Neurons can be modified with light-gated ion channels, which cause them to become excited upon illumination. This discovery has given rise to the field of optogenetics, with impressive examples such as triggering the beat of a heart with light. There is, however, one technical limitation: Light-gated ion channels are typically stimulated with blue-green light, which is heavily scattered by tissue. Thus, deep brain stimulation, focused on small regions, is a major challenge. On the article of Chen et al. Science, 2018, 359, 679 use transgenic mice implanted with upconverting nanoparticles (NPs) to locally activate light-gated ion channels and modulate neuronal activity, even deep inside the brain. This method might eventually lead the way for clinical applications to optically control neuronal dysfunctions, such as Parkinson's disease or even paralysis.