Optogenetics, a tool for controlling neurons with light, has given neuroscientists the ability to flip brain cells on and off more or less at will, revolutionizing neuroscience.

Yet the technique faces a fundamental challenge: To study all but the outermost part of the brain, researchers need to implant fiber optics or other invasive devices to deliver light deep into the brain.

Now, in Proceedings of the National Academy of Sciences, Stanford researchers report that they’ve found a less invasive way to do so: injectable nanoparticles that convert sound waves, which can easily penetrate into the brain, into light.