Optogenetics is a recently developed biological tool to artificially control the activities of targeted neural populations and neural projections in millisecond time resolution by intracerebral light irradiation in freely moving animals. In this review, we have briefly explained: (1) the mechanisms of light-gated microbial channels or pumps permitting light-induced membrane potential changes, and (2) the experimental procedures for expressing these proteins in target brain regions to activate

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... inactivate specific cell populations. We have also introduced recent findings revealed by optogenetics, particularly focusing on studies investigating how psychological theories are embedded in specific brain circuits. Optogenetics will greatly expand our understanding of neural mechanisms underlying various kinds of behaviors.