A macaque monkey sat in front of a computer. A yellow square—the target—appeared in the periphery on the left side of the screen. After a few seconds delay, a second target appeared on the right. The question was: Which target would the monkey look at first? So far so routine as neuroscience experiments go, but the next step was unusual. By non-invasively directing bursts of inaudible acoustic energy at a specific visual area of the brain, a team of scientists steered the animal’s responses. If they focused on the left side of the brain, the monkey looked to the right more often. If they focused on the right side, the monkey looked to the left more often.
The results of the experiment, which were presented last week at the annual Society for Neuroscience meeting, marked the first time that focused ultrasound was safely and effectively used in a nonhuman primate to alter brain activity rather than destroy tissue. A second study, in sheep, had similar results. “The finding paves the way to noninvasive stimulation of specific brain regions in humans,” says Jan Kubanek, a neural engineer at Stanford University School of Medicine and lead author of the macaque study. The technology might ultimately be used to diagnose or treat neurological diseases and disorders like Parkinson’s disease, epilepsy, addiction and depression. Other scientists are optimistic. “The idea that, with a very carefully designed dose, you could actually deliver [focused ultrasound] and stimulate the brain in the place you want and modulate a circuit rather than damage it, is a really important proof of principle,” said Helen Mayberg, MD, of Emory University School of Medicine, who was not involved with the study.
Ultrasound has long been used for imaging. When sound waves above the level that humans can hear (more than 20,000 hertz) are aimed at the body, some of the energy bounces back creating a picture of internal bodily structures. Focused ultrasound, or FUS, raises the energy level to accomplish other ends. Like using a magnifying glass to focus beams of light on a single point and burn a leaf, FUS concentrates as many as 1,000 sound waves on a specific target with precision and accuracy.
First approved by the Federal Drug Administration in 2004 as a treatment for uterine fibroids, focused ultrasound has gained an increasing variety of potential uses, generating excitement among many doctors. “There are 18 ways, or mechanisms of action, by which focused ultrasound affects tissue. That fact creates the opportunity to treat a whole variety of medical disorders,” says Neal Kassell, MD, former co-chair of neurosurgery at the University of Virginia and founder and chairman of the Focused Ultrasound Foundation, which seeks to speed the development and adoption of the technology.
A decade ago, FUS was being investigated as a treatment for three diseases or disorders. Today that number stands at more than 90. Thus far, however, it has only been used in humans to target and destroy tissue with heat. In addition to uterine fibroids, it is approved for four other therapeutic uses in the United States. Prostate cancer was added to the list in 2015, although some urologists have been lukewarm about its use, emphasizing in the Journal of the American Medical Association in 2016 that the long-term efficacy is not yet proven. In the brain, the FDA approved its use as an ablation treatment (removing tissue) for essential tremor in 2016. (In Europe, it’s more widely used.)