Final Answer:
In a chain, a stimulus can serve as both an SD (discriminative stimulus) and a conditioned reinforcer when it signals the availability of a reinforcing event and has acquired reinforcing properties through previous pairings with other reinforcers. For example, a light (SD) that signals the delivery of food during training can become a conditioned reinforcer itself through repeated pairings with food delivery.
Step-by-step explanation:
In behavioral terms, a discriminative stimulus (SD) signals the availability of reinforcement upon a specific behavior, while a conditioned reinforcer is a previously neutral stimulus that gains reinforcing properties through association with other reinforcers. In a behavioral chain, a stimulus can function as both an SD and a conditioned reinforcer when it not only discriminates the availability of reinforcement but has also acquired reinforcing properties through prior pairings.
Consider a rat in an operant conditioning experiment. Initially, a light (L) is presented before the delivery of food (R). In this context, the light serves as the SD, signaling the availability of food. After multiple pairings, the rat learns to associate the light with food, and the light itself becomes a conditioned reinforcer. Now, the light serves as both an SD, signaling the opportunity for food (R), and a conditioned reinforcer, reinforcing behaviors that precede its presentation. This dual function exemplifies how a stimulus can operate in a chain as both an SD and a conditioned reinforcer.
In summary, the dual role of a stimulus in a chain arises from its ability to both signal the availability of reinforcement (SD) and function as a reinforcer itself (conditioned reinforcer) through associative learning. This dynamic illustrates the complexity of behavioral chains and the intricate interplay between discriminative stimuli and conditioned reinforcers.