The nematode C. elegans shown here expresses Channelrhodopsin in its mechanosensory neurons (Pmec-4::ChR2::GFP ). When blue light shines on the the touch receptor neuron ALM, the worm experiences the illusion of touch and 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 http://colbert.physics.harvard.edu for more.