The pigeons were housed individually in their home cages (375 mm high by 375 mm deep by 370 mm wide), which also served as experimental chambers. On one wall of the cage, 200 mm above the floor, were three 20-mm diameter plastic keys set 100 mm apart center to center. Each key could be illuminated yellow or red, and responses to illuminated keys exceeding about 0.1 N were recorded. Beneath the center key, 60 mm from the perch, was a 40 mm by 40 mm magazine aperture. When food was delivered, key lights were extinguished, the aperture was illuminated, and the hopper containing wheat was raised for 2.5 s. The subjects could see and hear other pigeons in the room during experimental sessions; no person entered the room during this time.