Transcranial magnetic stimulation isn’t doing what we thought.

Science magazine has some interesting – and rather weird – findings about the new “brain-zapping” technology that uses magnets to affect brain states. Apparently, as a new cadaver study shows, the current shouldn’t be strong enough to go through a human cranium:

When [NYU neuroscientist György] Buzsáki and his colleague, Antal Berényi, of the University of Szeged in Hungary, mimicked an increasingly popular form of brain stimulation by applying alternating electrical current to the outside of the cadaver’s skull, the electrodes inside registered little. Hardly any current entered the brain. On closer study, the pair discovered that up to 90% of the current had been redirected by the skin covering the skull, which acted as a “shunt,” Buzsáki said.

Little is known about how these techniques might influence the brain. Yet many scientific papers have claimed that the techniques can boost mood, alleviate chronic pain, and even make people better at math by directly affecting neuronal activity. This has spawned a cottage industry of do-it-yourself gadgets promising to make people smarter and happier.

The new, unpublished cadaver data make dramatic effects on neurons unlikely, Buzsáki says. Most tDCS and tACS devices deliver about 1 to 2 milliamps of current. Yet based on measurements from electrodes inside multiple cadavers, Buzsaki calculated that at least 4 milliamps—roughly equivalent to the discharge of a stun gun—would be necessary to stimulate the firing of living neurons inside the skull. Buzsáki notes he got dizzy when he tried 5 milliamps on his own scalp. “It was alarming,” he says, warning people not to try such intense stimulation at home.

Neuroscientist Vince Clark of the University of New Mexico, Albuquerque, for example, has found that applying 2 milliamps of current to a person’s scalp for just 30 minutes can double the speed at which they learn a game in which players must detect a concealed “threat,” such as a bomb or sniper, in a video clip. Several labs have replicated those results, he says, adding that the idea that 10% or less of the current gets through to the brain is not new, and doesn’t necessarily mean the methods are ineffective. “If it works, you know 10% is enough,” Clark says.

Marom Bikson, a biomedical engineer at The City College of New York in New York City who uses computer models and slices of rat brain to study the mechanisms of tDCS and tACS, says that many in the field already accepted that the 1 or 2 milliamps the methods use don’t directly trigger firing. It can make neurons more likely to fire or form new connections, he and others believe.

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