodes on the brain. And there are electrodes that are right over — that we know from imaging- are right over various places like dorsal lateral prefrontal cortex in the brain. So, you can use that system as well to position your coil. These aren't great ways to do it because you're essentially blind to what's the actual location of the brain you're looking for. So, you can use things like MRI, say a structural MRI, of that person, and then be able to target based on the person's MRI. Since everybody's a little bit different, their brains are formed a little bit different, where everything winds up in the brain is a little bit different in each person. This allows you to target much more precisely the site that you're looking for to stimulate. You can do very nice targeting on the scalp in one sense. You can look for a spot, say over the motor cortex, that will stimulate a certain muscle, okay. One muscle rather than another muscle, just by moving the coil around and stimulating and watching the muscles move.
So, that's one of the very few times you can do this on the scalp in any kind of precise way. You can also do it in terms of phosphenes in terms of the visual system. You can create phosphenes in the brain by stimulating the back of the head over occipital cortex where the visual cortex is. And you can move it around and find the best spot to stimulate there. And you can do that actually in different places in the visual field. So, you can do it relatively precisely there too. The rest of the cortex is basically silent to that kind of procedure. So, we need imaging to really help guide us if we really want to precisely aim our magnetic field.
And so, other ways to do it are by looking at a structural MRI and targeting that way. And further, you can actually use functional MRI. You can have people, for instance, do a task — say a memory task — look what lights up in the brain, and target that spot that lit up in the brain as well. That's a little bit more precise in terms of being able to stimulate the part of the brain that is functional and associated with the function that you're trying to change in the brain. You can take that one step further because you can do that on a group basis, which is kind of how you'll see most MRI research. But you can do this on an individual basis, which is really where we'd like to go in terms of helping people and doing — finding therapies. Because each person's different in exactly where a certain function might be instantiated. And so, you can actually do imaging and do fMRI on an individual basis and look at where in the brain that person is using the brain for that given function and aim at that. And we think that's probably the most precise way to do this.
So, just in terms of EEG sites, you can see, for instance, that these are the kind of the general regions: this is based on a number of people that were brought into a CAT scan with the electrodes on they saw what part of the brain they are actually- a given electrode site actually winds up being over. And there's a kind of a large multi-centimeter area that the electrode could be over in any one person, what cortex it's over. The important one for depression, for instance, is — would be the site over the dorsal lateral prefrontal cortex. So, if you use the site F3 in the EEG 10-20 system, you may wind up somewhere — and I just lost it — there it is — somewhere over here. So, it's not quite precise, you know. It's not a very specific region. But, at least it gets you over dorsal lateral prefrontal cortex, and that's kind of the best you can do with scalp-derived targeting at the moment.
You can use what's called neuronavigation. You can use infrared cameras that can very, very precisely pinpoint exactly where the coil is relative to an MRI online with a person. So, you can have a coil be over a person's head and know exactly where you are. And even if a person is moving a little bit, you can see exactly where the coil is relative to the target in the brain. So, this is something that we all use now: this kind of frameless stereotaxy.
Just in terms of a comparison of these different methods: there was a group, Sack et al., in 2009, that ran a study to compare these. And I won't go over in detail the tasks they used. But they found a task, a Stroop-based task, and basically, they showed you two numbers. And the numbers could be the same size. That's the neutral condition. But they could also show you two where one was bigger than the other. Now, the way to answer this when these numbers come up, you say which one is bigger. And they mean bigger in terms of — like four is bigger than two. But in this case, it's actually also in size; it's bigger as well. And so that actually helps you answer faster. Whereas if the two is bigger, you kind of get slower because, in one sense, two is bigger, in another sense, four is bigger. And that slows you down a little bit. And you can just see that over here. If this is the reaction time, you can see that when there is no TMS that you answered about this many — about 480 milliseconds, you're a little faster in the congruent case when the four, for instance, is bigger. And you're much slower in the case where they're incongruent.
So, you have this normal effect without TMS in general. The TMS itself, when you stimulate the right parietal area, you actually get faster. It makes you faster in the incongruent stage — condition, rather. And so, this is a TMS effect that enhances your performance slightly. And we can look at how many people it takes to see this kind of effect — how many people it takes to statistically see how many it takes using the various targeting methods. And so, this was done. And so, they used the scalp 10-20 system. So, they went over right parietal cortex based on the scalp. And when they did, that's this group of five people here — five brains — you can see that they stimulated in different places in the brain when they actually looked at their MRIs. So, there was a variation in where they ended up over the brain if you just used a scalp-based system. That variation is just random in the sense of what's going on in the brain functionally. They used MRI-guided — just used their structural MRIs. Then you're going for a specific spot in the brain that they found in a group level activation. And the other way they tried to do this was to do the individual guided MRI.
