The latest edition of the Nature NeuroPod podcast is now available. It has the usual collection of cutting edge brain stories but is particularly good for an introduction to transcranial magnetic stimulation or TMS, a technique that allows researchers to temporarily ‘switch off’ bits of the human brain during experiments.
TMS is really just a large electromagnetic coil that can switched on and off very quickly, allowing a focused high intensity magnetic field to be directed into the brain from a few centimetres outside the skull.
As you may remember from high school physics, when a magnetic field passes over a conductor it causes an electrical current. In this case, the conductor is the area of your brain just at the focus of the magnetic field and the current is enough to trigger all the neurons in that small area.
Because neurons are all busy doing their thing, suddenly electrifying them all at once effectively ‘resets’ them, and so switches them off for a brief moment before they resume.
If you suspect that a particular brain area is involved in a task, you can get someone to do the task and switch the brain area off for a few hundred milliseconds with TMS. If the area is genuinely involved, the person should do it slightly worse or slightly slower, whereas, if it isn’t, there should be no difference.
TMS can also be used before someone is doing a task to make the area more or less excitable in general terms, by applying repetitive pulses to the area a few minutes before. Think of it like changing the mood of a crowd before the main event. It’ll affect how they react later on.
It’s a versatile and interesting technique for exploring brain function, but the exact detail of how it affected the electrical circuitry of the brain has been a mystery.
NeuroPod interviews neuroscientist Sven Bestmann, who recently published a paper on what we know about TMS and the brain, where he discusses the latest discoveries and explains the technique in more detail.