Anyone who remembers high-school physics knows that a fluctuating magnetic field can induce an electrical current. That's the principle behind transcranial magnetic stimulation (TMS), where an electromagnet is held over the head and pulsed rapidly. Depending on the frequency of the pulses, this can either enhance or suppress activity in neurons a few centimetres under the skull.
TMS is seen as one of the safer forms of brain stimulation, as it requires no surgery. Yet it is not completely risk free: some people experience pain in the scalp, headaches or facial spasms. More concerning were the 10 cases of seizures triggered by TMS in the first few years of its use.
Fortunately these became very rare once those administering TMS learned to limit the intensity and frequency of the stimulation and give patients regular breaks in treatment. TMS was leapt on as the perfect research tool. Much knowledge of the brain has come from people who have had a stroke or head injury - the mental abilities they lack reveal the role of the damaged area. TMS allows researchers to disable parts of the brain at will in a way that is completely reversible.
The method has also been tried out in numerous medical conditions and forms of enhancement. But many of the studies are regarded sceptically, because it is hard to control for the placebo effect. Researchers have typically tried to give half the volunteers fake therapy with the TMS machine turned off, but people often know if they are getting real treatment or not by the presence or absence of the characteristic physical signs.
Last year, however, a group at Emory University in Atlanta found that a month's worth of treatment could be crammed into a few days with no apparent ill effects (Depression and Anxiety, vol 27, p 960). That approach might lead to wider use.
The big drawback of TMS is that at the moment it is only offered at major hospitals. But US firm Neuralieve is developing a handheld device for people to use at home for treating migraine. It delivers only a single pulse, but if its safety is proven, who knows how long that will be the case?
VERDICT Approved for depression, but impractical for less serious illnesses or enhancement until equipment becomes more portable.