So, they found in each of these brains where that task most activated in the parietal area, and they stimulated that, okay. And what they found was — they did an analysis, and this is basically the alpha, the statistical level. And what you want to do is see when the effect is significant. And this is like how many people it takes to see that significance. You can see that it takes about 47 people if you use a scalp-based method, all right, to see that TMS effect, okay. But it only takes five people if you use the individual targeting method. And this is just an example of being able — that you really want to use this neuronavigation on an individual basis that allows you to most effectively, in terms of spatial parameters, stimulate the brain. And you can increase that precision, by the way. Humans — human operators are trying to hold a coil over the head and keep it right over a head. That's not so easy in itself, even using neuronavigation. And then the subjects receiving the TMS also move.
So, if you're sitting there for a while, even unconsciously, even if you're trying hard not to move, you still move. There are now robot coil holders that stay with the person, stay on target, and position it with millimeter accuracy that allows incredible precision in terms of putting the magnetic field in the brain where you want to put it.
Now, having said that, if you stimulate any part of the brain and you create action potentials in the brain using the TMS, you're actually stimulating whatever that area of the brain stimulated is connected to. You stimulate a network. So even though you have great precision in stimulation, you have less precision in the sense of you're stimulating a number of other areas too. So, you really kind of want to know what you're stimulating in the brain when you stimulate any one part. And there are ways to do that. You can actually use fMRI, for instance. You can use — you can put people in a scanner and stimulate them with TMS in the scanner and using the fMRI imaging, see where in the brain they actually wound up getting stimulated or activating.
This is something that can be used, by the way, to defeat one of the problems of TMS which is TMS itself — here's a coil, okay, at the scalp of the head, all right, and it creates an effective stimulation to a certain depth in the brain. The depth is only right about here into the brain, a few centimeters in that you're effectively stimulating neurons and creating action potentials. But say you wanted to reach a much deeper place, now, for instance, area — Brodmann Area 25, subgenual cortex, is a target now in depression research. But you can't get there directly with TMS, all right? The question is, can I get there indirectly, transynaptically? And the answer we found was yes.
So, if I stimulate a certain place, can I reach this area and aim for it? And you can by doing imaging and doing a thing called diffusion tensor imaging or DTI, and if you see this area of interest, all right, and see what this DTI does is map out the white matter, the wiring of the brain, the white matter tracts, okay. And you can see where they come out to the surface where you can stimulate with TMS. And so, we did that just to show this works. We did that. We seeded it and saw where the white matter tracts came out on each individual person. And then, we put people in the scanner, and using fMRI, we stimulated people in the scanner to see if we could activate that area by activating where those white matter tracts came out. And when we did that — so here's like 10 people. Here are the sites on their heads, in the front of their heads where the white matter tracts came out. Okay, when we did that, we showed that as you increase the strength of the TMS, okay, the dosage of the TMS, you actually increased — oops, didn't mean to do that — you actually increase the activation in Brodmann Area 25. And you can see that here that we actually increased the activation, the number of voxels being stimulated as we increased the strength of the magnetic field, as opposed to other areas of the brain that did not see that kind of increase.
You can also, by the way, look at the — through imaging, in this case, EEG — look at what's happening in the brain over milliseconds, over time. So, if you can use high-density EEG — and I, say, stimulate the motor cortex — I can see its effect in the motor cortex in the first five milliseconds. When I look 15 milliseconds or so afterward, I can see that that activation has moved a little bit to premotor areas in front of that. And in fact, after 24 milliseconds, I can see it's moved — the activation's moved to homologous areas on the other side of the brain. So, I can actually watch the activation happen over time using EEG as well as over space using MRI. And you can see the same thing in the visual cortex in the back of the head. You can see that after 28 milliseconds, it's moved across the corpus callosum and into the other hemisphere, the activation caused by the TMS. So, we are very good at precisely locating where we want to put stimulation into the head. And so, we have a very good control over the spatial parameters.
Another aspect of that is to look at coil shape, okay. Different shapes of coils — one on here, too — for instance, a figure eight coil, which is most commonly used, a double cone coil right here, which is less commonly used but it can get a little bit deeper in the head; a round coil — those are used much less nowadays — metal core coils and the H-coil, which is essentially a very large round coil, produce different effects in the brain. Dr. Zhi Deng will be talking about that in later lectures in terms of the trade-offs that you see, in terms of how deep you can go, versus the focality.
