It’s one of the boldest treatments in medicine: delivering an electrical current deep into the brain by implanting a long thin electrode through a hole in the skull.
Such “deep brain stimulation” (DBS) works miracles on people with otherwise untreatable epilepsy or Parkinson’s disease – but drilling into someone’s head is an extreme step. In future, we may be able to get the same effects by using stimulators placed outside the head, an advance that could see DBS used to treat a much wider range of conditions.
DBS is being investigated for depression, obesity and obsessive compulsive disorder, but this research is going slowly. Implanting an electrode requires brain surgery, and carries a risk of infection, so the approach is only considered for severe cases.
But Nir Grossman of Imperial College London and his team have found a safer way to experiment with DBS – by stimulating the brain externally, with no need for surgery.
The technique, unveiled at the Society for Neuroscience conference in San Diego, California, this week, places two electrical fields of different frequencies outside the head. The brain tissue where the fields overlap is stimulated, while the tissue under just one field is unaffected because the frequencies are too high. For instance, they may use one field at 10,000 hertz and another at 10,010 hertz. The affected nerve cells are stimulated at 10 hertz – the difference between the two frequencies.
So far the work is at an early stage. Grossman has shown that it works in principle in mice, which can be killed after the treatment to see which neurons were stimulated.
His team has also tested it on nine healthy people as they lay in an fMRI brain scanner; the target region of brain tissue became more active when the stimulation was turned on, he told the conference.
“It’s safer than putting holes in someone’s head,” says Peter Steinmetz, at the Nakamoto Brain Research Institute in Tempe, Arizona. “That’s the beauty of it.”
But it’s not yet as precise as regular DBS, says Grossman. “As we get deeper, the size of the stimulation becomes larger.” And it cannot yet reach as deep as the brain’s movement control centres targeted in Parkinson’s disease, which are about 10 centimetres below the skull.
Grossman plans to target the hippocampi, a pair of elongated structures about 5 centimetres under the skull, which are crucial in memory. At that depth, the volume stimulated would be about right for the 3-centimetre-long hippocampi, he says.
Grossman says this could be useful in conditions involving memory loss, such as Alzheimer’s disease. He also plans to target areas involved in depression and addiction.
Other groups are investigating stimulating the brain externally with ultrasound – which is thought to make neurons fire by knocking open electrically active channels in their cell membranes. But Grossman says it’s safer to use electricity. “With ultrasound we are hitting the cell so there’s more chance we are damaging it. Electrical fields have been used for decades.”
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