Have you ever wanted to stimulate some part of your brain, only to be thwarted by that pesky skull ensconcing it? Don’t want to drill holes and implant electrodes? Well lucky it’s the 21st century so now you don’t have to. Let’s take a dive into the weird science of transcranial magnetic stimulation.
1. Some background on brains
Throughout history people have consistently tried to make sense of the brain in terms of the most advanced technology available at the time. This has involved comparing it to, at various eras: a series of hydraulic pipes, a gear-powered automaton, a telephone switchboard, and most recently a squishy computer.
Time will tell whether this brain-as-computer metaphor holds up or eventually gets superseded by some future technology. What is clear though is that brain function is dependent on neurons being networked in certain patterns, and electrical signalling within these networks.
Imagine if we could custom modify these networks and electric signals. Such a technology would open up immense possibilities: we could repair disruptive firing patterns like those seen in epilepsy and depression, effortlessly gain skills and knowledge; or, conceivably, experience really anything imaginable (and maybe things unimaginable). This is the technological dream.
Even if this technology existed however, most of us probably wouldn’t love having a giant socket installed in the back of our head. Consequently, science has been left with the unenviable task of trying to tweak the brain without physically opening it up. A few decades back this dubious challenge was solved (well, sort of, as we shall see) with weird physics. Time for us to meet transcranial magnetic stimulation (TMS).
2. The science of TMS
TMS triggers electrical activity in a chosen part of the brain via the outright magic of ‘electromagnetic induction’. Essentially, a plastic wand packed with coiled wires is held against the scalp and an electrical current is passed through the wires. This generates a brief but powerful magnetic field. The field lasts less than a second and packs around 2-3 Tesla, about the same amount of energy as an fMRI machine.
This magnetic field zips through the skull as effortlessly as a carefree otter through water until it hits a conductive material: neurons. The magnetic field is then converted back to an electrical current, which ripples onward in a way which is currently impossible to accurately quantify. It works something like this:
While this sounds pretty groovy, what does it mean for the subject?
Three decades of TMS experiments have shown that if you zap a random part of the brain, most of the time it does… nothing. At least, nothing that the subject can consciously detect. This supports findings from social psychology that a tremendous amount of what the brain does is unconscious. Our waking minds may be little more than the tip of iceberg. What mysterious sub-processes and whispers of consciousness may be swirling away down below?
There are exceptions of course. Stimulating the occipital cortex – the main image processing region of the brain – causes flashes of light to be “seen” by the subject. Numerous regions can cause subtle behavioural changes when stimulated (mostly not-very-exciting things like slight delays in performing some task), even if the subject isn’t aware of anything. The technique can also be used to determine whether nerves have been damaged following a damaging event like a stroke.
On the safety front, TMS is thought to be A-ok and won’t leave any lasting traces on your brain – unless, of course, you want it to.
The brains of depressed people show characteristic activity changes in certain regions including the prefrontal, cingulate and parietal cortices (some of which may by involved to negative ruminative thought).
Because TMS can alter brain activity, it offers an exciting non-chemical avenue for treating depression. We’ve discussed in a previous post how learning anything requires repetition and time so that the necessary physical circuits can be linked up in the brain. It seems that the same goes for unlearning depression. Repetitive TMS can effect long-term increases or decreases in the activity of brain regions. And sure enough, some sufferers of major depressive disorder have experienced substantial decreases in symptoms following such treatment (although admittedly it fails for most people).
3. The first-person experience
Being keen to zap my own brain, I was lucky enough to take part in a TMS study. I showed up at the testing centre, completed an odd facial recognition task (I always try not to ponder the research question in case it biases my performance), and it was time for the treatment.
I was led into a room cluttered with treadmills and brain-scanning devices and invited to take a seat in a reclining dentist-style chair. The TMS apparatus squatted blandly beside me.
On a screen across from me hovered a 3D rendering of my own damn brain, captured in an earlier fMRI experiment. A red dot superimposed somewhere near the occipital lobe indicated the area they would be targeting.