Okay, so, the deeper you go with TMS, usually, the less focal the stimulation is unless you do this transsynaptic trick that I just mentioned before. Another very important aspect is in terms of, for instance, the orientation of the coil — so if you have a figure eight coil, it actually has kind of a horizontal axis to it because it has two loops, okay. And you have a handle here. All right, you can move the handle, all right- without moving the center of the coil- you can move the handle so that the whole shape of the coil moves around. When you do that, that actually changes where in the — what in the brain is getting stimulated and where it's getting stimulated. And that matters a whole lot.
So, the coil orientation is very, very important, too, as well. And that's something that people aren't paying as much attention to but need to. And we're showing that in two different ways here. We're showing that in terms of — don't try to read this table here. But this is 13 people being stimulated with the coil at various angles. And finding how much it takes to stimulate their motor cortex to produce a certain level, what's called a motor threshold, which will also be talked about later in terms of dosing. But, you can — as you change the orientation, you can see that there's an optimum spot, an optimum orientation to stimulate that coil. And it has most to do with exactly how the motor cortex in its gyrus is situated. And what you want to do is stimulate so that the current from the coil is — the current direction is going into the gyrus as opposed to going with the gyrus. And depending on how you turn the coil, that will change the current direction. So, current direction and its accompanying motor — the orientation of the coil matters quite a bit.
Another — I class this parameter in with spatial parameters because intensity determines how deep the stimulation goes and how wide its spread, okay, spatial — which is spatial parameters. And intensity is one of the first and easiest things there are to manipulate with TMS. It's just a knob on the machine, and you can turn it up to 100 percent of the machine's capacity or not. However, intensity — what we found early on, the higher the intensity, the more chance of something like a seizure happening. And so, we — there have been actual studies that have shown, given a frequency that you use, higher frequencies also lead to seizures. And this is the intensity based on the motor threshold I just mentioned before. So, increasing intensity means that you can see that if I stimulate at 10 hertz, you actually have to make the trains of TMS, how long you can stimulate before you risk seizure — those numbers go down with increasing intensity. So, we know a lot about what happens with high intensities. We know a lot less about what happens with low intensities. But, subthreshold intensities actually can also affect neurons as well. And we're just starting to learn about that, too. But that is something we're finding out more about now as well.
So, anyhow, I've gone over spatial targeting. Now, I'd like to talk a little bit more about the temporal parameters that are involved as well, which include waveform, frequency, intertrain interval, and cumulative effects. So, just in terms of the waveform of the TMS — so TMS itself — a single pulse- can look like this particular kind of pulse. The other standard pulse is a bipolar pulse, okay, where the current goes this way, that way, this way, that way again. And so, those are the standard kinds of pulses. Now, I can use these pulses over the motor cortex and create responses in a given muscle, ll right, and I can just get an average of those responses ahead of time. And then, I can stimulate with a different kind of waveform each time. So, for instance, I can use the standard biphasic waveform, which is a basically dampened cosine wave. But I don't have to do a cosine. I can actually use something that's more like a square wave. And it turns out, neurons like square waves better. They respond to them better. And I can also form a square wave, okay, monophasically instead of just biphasically.
So, I can try these different sorts of waves, all right, and then I can test again to see if I created an effect. So, if I stimulate for about 15 minutes with 1000 pulses — say, one a second, okay- I create an effect in the brain that can last on the order of 15 minutes or so before it goes away, it goes back to baseline. This is, by the way — and I'll get to this later — is one of the very standard pre/post protocols that one uses to see TMS effect. So, if I look at the various kinds of waveforms I use across, in this case, 13 people, what happens if I do one hertz stimulation — and I'll talk about this in a second — is that you tend to inhibit. You tend to reduce the response in the motor cortex. And so, I'm expecting a decreased response, all right. This is the people that got the standard waveform in this group that you actually didn't see an inhibition. I didn't see a decrease of response below the baseline. But with the various square wave waveforms, I did. All right, and those will all gradually go back to baseline over time. This is 10 minutes, 20 minutes, 30 minutes.
And so, it gradually goes back to baseline. But the point is that I — that the square wave was much more efficacious and created an effect whereas the standard form didn't. So, the waveform can actually create different effects of TMS. And that's actually something that we're exploring now as well.
One of the first kind of parameters that we found that had very different effects was the frequency. And sort of the rule of thumb that we found was that if you use high frequency, which means greater than three times a second — and we tend to go from — high frequency tends to go from- three times a second up to when you're doing trains, up to about 20 or 25 hertz, okay. That tends to make the brain more excitable. And you can see here with the PET image, that stimulating right here activated a very large amount of cortex when we actually measured the activation of this high-frequency stimulation. Low frequencies tend to down-regulate cortical excitability. They tend to inhibit the response. And so, the stimulation was done about — well, up here — done about here.
