Transcranial magnetic stimulation (TMS) is a form of brain stimulation that was invented in the mid-1980s and has become an established treatment for depression. TMS uses a powerful, highly focused magnetic field to stimulate brain cells in a region near the temple of the forehead called the dorsolateral prefrontal cortex.
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Stimulating these cells at specific frequencies alters how they communicate with their neighbours.
It does this either by a rapid series of pulses to encourage more crosstalk ("cortical excitability"), or causing the cells to quieten down and keep to themselves ("cortical inhibition") with a slower, more leisurely succession of zaps.
By tapping into the correct spot, the effects of TMS can propagate through entire networks that we know are dysfunctional in disorders such as depression.
TMS normalises their activity back to a healthy baseline, reversing the symptoms of depression.
How do we find and target that specific bit of the brain to improve mood?
The simple, low-tech solution uses elastic caps originally designed for electroencephalography (EEG), which measures electrical activity in the brain.
These caps have evenly distributed markings across the scalp - imagine a chessboard printed onto a swim-cap, where the centre of each square contains a mark - and can be used to match locations according to a person's head shape and size.
One of these cap markings - called "F3" for the left-side of the head and "F4" for the right side - sits over the prefrontal cortex and so has been used as the target for TMS treatments.
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An improvement on EEG caps at the forefront of research is the use of functional magnetic resonance imaging (fMRI) to fully individualise targeting of the prefrontal cortex.
fMRI measures brain activity by detecting subtle changes in blood flow. Using fMRI, researchers have been able to identify the dysfunctional brain networks associated with depression at an individual level.
The network map created this way allows targeting of the prefrontal cortex with high precision.
Finally, neuronavigation equipment - expensive infrared cameras that can match and display an image of the fMRI brain scan onto a patient in real-time - are used to position the TMS device directly over the patient's prefrontal cortex.
Although more work is needed, early studies suggest that TMS treatment using fMRI-based targeting leads to better efficacy.
It's possible that, in a few years, this may become the new standard in clinics and research labs.
- Dr Stevan Nikolin is a neuroscience fellow at the UNSW School of Psychiatry, a partner of the Black Dog Institute
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