Scientists can now move a mouse’s whiskers, ears, and a paw using just electrodes on the outside of the animal’s head. This new method can stimulate deep parts of the brain without surgery and — if it pans out in humans — it could be very helpful for people with neurological conditions like Parkinson’s, depression, or epilepsy.
The brain has surface regions located near the skull and deep regions further inside, such as the motor cortex, the area of the brain that controls movement. Until now, the only real way to stimulate the regions deeper inside the brain was to cut open the skull and directly implant electrodes. (This is called deep-brain stimulation.) But surgery can be dangerous and the electrodes can cause damage inside the brain. In a study published today in the journal Cell, a team of scientists at the Massachusetts Institute of Technology showed a way to sidestep the dangerous surgery and stimulate the motor cortex from the outside.
To do this, the team took advantage of how neurons process electrical signals. Neurons only activate when they receive low-frequency electricity signals. If high-frequency signals are applied to the brain, the brain just ignores them. “It can’t keep up,” says study co-author Ed Boyden, a cognitive scientist at MIT.
But something interesting happens when you send two high-frequency signals that are just a little bit different, like 3000 hertz and 3001 hertz. Most parts of the brain ignore the signals. But when the two frequencies meet at the target site — in this case, the motor cortex — and interfere with each other, the neurons pay attention. They interpret the difference in frequency as if it were a low-frequency wave. This technique, called “temporal interference,” makes it possible to stimulate just one part of the brain and not all the other parts on the way there.
That’s exactly what the team did. After calculating the right frequencies to target the motor cortex, they sent the two frequencies to the mouse brains. In this way, they made the animal wiggle its ears, whiskers, and a paw.
The technique could potentially help humans with certain conditions. In a disease like Parkinson’s, for instance, the motor cortex sends and receives abnormal electrical signals. This causes people to have tremors and their muscles to weaken. With traditional deep-brain stimulation, the electrode implanted in the brain blocks these abnormal signals, which helps control the tremors for up to five years. Ideally, says Boyden, with the new technique one day you could stimulate a human brain from the outside for a brief amount of time, and have the effects last for the rest of the day or the rest of the week.
“This technique has potential to be an extremely useful tool to both probe and potentially change the functioning of brain regions and brain circuits that are very important to a lot of human illnesses,” says Ben Greenberg, associate director of Providence VA Hospital’s Center for Neurorestoration and Neurotechnology, who was not involved in the study. But he notes that it’s simply “too soon to say” how this stacks against deep-brain stimulation, which has been researched since the 1980s.
There are many questions that need to be answered. We don’t know yet how precise this new method can be; for now, we just know it’s nowhere near as precise as implantable devices. Safety is another concern: early tests showed that this type of stimulation didn’t harm the animals, but more research needs to be done, according to Boyden.
And, of course, the big question is how useful this could be in humans. For one, the human skull is thicker than a mouse’s, which will change the types of frequencies needed. Plus, different people might respond to frequencies differently, says Greenberg. “Even in the mice, there were differences in how responsive their cells were.”
Boyden’s team is already doing more safety experiments with animals, and early studies with human volunteers. “We want to be as careful as possible, of course,” Boyden says, “but the good news is that since we’re building from decades of research on electric fields, so it’s not like we’re starting from scratch.”