And you can see that this blue means that there is a deactivation. And there's — around where we stimulated and here it is — around where we stimulated, there's a deactivation. But that's kind of the rule of thumb that we use. Low frequencies around one hertz tend to inhibit activity. Whereas high frequencies tend to excite. That may have a lot to do with the actual frequencies of the oscillations that are going on in the brain, for instance, the alpha rhythm or the theta rhythm- that are going on in the brain that seem to have a lot to do with function. And this was tested out, for instance, even 20 years — almost 20 years ago — in a study where Klimesch et al. in this case stimulated at the person's own alpha frequency with the TMS over parietal and prefrontal areas while they did a task. All right, and what they found was that the accuracy in the task increased quite dramatically: 15 percent's pretty good. They enhanced the people's response by stimulating at their own alpha frequency. Whereas, when they stimulated at three hertz less than their alpha frequency or stimulated just at 20 hertz, they didn't see this enhancement. So, there may be something to do with how frequent — the reason that the frequencies affect the brain the way they do, interact with the brain the way they do, has to do with the brain's own oscillatory dynamics.
You can look at this in another way, for instance. Remember that — the protocol I showed you with the waveforms, okay, where we do a motor evoked potential, okay. That's the response of a muscle of a target muscle when I stimulate the motor cortex. If I look at the average MEP, motor evoked potential, and get kind of an average of that, and then I stimulate, all right, and then I look at the motor evoked potentials — I'm looking at pre-post changes in that response to stimulation of motor cortex. If I just do the normal one hertz, remember that should just create an inhibition. It did, okay, in this particular study. Here's the baseline. It went — the response after ten minutes of one hertz stimulation did indeed go down and return slowly to baseline. It's back up around 30 minutes or so. If I stimulated at high frequency first — so I do about 10 minutes of that first and then stimulate at one hertz and then measure it again, I created a much more — sorry — much more enhanced inhibition and much more long-lasting inhibition.
So, there's an interaction between different frequencies that I used. Just trying to show here that it's very complex how all the different frequencies actually work with each other. In this particular case, it may have worked because this high frequency would excite the cortex, all right. And then after a little bit, you're –, it's returning to baseline. So the brain is actively pushing back towards baseline, a mechanism that does that. So, it's doing that, and at the same time, I next add the one hertz, which also decreases activity. So, it's already going down, and this shoots it even further down.
So, we're playing with the brain's own homeostatic mechanisms to create even larger effects. Another example of these kinds of interactions is with that theta burst stimulation, TBS, I had mentioned before. Remember, TBS is little bursts of high-frequency stimulation given every 200 milliseconds so that I'm doing it at theta — so that's the theta part of the theta burst. And this patterned stimulation, if I do a continuous amount of that patterned stimulation, for about 90 seconds, I create an inhibition of these motor — if I stimulate motor cortex — I create an inhibition of these motor evoked potentials afterward. If I put a little time in between, if I do a train of theta bursts for two seconds and then I wait eight seconds then do it again and keep doing that for about four minutes, I create an opposite effect, all right. Suddenly, I'm exciting the motor cortex, and it's producing exciting higher amplitude motor evoked potentials. So, I'm able to create opposite effects, inhibition, and excitation, all right, just by adding in a little time between the trains.
So, there's this interaction between the intertrain interval, the duration of the trains, all right, and the frequency of the trains that are very, very complex. And we're just starting to learn how that all works.
Okay. And the last parameter I want to talk about is the duration of the stimulation. So, you can create — and I'll be using this particular slide quite a bit now as I go into protocols — excuse me — is that you can create acute effects. Namely, you can create effects that happen right away in the brain that you can see. For instance, you can — if I stimulate the occipital cortex with a single TMS pulse, all right, and I do it at a certain time after I show say three letters. So, I'm showing you three letters and I'm asking you to repeat the three letters, what they are, and I show those three letters very briefly, like 10 milliseconds.
So, if I do — I lost my — there it is — if I do the TMS pulse, okay — and these are the tracings of three different people. But if I do the pulse at the same time as I show those three letters, nothing happens. People are getting about three letters right, okay, three out of the three letters right. If I do the TMS pulse at 80 or 100 milliseconds after I show the three letters, okay, in other words, the time it took to go from the eye, the retina, back into the brain and back into the occipital cortex is about in this range, okay — then I can actually interfere with people's ability to see it. In fact, they can't report the letters at all. This was originally called a virtual lesion effect. But, the idea is that for — there's a little time period, say for 50 milliseconds, when I can interfere with what's going on in the brain very, very precisely in time. So, I can create what's called an acute effect, a direct effect.
