Signalled delay of reward: Overshadowing versus sign-tracking explanations

Abstract

Four experiments used hungry rats leverpressing for food reinforcement. The effects of different stimuli serving to signal a delay of reinforcement on responding maintained by a variable interval (VI) 60-s schedule with a 5-s delay of reward were studied. In the initial two experiments, groups of rats responded faster with a discrete visual stimulus (Experiment 1) or diffuse auditory stimulus (Experiment 2) that either signalled the whole delay period or signalled the first part of the delay period relative to groups with an unsignalled delay. If only the second half of the delay period was signalled with the visual stimulus, then performance relative to the unsignalled condition was attenuated (Experiment 1). In contrast, performance with the diffuse auditory signal in the second half of the delay period was similar to that with no stimulus (Experiment 2). The results obtained with a stimulus filling the second half of the delay period were confirmed in Experiments 3 and 4. These data suggest that the effect of signalling reinforcement delays will depend upon the degree of interference produced by sign-tracking behaviors, and not on the blocking of response-reinforcer associations by intervening stimuli.