Controlling activity of defined populations of neurons without affecting other 
neurons in the brain is now possible by a new gene- and neuroengineering 
technology termed optogenetics. Derived from microbial organisms, opsin genes 
encoding light-activated ion channels and pumps (channelrhodopsin-2 [ChR2]; 
halorhodopsin [NpHR], respectively), engineered for expression in the mammalian 
brain, can be genetically targeted into specific neural populations using viral 
vectors. When exposed to light with appropriate wavelength, action potentials 
can be triggered in ChR2-expressing neurons, whereas inhibition of action 
potentials can be obtained in NpHR-expressing neurons, thus allowing for 
powerful control of neural activity. Optogenetics is now intensively used in 
laboratory animals, both in vitro and in vivo, for exploring functions of 
complex neural circuits and information processing in the normal brain and 
during various neurological conditions. The clinical perspectives of adopting 
optogenetics as a novel treatment strategy for human neurological disorders have 
generated considerable interest, largely because of the enormous potential 
demonstrated in recent rodent and nonhuman primate studies. Restoration of 
dopamine-related movement dysfunction in parkinsonian animals, amelioration of 
blindness and recovery of breathing after spinal cord injury are a few examples 
of such perspectives.