And for instance, I can do that with a muscle. I can make a muscle twitch. That happens right away. I can do this kind of visual masking. I can actually show that I can speed — I can make people better or worse, all right in an immediate kind of way. These effects only last on the order of milliseconds to seconds, okay. There are effects directly on the membranes, the electrophysiology of the brain, immediately. But if you keep stimulating, if you stimulate, for instance, for 10 minutes with say 10 hertz as I showed you before, you can create lasting effects that go on after I turn off the TMS.
And so, if I stimulate for around 10 minutes or 15 minutes or 20 minutes with say 10 hertz, I create enhanced activity say in the motor cortex that lasts on the order of 20 minutes to an hour. And then it goes away. That kind of effect, you're changing synapses, okay, in a way that the synapses are responding to the stimulation and they're kind of adapting to it. And they're actually changing their properties, all right, and adapting to it. And that kind of lasts for a little while until it all returns to baseline. You can make those changes last a lot longer by repeating over and over these 10, 20, 30 minute, you know, bouts of stimulation, all right. And that's what we saw right in the beginning when we were looking at this was weeks of stimulation stimulating every single day for 37 minutes, okay. And you did that from week — after two weeks, after four weeks, after six weeks. And you can see the effect growing and growing. And those effects can last months to years, okay. And what you're doing there is creating epigenetic changes in the brain, and you're creating — in the synapses rather — you're creating changes that last much, much, much longer because you're programming them in through the DNA in a way that lasts, you know, even a year, but basically, at least months.
So, there are a number of temporal scales that TMS can act on depending on what you do and how long you do it that you can look at. So, there are all kinds of levels of cumulative effects that you can create. And this is our great hope for therapy is that we can create these long-lasting effects.
Okay, so that's basically the, you know, the parameters of TMS that we can manipulate, through neuronavigation, through brain imaging, through robotic coil positioning, through realistic head modeling and e-field modeling we can deliver, very, very specific dose right into the brain right where we want it to go. And that allows us to affect very specific networks in the brain, very specific sites and networks that lets us learn, for instance, their functions, give us the ability to diagnose when you can disease populations, for instance, and gives us a really great handle over spatial parameters. Although again, we have a tremendous control over the first millisecond. And then, after that, it's how the brain responds to things. And that matters quite a bit because what the brain is doing at the time and place that you're stimulating, at the point of stimulation, really determines the effect of the TMS.
So, it's not just the parameters that we use externally but what the brain is doing too. There's a big interaction there that we're just trying to learn how to handle and control. And indeed, what happens next is something that we can look at in terms of the temporal parameters. And so I mentioned the TMS waveform and how it really can have affect literally on the membrane time constants, you know, using them and using a square wave, for instance, versus a cosine wave can matter quite a bit in terms of how the neuron responds. Also, the TMS frequencies, how they're related to the endogenous oscillations of the brain can matter in terms of what TMS effects we create. The train duration and the ITI, the intertrain interval rather, can have big effects. You saw that with, for instance, theta burst. Just changing the train — the intertrain interval can change whether things are excitatory or inhibitory. And if you do durations long enough of TMS, you're changing long term potentiation in the synapse. You're changing — you're effecting plasticity mechanisms. Repeating, creating cumulative effects by repeated TMS sessions hopefully create long-term network changes that we can use therapeutically.
Okay. So, given all those parameters, I'd just like to talk a little bit about the basic TMS protocols that we use. And they're kind of divided into two separate kinds of protocols. One is online protocols, looking at acute effects. The other is offline protocols, looking at lasting effects. So, online protocols are stimulating and seeing an effect, okay, right away. These effects are very short-lasting. And it's usually a matter of probing and exploring what the brain is doing at certain points. And it can also be used for biomarkers and diagnostics. And I gave you an example of that already.
For instance, the acute effect here is only a 40 or 50 millisecond effect in the brain- in the visual system using this Amassian study. This is actually, by the way, one of the very first studies ever done- in 1989- in TMS. So, to do these kinds of acute effects, to do these online protocols, stimulating at a certain time, for instance, relative to a task that you're giving people at the same time so you can functionally see changes, you can do single pulses as a probe. You can also do two pulses, all right, for instance, or multiple pulses that are occurring very, very close together in time on the order of a few milliseconds between them. Those produce various effects. And I'll talk about that in a second. Or you can just do short trains of TMS, say two, three, four seconds of a certain frequency of TMS. All these can create acute effects that you can use to study the brain.
