The time course of each trial is illustrated in Fig. 1. This procedure was a slightly modified version of a previously reported DD task called an adjusting amount procedure and has been previously described (Richards et al., 1997). Briefly, the center nosepoke cue light is illuminated until the mouse initiates the trial by nosepoking. After the nosepoke, the center light goes dark, and the lights illuminate above both the left and right lever, signaling availability of a choice. Both lights remain illuminated until a lever press is recorded. Once a lever press is made, the nonchosen lever light extinguishes, and the sipper containing saccharin solution descends into the drinking position. Saccharin solution was used to promote higher responding. One lever was always assigned to “delay” and the other was assigned to “immediate.” A lever press on the delay side resulted in delivery of the standard, 2-second reward after the programmed delay interval, during which time the light above the delay lever remained illuminated. An immediate-side lever press resulted in delivery of the adjusting reward without any programmed delay. The immediate
