Studying deep brain tissues noninvasively is difficult. Now scientists have developed a way to send light deep into the brain without invasive optical fibers. The method uses infrared light outside the head to activate upconversion nanoparticles (UCNPs). When these nanoparticles absorb near-infrared laser light, they emit visible photons to deep areas in the brain, allowing remote optogenetic stimulation or inhibition of neurons in the brain.