The offline protocols, where you're trying to create lasting effects, cumulative effects the best — the prototype example we have of that, obviously is what we see in depression trials whereas we increase the number of sessions, we're increasing the effect — the overall effect of the TMS. And you can see these lasting effects by the kind of protocols you basically use, for instance, in a given session of TMS is pre/post testing. And I've given you a couple of examples of that already where we stimulate — we do, for instance, motor evoked potentials and then, a baseline. And then we do some intervention with TMS. And then, we look at what changes that did to the MEPs. You do a pre/post kind of test. And looking for these kinds of short-term plasticity effects. And you can also do that over multiple sessions and then look at what — look at pre/post, the whole series of sessions. Have you made any changes in people's behavior, in their disease, or for instance, in their imaging and the actual networks of the brain through imaging? Okay, and in fact, the idea in these protocols is well, how do I see a TMS effect?
All right, so I can see it, for instance, online by looking at behavior — so looking in this case in people's accuracy. Or I can do a pre/post effects over time. So, I can actually look at various milliseconds apart, right, and do stimulations at various times and see if there's a difference in effect. So, I can actually do a control in time of this time — oh, there's no effect. This time there is an effect, all right. And just be able to use my statistics that way, for instance. Or I could do these pre-post studies like you saw with looking at the waveform where we do look at MEPs ahead of time and look at their changes depending on what stimulation I give. So, I could look across time in that sense as well. I can also stimulate in different places. So, if I expect to have an effect on say a visual task, all right, by stimulating occipital area, I might stimulate the vertex of the head where there is no visual area, right. And there presumably should be no effect on the visual task that I'm doing. So, I can compare across different sites. That can have a problem because it's kind of hard always to find a place in the brain that isn't somehow involved with your various tasks and behaviors. But those are two different ways to control.
But you can also look across other parameters as well. For instance, you can look across frequencies. And so, you saw that in that PET study where you use high frequency versus low frequency and saw dramatic differences in the brain imaging. Other parameters like intensity, you can do that as well. So, for instance, we did that with the-doing that transsynaptic depth study where we were able to stimulate Brodmann's Area 25 deep in the brain and were able to show a dosage effect that we had control over the activation of that deep area. You can also use — and actually, the first thing people might think of is well I can do TMS and then not do TMS, all right, no TMS and compare what happens when you don't do TMS and when you do. The problem there is TMS itself has its own effects, peripheral effects, all right. When you stimulate on the head, it feels like something, all right. And the more you increase the intensity, the more it feels like something, all right. So, contract muscles on the head. You contract, and you affect peripheral nerves and things like that. It's usually just kind of startling. It feels like a little bit of a thumping. It can be painful sometimes in certain places. But it creates — and for instance, if you hit the right superficial nerve, you can create an eye twitch. You can twitch your jaw. But there are peripheral effects that you create, all right. So, if you do no TMS versus TMS, you're creating effects in one case that can, you know, interfere with tasks that have nothing to do with affecting the brain. So, that's something of a problem. And by the way, the other peripheral effect is it makes a sound, a clicking sound, all right.
So, every time you do TMS, there's always a clicking sound. The clicking sound gets louder with increases of intensity. So, all these things can create other effects, and they're the reason you really don't do no TMS to compare with. What you try to do in this kind of situation is sham TMS. And it has its own problems because how do you create that clicking sound and the other kind of peripheral effects, the thumping, for instance, and all those without actually giving people TMS? And people have been working on that. There are ways you can at least make the sound. You can have a coil on the head and not apply the magnetic field into the brain but still make the sound. And people actually put little electric currents — they put a few electrodes right under the coil and give people a little bit of a surface jolt of electricity right when the pulse happens to kind of mimic the TMS thumping effect, okay. It's not quite the same, but it's similar enough. But there are problems with it, but the idea is to create superficial stimulation that mimics TMS without affecting the brain and then comparing that with the TMS.
So, if you do various combinations of these controls, you're going to actually — you can find ways to compare TMS with no TMS either a — or TMS at a different time or a different place. And in these ways, you're able to pull out TMS effects.
And what are the TMS effects? How do I measure them? So, there are changes in behavior. I mentioned that first- there are changes in performance, for instance, in your accuracy in this case with the visual stimulation or reaction time. That's a good one to use too. There could be changes in clinical ratings. So, the response, as you see here, is in terms of people's ratings in MADRS Scores or in Hamilton Scores in depression. So, you can actually look at those kinds of changes. You can actually look at physical changes that are occurring right away by looking, for instance, at motor evoked potentials. And you can actually do that with EEG and look at TMS evoked potentials right on the head, you know, right underneath, for instance — where you're stimulating you can actually see the response to that stimulation. You can also look at brain imaging changes. I've talked a lot about MRI changes as well as PET changes in this study down here, for instance. You can use high-density EEG. I showed you an example of that as well. So, there's a number of ways you can measure the TMS effects both physiologically and behaviorally.
