Scientists can now switch individual brain cells on and off with light

Optogenetics inserts light-sensitive proteins borrowed from algae into specific neurons, so researchers can flip them on or off with a precisely timed flash of light. Since the technique matured around 2005, it has let neuroscientists trigger or silence single circuits in a living brain and watch exactly what behaviour changes — something no drug or electrode could do with that precision.

— Edward S. Boyden, Feng Zhang, Ernst Bamberg, Georg Nagel & Karl Deisseroth, Millisecond-timescale, genetically targeted optical control of neural activity — Nature Neuroscience, 2005