Figure 1. Schematic of the experimental approach and analysis procedures. A, Setup used to analyze head movement. B, Apparatus for tethered flight experiments. 1, Thin light fiber; 2, matched achromatic doublet pair; 3, tether; 4, infrared LED; 5, camera; 6, head positioner; 7, wing beat analyzer consisting of a mask, two prisms (white), and two photodetectors (not shown). Ci, Head movement analysis from raw images of fly head. The light spot for unilateral optogenetic stimulation is indicated with an arrow. Cii, A box is fitted around the contours (green) of the thresholded image shown in Ci. D, Example wing beat trace for clockwise and counter-clockwise visual rotation (stimulus time interval indicated by gray boxes). The LWB is in red, the RWB is in blue, and the LWB–RWB is in green. E, ChR2–C128S-EYFP expression using R27B03-Gal4. The micrograph shows a collapsed confocal image stack of one brain hemisphere (posterior view). In the optic lobe, expression is largely confined to the three HS cells: HSN (top), HSE (middle), and HSS (bottom).