Editor’s note: The brain is densely packed with interconnected neurons, but until about six years ago, it was difficult for researchers to isolate neurons and neuron types to determine their individual roles in brain processes. In 2004 however, scientists, including author Edward S. Boyden, Ph.D., found that the neural expression of a protein, channelrhodopsin-2 (ChR2), allowed light to activate or silence brain cells. This technology, now known as optogenetics, is helping scientists determine the functions of specific neurons in the brain, and could play a significant role in treating medical issues as diverse as sleep disorders and vision impairment.