So, other ways — and I mentioned before other ways to actually stimulate, to do kinds of different protocols that create different kinds of TMS effects, okay. So, for instance, here's an example of an MEP, right? So, I do a TMS stimulation, right, and this is the MEP, the motor evoked potential I might measure in hand muscle that I was targeting. If I do a second pulse, all right, or rather a conditioning pulse, a pulse right before the — that pulse that I did– and by right before, I mean, for instance, like two milliseconds, okay. If I do it two milliseconds before- and this conditioning pulse isn't even strong enough to contract its muscle or create its own MEP, for instance, I can [coughs] — excuse me — I can create an effect. So, this is the response I see, you know, with just a single pulse. And we've traced that again here. If I do a little conditioning pulse ahead of time, okay, I've actually inhibited or quite dramatically decreased the response. If I extend that time, say to 10 milliseconds, I can actually create a facilitation, an increase, all right, against the baseline. All right, so here's a baseline response, and here's the increase that I see, okay.
So, the time, the ISI or the interstimulus interval, that I choose to use can play with what's going on there. What we think we're playing with is the response of different neural systems in a very local location, some that respond to GABA, some that respond to glutamate, so it's inhibitory and excitatory neurons. And you're playing with their dynamics with each other. Okay. And so, you're able to actually look at separate GABA systems and glutamate systems inside one particular area, in this case, motor cortex, by using what we're calling paired-pulse stimulation. And you can increase, for instance, the paired pulses-this is called long interval cortical inhibition-if I stimulate with the conditioning pulse say right here, all right, and with one that's strong enough to actually create a motor evoked potential, and then I wait 50 to 200 milliseconds, I can create — this would be with the test stimulus-if I didn't have the conditioning stimulus I get a nice healthy MEP. But if I have this conditioning pulse, I have a much more inhibited response. That's long-term cortical inhibition. And that's actually — that separates GABA A and GABA B systems, for instance, in the brain.
So, you're actually able to look at different neurotransmitter systems in the same area just by playing with paired pulses and the time between them. And in fact, you can see that, for instance, if I look over a period of one to 12 milliseconds, that I get — relative to the baseline, the test pulse, all right, I get inhibited responses if I have short-term — short interstimulus intervals, ISIs. If I have longer ISIs, I start creating a faciliatory. If I extended this out further, I create inhibitions again and so on. But you're seeing nice and very reliable responses that have to do with neurotransmitter systems locally. And so that's a very useful stimulation. And people do also — you can — you don't have to just do pairs. You can actually do say four, all right, or quadripulse, all right. There's even octopulse stimulation. They're essentially very high frequency, right? If there's only two milliseconds, that 500 hertz, right? But they're very closely spaced stimulations or short trains, all right, and those create their own kinds of effects. If I do short trains of stimulation, this is an example, for instance: if I look at a memory task, all right — so I can show you a letter or — and then I can wait a little bit of time and then show you another letter, and you tell me as fast as you can is that letter one of the letters I just showed you, that's a very typical — it's called a Sternberg task and a working memory task that people use.
Here's another example. I show you six letters, wait a little bit of time, and show you a letter — in this case you'd answer, "No," because it wasn't one of the stimulations. I can do a train, all right, of rTMS, say in that time between the two sets of letters, all right. When I do at the right place, in this case, the parietal area, and I use the right frequency, in this case, five hertz, okay, so the solid areas are active. The lighter areas are sham comparisons, okay. So, we're doing active and sham comparisons here, and we're using different frequencies. So, one hertz and 20 hertz doesn't show very much. This is with a set size of six letters, okay. You can see that I've speeded up in the reaction time two of these letters, all right, if I stimulate the parietal area, okay, at five hertz during this delay period. And that also happens at a set size of one as well. If I stimulate later, like during the probe period, I don't see a difference between active and sham. But I was able to replicate in a different group the same facilitation of reaction time, enhancement of performance, okay, by stimulating during the retention period. So, you can play with trains of TMS, and you can create enhancement for disruptive effects with those trains if they're placed at the right time relative to how the task is going on and in the right place in the brain and with the right frequency.
So, this is an example of an online protocol where you're directly affecting, acutely affecting behavior. And by the way, we're able to show that if we sleep deprive people for two days — and here's a network that activated with the working memory task — and we stimulated inside that network these people that were sleep deprived for two days, and we also stimulated outside the network the same people and stimulated that same parietal area (That's actually inside the network, but you can't see that from here.) -and what we saw was people's reaction time when it was just sham TMS — so again, we're using a sham control, okay, and we're using a space control. We're using different locations, okay. But we see that if I stimulate right outside, nothing happens, right? But if I stimulate inside the network here and here, I actually decrease the reaction time, okay. So, I make people a little bit better and help them recover a little bit from their sleep deprivation. What happens when you're sleep deprived is you get slower, right? And so, we're able to help them recover a bit.
