How does TMS work?
Dr. Rustin Berlow discusses the mechanism of action behind TMS and its current understanding in the brain stimulation research community.
The process involved in using TMS
Can you explain to us how exactly TMS works?
Nobody knows exactly how it works but what I can show is what we do know, or what we think happens that allows it to work. To understand, we can start at the wall plug and go to the person. The first part of the TMS machine is a power supply. It takes the power supply and stores it up in capacitors. The other part of the TMS machine has a pulse shaper and what that does is it makes the magnetic field change at a faster rate or a slower rate, and we can turn it up or down. We know that the frequency of a magnetic field-change affects the current that is induced, which we’ll talk about in a second.
After it goes from the power supply to the pulse shaper, it goes to the coil. The coil is basically a wrapping of wire. When electricity goes through it, a magnetic field is generated. The magnetic field is invisible but if you hold your keys up to it, you can feel them move. You can feel force being exerted on those keys. If you hold your hand up to it, it feels like an electric shock because you have nerves in your hand, so you can’t really feel the magnetic field that way.
The coil is placed against the person’s head and when the TMS machine is turned on, that magnetic field goes through the skin, scalp, and skull and interacts with the brain. When it does, a current develops in the brain. That current allows, creates or causes nerve cells to fire. You could look at it the other way around, that nerve cells fire because of the magnetic field and that creates a current. It actually changes in currents. So the part of the brain that you hold it up to has a big impact on how it works because conditions may need different locations of stimulation. Most TMS occurs and is done for depression.
Dorsolateral Prefrontal Cortex
*Dr. Rustin Berlow picks up a model brain to demonstrate*
This is the frontal lobe of the human brain (points to the location on model). If you look at the frontal lobe, you can see the different parts. There’s the anterior cingulate in the middle. There’s the orbital frontal cortex here. There’s the central lateral frontal lobe and then there’s the dorsolateral frontal lobe.
Dorsolateral prefrontal cortex is usually the area that’s stimulated in TMS. The reason that is stimulated is that, in some ways, it’s the highest part of the brain. It’s the part where sensory information comes in and gets processed. It goes all the way up to the top of the dorsal lateral prefrontal cortex and then from there, the DLPFC can pause, take all the information into account and then make a decision and initiate a sequence of events that can be like a plan of multiple muscles moving.
The DLPFC is the part that has the cells that respond to what just happened and another set of cells that trigger what’s about to happen. So the DLPFC, in some ways, is the location of now. So when you pause and you can deliberate and think about things, it’s your DLPFC that’s doing that.
When you think about the ability to pause and think about what’s going on, that’s kind of deliberation. That’s kind of the opposite of impulsivity, at least in what I’ve seen. And that ability to hold things in your mind and to think about different things underlies what’s called cognitive control. The people up at Stanford have developed this idea that what’s going on in the mind of psychiatric patients, is there may be a lack of cognitive control. They’re not able to control what they’re thinking about and there’s too much rumination or they can’t remember things. So we are increasing the activity and the strength of the DLPFC and the network that it is part of, which is called the executive network or the cognitive control network. By doing that, you reduce the symptoms of depression and you help a person to return to living a life that can be enjoyed and to be found meaningful.
Now, what I described was not the original intention of why the DLPFC was chosen. But I think the best current knowledge is that the DLPFC is part of a network which underlies an important function that includes cognitive control, executive function, attentional control as well as holding information in memory. One of the interesting things about DLPFC is that when you do one exercise for DLPFC – for example, if you have a person memorize a ten digit number, you start with a seven digit number and get them good at that and then get them up to eight and then nine and then ten digits – that’s working memory that they’re strengthening. But it not only improves working memory, but it also improves the other functions of the DLPFC.
So one brain area does many things and anyway that you strengthen it, it can do the other things better too. And this is obvious if you think about a muscle. If a person does bodybuilding and they can lift weights, they obviously could lift something else or they can use muscular force in another way. It’s the same thing with the brain. The study that I’m referring to there is Bickel and colleagues who looked at digit sequencing, numbers of digit span, learning that and then relating it to delay discounting which is a form of delayed gratification that’s been mathematically worked out in great detail.