A new optogenetics method can selectively stimulate inhibitory neurons and so reduce activity in a specific brain region. To test this method in macaques, De, El-Shamayleh and Horwitz injected the viral vector Dlx5/6 carrying channelrhodopsin – which activates inhibitory neurons (pink) – into the primary visual cortex of primates. Upon stimulation with light (torch), these inhibitory neurons were activated (top graph, blue line), subsequently leading to a deactivation of the excitatory neurons (grey: bottom graph, blue line). In a visual task, the monkeys were presented with a point that could appear randomly in different locations, and the subjects had to detect this point using an eye movement (green arrows). Activation of inhibitory neurons induced a blind spot in the visual field of the monkey; hence, monkeys were less likely to detect the point when it appeared in the blind spot (as indicated by the red cross). IPS: impulses per second (which corresponds to the firing rate of the neuron).