Another kind of offline TMS that you can see — it's a very interesting one — is called paired associate stimulation. In this case, people — again, we're using these motor evoked responses to see this. So, here's a typical one you see by stimulating motor cortex, an averaged one you see. But what I'll do is I'll do 90 pairs of stimulation where I stimulate the median nerve in the hand that's associated with this muscle, okay, and 25 milliseconds later — in other words, the amount of time it takes for this stimulation to get to that part of the brain, okay — and stimulate at that time just when it arrives, okay. I stimulate with the TMS, all right. So, I'm doing paired-associate stimulation. I'm creating a Hebbian-like change of fire together, wire together situation. And I do that 90 times; then I test this area again. I get an enhanced response. Okay, so you can actually change the wiring a little bit. And it's temporary unless again, you do cumulative effects. But you can show these plasticity effects directly. It's a nice way to measure plasticity in the brain, in fact.
Another kind of offline protocol is again that theta burst that I've talked about a few times. So, if I look — and to do a pre/post measurement, okay — that's what you do offline now — is I'm doing a pre/post measurement just like the previous — just like the paired-associate, I do a pre/post with the motor evoked potential Here, I'm doing that again, I'm looking at motor evoked potentials and seeing the changes after I do, say continuous theta bursts or if I do intermediate theta bursts, namely theta bursts where I put that eight seconds in between each two second train. And look at the differences in the post response in the MEPs, all right, with the inhibitory and the excitatory — I mean, sorry — excitatory and inhibitory TBS. So again, looking at theta bursts, you do this pre/post, with the intervention in the middle, measurement. And that's your typical offline kind of protocol.
Another example of that is using, again, a pre-post measure, okay. This time instead of using MEPs or motor evoked potentials, I'm using TEPs, TMS evoked potentials. And I'm looking at evoked potentials of the EEG directly in the brain, all right, when I do this intervention between the pre-post measurements, okay, of stimulation. In this case, it's five hertz stimulation, okay, over I believe it's like 15 minutes, okay. And when I do this 15 minutes of five hertz stimulation, I can see — and this is the pre-TEP responses, okay, averaged in a group and across the brain — you can see that I've enhanced the amplitudes, all right, of the response, okay, after all the stimulation. All right, so again, it's this pre/post measurement that I do when I'm doing this kind of offline sort of protocol.
Here's another example of kind of putting a lot of that together. So, again with that sleep deprivation protocol that I showed you before, this time during the two days of sleep deprivation here, okay, we had four sessions. So, we're trying to create a cumulative effect, right? We're doing four sessions of TMS for an hour each while people are doing the memory task, the working memory task, all right. So, they've done four sessions of it. And then at the end of the sleep deprivation period, we measure — we have them do the task again, all right, with no TMS. So, it's like almost a day later now and no TMS.
So, we're looking for a lasting effect. And so people that got sham TMS — this is reaction time and against baseline, against the pre baseline — the pre-sleep deprivation baseline. You can see that the people that got just the sham TMS, no TMS, slowed down like you usually do with sleep deprivation. But people that got the active TMS look like people that weren't sleep deprived, all right. They get a little bit faster, all right. And they're a little bit faster because they've had a little more practice. But practice didn't help these people with the sham. But it did help people with the active TMS. So, they're not showing the effects of sleep deprivation in this task. In fact, people that are sleep deprived also lapse. They don't answer. They have little microsleeps. And the number of lapses that actually occurred with people with sham was pretty typical. But there were much, much fewer — almost none in most people — in people that got active TMS.
And in fact, the coil was right here over the brain, and the only difference between people that got active and sham was in that area. In that area, that was a more activated area right under the coil that was associated with people's ability to be able to defeat, to be able to — sleep deprivation effect in this task. So, we were able to actually show pretty much complete remediation in this task of the effects of sleep deprivation just by the cumulative effect of using TMS. And so, again, a nice offline strategy, in this case, to try to create a therapeutic effect.
So, overall then, in terms of protocols, all right, you have online protocols that create acute effects. These can be used to both probe and explore the brain and its networks and to diagnose, all right, to look for biomarkers. Repetitive TMS lends itself to both online and offline. You saw the last example I gave was repetitive TMS, five hertz stimulation, and short trains, okay. But you also, with repetitive stimulation and things like TBS patterned stimulation, can create cumulative effects as well. And so, and these are the offline effects where you're looking pre and post something, all right, some kind of change. And you know, I mentioned earlier about the changes themselves. Okay, that actually concludes my exploration of parameters, TMS parameters, and basic ideas, the basic physical effects of TMS, and how we measure them through various TMS protocols.