The contingency between responding and stimulus change on a chain variable-interval 33-s, variable-interval 33-s, variable-interval 33-s schedule was weakened by interposing 3-s delays between either the first and second or the second and third links. No stimulus change signaled the delay interval and responses could occur during it, so the obtained delays were often shorter than the scheduled delay. When the delay occurred after the initial link, initial-link response rates decreased by an average of 77% with no systematic change in response rates in the second or third links. Response rates in the second link decreased an average of 59% when the delay followed that link, again with little effect on response rates in the first or third links. Because the effect of delaying stimulus change was comparable to the effect of delaying primary reinforcement in a simple variable-interval schedule, and the effect of the unsignaled delay was specific to the link in which the delay occurred, the results provide strong evidence for the concept of conditioned reinforcement.

Melioration theory entails that matching in concurrent schedules occurs because the subjects equalize the local reinforcement rates (reinforcers received for each alternative divided by the time allocated to each alternative). The role of local reinforcement rates was tested by using multiple schedules in which one component involved an alternative with a high absolute rate of reinforcement and a low local reinforcement rate while the second component involved an alternative with a low absolute rate and a high local rate. These alternatives were then presented simultaneously in probe trials to determine preference between them. Contrary to melioration, the absolute rate of reinforcement, not the local rate, was the controlling variable.

Pigeons were presented a concurrent-chains schedule of reinforcement that had terminal links of equal duration. The initial links of the schedule were periodically interrupted by 15-s periods during which an extinction schedule was in effect. The extinction periods were presented on either a responsecontingent or a noncontingent basis. Relative response rate for the left alternative decreased when the extinction periods were accompanied by the left terminal-link stimulus. Relative response rate for the right alternative decreased when the extinction periods were accompanied by the right terminal-link stimulus. Relative response rate varied inversely with the frequency of presentation of the extinction periods but was unaffected by presence versus absence of the response contingency in the schedule of extinction-period presentation. Furthermore, relative response rate was unaffected by presentation of extinction periods accompanied by a novel stimulus. When the extinction periods were presented after reinforcement in the left terminal link instead of as interruptions of the initial links, relative response rate for the left alternative was reduced if the postreinforcemcnt extinction period was accompanied by the terminal-link stimulus for the left chain and reduced less if the extinction period was accompanied by the terminal-link stimulus for the right chain. The results demonstrate that the correlation between the terminal-link stimulus and extinction influenced the relative response rate in the initial link.Key words: conditioned reinforcement, stimulus value, extinction, concurrent chains, choice, pigeonsThe concept of a conditioned reinforcer entails an initially neutral stimulus acquiring properties of a reinforcer via a history of correlation with a primary reinforcer. One of the most common procedures to study the properties of conditioned reinforcers uses chain schedules in which the initial links are concurrently available (concurrent chains), and the terminal-link stimuli are correlated with different parameters of primary reinforcement. The critical measure is the relative rate of responding ("preference") strength can then be assessed by the effects of various parameters of the terminal-link schedules of primary reinforcement on preference.Early research with the concurrent-chains procedure suggested that preference was determined by the rate of reinforcement during the terminal links. For example, Herrnstein (1 964b) demonstrated that the relative rate of responding in initial links matched the relative rate of reinforcement on a variable-ratio schedule in one terminal link and a variable-interval (VI) schedule in the other despite the differing response requirements. But subsequent research demonstrated that a simple arithmetic average of the reinforcement rates in the two terminal links did not predict preference. Herrnstein (1964a), Fantino (1967), and Killeen (1968) compared fixed versus variable terminal-link schedules, and all reported higher rates in the initial link leading to the variable...

Six pigeons responded on concurrent-chains schedules with either independent or interdependent equal variable-interval schedules in the initial links and unequal variable-interval schedules, always in a 2:1 ratio, in the terminal links. Relative response rates in the initial links increased across conditions as initial-link duration was shortened and decreased across conditions as terminal-link duration was shortened, replicating previous findings. Responses in the initial links were recorded in 5-s bins, and local or molecular relative response rates were calculated in order to ascertain how relative response rate varied as a function of time since the onset of the initial links. Two distinct molecular patterns were found. With interdependent initial links, relative response rates for the preferred key were elevated for the first 10 or 20s of the initial links and then declined to an asymptotic value. With independent initial links, a negative recency effect was found similar to that reported by Killeen (1970). These two molecular patterns were related to the different momentary reinforcement probabilities resulting from independent and interdependent scheduling.Key words: choice, molecular analysis, conditioned reinforcement, delay-reduction hypothesis, concurrent-chains schedules, variable-interval schedules, forced-choice procedure, key peck, pigeonsThe concurrent-chains schedule has been used widely in research during the last 25 years (see Fantino, 1977, for a review). Although it was originally developed for study of conditioned reinforcement (Autor, 1960(Autor, , 1969, the concurrent-chains schedule has more often been used to investigate preference between different schedules of reinforcement, a task to which the procedure seems particularly well suited because it allows a measure of choice uncontaminated by the particular patterns of responding generated by the schedules that are chosen. In this procedure, the subject responds on two concurrently available alternatives (the initial links or "choice phase") usually correlated with equal variable-interval (VI) schedules. Responses on each alternative occasionally produce another stimulus, correlated with entry into the terminal link of the chain on that alternative (the "outcome phase"). Responses in the terminal link are reinforced with food. The independent variable has generally involved some difference in the conditions arranged during the terminal links, such as differing rates of reinforcement. in-145 1987, 48, 145-159 NUMBER 1 (JULY)

Pigeons were trained on three-component chain schedules in which the initial component was either a fixed-interval or variable-interval schedule. The middle and terminal components were varied among fixed-interval fixed-interval, variable-interval variable-interval, and an interdependent variable-interval variable-interval schedule in which the sum of the durations of the two variable-interval components was always equal to the sum of the fixed-interval fixed-interval components. At issue was whether the response rate in the initial component was controlled by its time to primary reinforcement or by the temporal parameters of the stimulus correlated with the middle terminal link. The fixed-interval initial-link schedule maintained much lower response rates than the variable-interval initial-link schedule regardless of the schedules in the middle and terminal links. Nevertheless, the intervening schedules played some role: With fixed-interval schedules in the initial links, response rates were consistently highest with independent variable-interval schedules in the middle and terminal links and intermediate with the interdependent variable-interval schedules; these initial-link differences were predicted by the response rates in the middle link of the chain. With variable-interval schedules in the initial links, response rates were lowest with the fixed-interval fixed-interval schedules following the initial link and were not systematically different for the two types of variable-interval variable-interval schedules. The results suggest that time to reinforcement itself accounts for little if any variance in initial-link responding.