The testing chamber was a 6 cm cube with a mesh Y-maze in the middle (Aladin Enterprises, Inc., San Francisco, CA). Odors were streamed in through opposite arms of the Y (each 6 cm). Vials of flies were placed at the lower Y arm and flies climbed up the mesh cylinder and chose between opposing arms of the Y to 2.5 cm diameter, 9.5 cm height vials. After 2 min, vials were removed and capped. The number of flies that moved into the odor 1 and odor 2 vials were counted. A preference index for the odor paired with ethanol was calculated as (# flies in paired odor vial - # flies in unpaired odor vial) / total # flies. A performance index for conditioned odor preference or aversion (CPI) was calculated by averaging the preference indexes for reciprocally trained groups of flies.
