CONTIGUITY AND CONDITIONED REINFORCEMENT IN PROBABILISTIC CHOICE

In a baseline condition, pigeons chose between an alternative that always provided food following a 30-s delay (100 % reinforcement) and an alternative that provided food half of the time and blackout half of the time following 30-s delays (50 % reinforcement). The different outcomes were signaled by different-colored keylights. On average, each alternative was chosen approximately equally often, replicating the ®nding of suboptimal choice in probabilistic reinforcement procedures. The ef®cacy of the delay stimuli (keylights) as conditioned reinforcers was assessed in other conditions by inter-posing a 5-s gap (keylights darkened) between the choice response and one or more of the delay stimuli. The strength of conditioned reinforcement was measured by the decrease in choice of an alternative when the alternative contained a gap. Preference for the 50 % alternative decreased in conditions in which the gap preceded either all delay stimuli, both delay stimuli for the 50 % alter-native, or the food stimulus for the 50 % alternative, but preference was not consistently affected in conditions in which the gap preceded only the 100 % delay stimulus or the blackout stimulus for the 50 % alternative. These results support the notion that conditioned reinforcement underlies the ®nding of suboptimal preference in probabilistic reinforcement procedures, and that the signal for food on the 50 % reinforcement alternative functions as a stronger conditioned reinforcer than th