Pigeons' choices with token stimuli in concurrent variable-interval schedules

The several roles of stimuli in token reinforcement

Three experiments were conducted with pigeons to identify the stimulus functions of tokens in second-order token-reinforcement schedules. All experiments employed two-component multiple schedules with a token-reinforcement schedule in one component and a schedule with equivalent response requirements and/or reinforcer density in the other. In Experiment 1, response rates were lower under a token-reinforcement schedule than under a tandem schedule with the same response requirements, suggesting a discriminative role for the tokens. In Experiment 2, response rates varied systematically with signaling functions of the tokens in a series of conditions designed to explore other aspects of the temporal-correlative relations between tokens and food. In Experiment 3, response rates were reduced but not eliminated by presenting tokens independent of responding, yoked to their temporal occurrence in a preceding token component, suggesting both a reinforcing function and eliciting/evocative functions based on stimulus-food relations. Only when tokens were removed entirely was responding eliminated. On the whole, the results suggest that tokens, as stimuli temporally correlated with food, may serve multiple stimulus functions in token-reinforcement procedures--reinforcing, discriminative, or eliciting--depending on the precise arrangement of the contingencies in which they are embedded.

Rats' choices with token stimuli in concurrent variable-interval schedules

Four rats responded on concurrent variable-interval schedules that delivered token stimuli (stimulus lights arranged vertically above each of two side levers). During exchange periods, each token could be exchanged for one food pellet by responding on a center lever, with one response required for each pellet delivery. In different conditions, the exchange requirements (number of tokens that had to be earned before they could be exchanged for food) varied between one and four for the two response levers. The experiments were closely patterned after research with pigeons by Mazur and Biondi (2013), and the results from the rats in the present experiment were similar. Response percentages on the two levers changed as each additional token was earned, and these patterns indicated that choice was controlled by both the time to the exchange periods and the number of food pellets that were delivered in the exchange period. In some conditions, the exchange requirement was three tokens for each lever, but the token lights were not turned on as they were earned for one of the two keys. The rats showed a slight preference for the lever without the token lights, which may indicate that the token lights were not serving as conditioned reinforcers (a result also found by Mazur and Biondi with pigeons). Overall, these results suggest that, in this choice procedure, the token stimuli served primarily as discriminative stimuli that signaled the temporal proximity and quantity of the primary reinforcer, food.