When asked about a recent study that questioned the effectiveness of another noninvasive brain stimulation technique, transcranial alternating current stimulation (tACS), Walker and Mayberg attributed the failures of tACS to its lack of localization within the brain. While tACS and its sister technique, transcranial direct current stimulation (tDCS), rev up a “large swath of brain,” Walker explains, TMS is “much more specific,” able to target particular groups of neurons.
“It’s location—all of this stuff requires knowledge of the circuits,” Mayberg agreed. When it comes to tACS and tDCS, researchers are still “trying to figure out how to steer it.”
Meanwhile, other researchers are working to develop yet another type of noninvasive brain stimulation that’s even more precise than TMS—transcranial focused ultrasound (FUS). Primarily tested on rodents thus far, Jan Kubanek of Stanford University School of Medicine described a recent study from his group that demonstrated the safety and efficacy of FUS in nonhuman primates.
Using the technique to stimulate the frontal eye fields (FEFs) in the frontal cortex of two macaque monkeys, Kubanek and colleagues showed that they could alter the animals’ behavior. The monkeys were trained to select whichever of two visual targets appeared first, but stimulation of the left FEF biased the monkeys to choose the target on the right, whereas stimulation of the right FEF made the monkey tend to choose the target on the left, regardless of the order of appearance. Examination of brain slices from these regions showed no signs of damage, suggesting that the technique is safe. These results “really pave the way toward using this method as a new tool to diagnose the neural circuits in neurological and possibly also psychological disorders in humans,” Kubanek said at the press conference.
Importantly, not only is FUS more localized than TMS—targeting regions less than a quarter inch in diameter, as compared to several inches, noted Seung-Schik Yoo of Harvard Medical School—but the technique also allows direct stimulation of much deeper areas. “Ultrasound can go pretty deep into brain,” Yoo said during the press conference. His group is now working to develop a wearable device that could be used to deliver the ultrasonic stimulation.
Mayberg pointed out that FUS has previously been used in clinical practice to induce lesions deep in the brain. So this newer work aiming to use the technique to “stimulate and modulate a circuit, rather than damage it, is a really important principle,” she said.