A technician began swabbing various parts of me with alcohol pads, attaching electrodes to my temples, nose, and three around my left thumb. The lead researcher explained that each person has a different “resting motor threshold”, and that this has to be determined before TMS can be performed. In other words, they need to figure out how high to crank the power to get a decent current but simultaneously not… over-zap things.
To do this, they usually stimulate the motor cortex to make the subject’s thumb twitch, and get the intensity just right on that spot. The electrodes on my thumb were connected to a “myocardiograph”, the researcher explained, which would measure any muscle contractions.
The researcher took the figure-eight wand, placed it against my scalp, I tried to be calm, and he pressed the button. A click emitted and a minute sensation hit my scalp, like being flicked by a belligerent mouse. My thumb sat there motionless. Everything seemed normal. He shifted the wand to a slightly different position and pressed the button again. Another tiny mouse flick, another click, and another nothing happening. It turns out TMS is more a “poke around and try to find the right spot” kind of science.
Two or three minutes elapsed in this fashion, with a couple dozen more mouse flicks taking place. The researcher managed to hit my index finger once or twice, which was a bit exciting. How to explain the odd sensation of watching your muscles, wondering if they’ll suddenly contract without apparent reason, and having them occasionally fulfil that expectation?
At one point I became concerned as I had started hallucinating the sound of bubbling water, but it turned out to actually be coolant fluid. And then, quite unexpectedly, I started feeling uncomfortably hot, and speaking seemed very hard to do, then little black specks were floating in my vision, then I was come to on the floor. The electrodes were all off, the technician was bustling around, and the researcher informed me I’d had a “vasovagel syncope”, which is a nice way of telling someone they passed out. It’s the exact same thing people get giving blood. The researcher explained as he cheerily hauled me back into the chair that fainting is an extremely rare side effect of TMS, and no one really has a clue why it happens. It seems that part of the brain stem that controls the sympathetic and parasympathetic nervous system gets activated, blood pressure plummets, and your body decides it wants to be sideways so more oxygen can get to the brain.
Alas, he refused to plug me back in for safety concerns, so my adventure came to a most anticlimactic end.
4. The future of your brain on TMS
How close are we to the techno-utopian/dystopian dream of custom modifying our brain circuits? The answer is probably “not very close at all”.
The neural changes that take place while learning something new (say, kung fu) are exquisitely specific, and at the moment TMS is far too blunt a tool. If old fashioned learning is like making a fine-brushed oil painting, TMS is upending a barrel of paint over the canvas. In the best case, the magnetic current stimulates several cubic millimetres of neurons. Given a cortical density of around 100,000 cells per mm³ in primates, “inaccurate” is an understatement. It’s also hard to see, given current understandings of physics and the complexity of the brain, how this performance could be significantly improved.
Still, at one point in history it was inconceivable that we would ever see individual atoms under a microscope, or control another person’s movements with the sheer power of thoughts sent over the internet. So, y’know, let’s not be too quick on the naysaying.
While we can’t predict with any confidence how useful techniques like TMS may become or when, it’s definitely worth watching this space in the near future. Especially if there’s some part of your brain that you’d really like to zap.
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- The Unconscious Mind
- A practical guide to diagnostic transcranial magnetic stimulation: report of an IFCN committee.
Cognitive Deficits in Depression and Functional Specificity of Regional Brain Activity
Repetitive transcranial magnetic stimulation in psychiatry
- Clinically meaningful efficacy and acceptability of low-frequency repetitive transcranial magnetic stimulation (rTMS) for treating primary major depression
- Evidence-based guidelines on the therapeutic use of repetitive transcranial magnetic stimulation (rTMS)
- A sham controlled study of repetitive transcranial magnetic stimulation for posttraumatic stress disorder
- Repetitive transcranial magnetic stimulation (rTMS) for obsessive-compulsive disorder (OCD)
- Efficacy of repetitive transcranial magnetic stimulation for the treatment of refractory chronic tinnitus
- Transcranial magnetic stimulation (TMS) for schizophrenia
- Neuronal and Synaptic Packing Densities