Apparatus.

Sessions were conducted in mouse operant-conditioning chambers designed for 5-choice serial reaction-time procedures (MED-NP5M-B1; Med-Associates Inc., St. Albans, VT, USA). On one curved wall of the chamber, 1.43 cm above the grid floor, were five 1.27-cm diameter circular holes equally spaced across its length. Behind each hole were an infrared beam and photodetector that bisected the hole opening and a light-emitting diode that could illuminate the hole. Interruption of the light beam and was defined as a response, with casual observations indicating that interruptions virtually uniformly occurred with a nose poke into the hole. Centered on the opposite wall was a food tray into which 20-mg food pellets (BioServ, Frenchtown, NJ) could be delivered as reinforcement for responses. A light-emitting diode within the food tray was used to illuminate the food tray during pellet delivery. At the top of the opposite wall was a light bulb (houselight) that provided general illumination of the chamber. The chambers were contained within light-proof, ventilated enclosures that provided sound attenuation. White noise was delivered to the chamber at all times to mask extraneous noise.