Ultrasound can make visible what is hidden, for example, to give parents a glimpse of their child before birth. We are now putting these high-frequency sound waves – beyond the range of human hearing – to use for a wholly different purpose: to manipulate the activity of the brain. Conventional brain stimulation techniques use electric currents or magnetic fields to alter brain activity. But these techniques have limitations. They can only reach the surface of the brain and are not very precise. By contrast, beams of ultrasound can be safely focused at a millimetre scale, even deep within the brain. Ultrasound thus has the potential to provide new insights into how the brain works.
Most studies of ultrasound stimulation have looked at what happens to the brain during the stimulation itself. But could ultrasound also induce longer-lasting changes in brain activity? This would be valuable for research but would also make it more likely that we could use ultrasound to treat brain disorders by changing brain activity.
We used an MRI brain scanner to measure brain activity in macaque monkeys after ultrasound stimulation. The results showed that 40 seconds of repetitive ultrasound changed brain activity for up to two hours. Ultrasound caused the stimulated brain area to interact more selectively with the rest of the brain. Notably, only the stimulated area changed its activity in this way. This helps rule out the possibility that the changes reflect non-specific effects. If the monkeys had been able to hear the ultrasound, for example, or if the stimulation had interfered with the measurement, this would have produced very different or more widespread changes. Most important of all, the changes were reversible and did not harm the brain.
Our results show that repetitive ultrasound can induce long-lasting alterations in brain activity. It can target areas deep within the brain, including those that are out of reach with other techniques. If this procedure also shows longer-lasting effects in people, it could yield valuable insights into the links between brain and behaviour. It could also help us develop new treatments for neurological and psychiatric disorders.