This new technology seems insanely brilliant. By using viruses to deliver the genes for various opsins into neurons in the brain, light can be used to activate single neurons at a time so that the functional role of the neuron can be determined. The neuron, once the opsins are expressed, can be essentially treated like a light switch.
Dr. Karl Deisseroth of Stanford University has used this technology in the amygdala to reduce learned fear in mice.
New York Times article describing this phenomenon.
TED talk by Ed Boyden. At the end, Ed describes how people who have suffered from a loss of photoreceptors in the retina could potentially have opsins genetically implanted into their bipolar cells to regenerate vision in some way.
Here is a short video from youtube (which is narrated with a delightful English accent) about optogenetics. The narrator discusses the proteins that can activate and deactivate cells, simply using a focused beam of light. Nature Methods video