The nematode C. elegans shown here expresses Channelrhodopsin in its mechanosensory neurons (Pmec-4::ChR2::GFP ). When blue light shines on the worm's anterior it stimulates the worm's anterior touch receptors (ALM and AVM). The worm experiences the sensation of being touched and the worm responds by reversing. The laser is on when the words "DLP On" appear.
This is an example of the CoLBeRT system from the Samuel Lab at Harvard University from the paper Leifer et al, "Optogenetic manipulation of neural circuits in freely moving Caenorhabditis elegans," Nature Methods, in press, 2011.
See colbert.physics.harvard.edu for more.