Responding by pigeons on one key of a two-key chamber alternated the color of the second key, on which responding produced food according to a variable-interval schedule of reinforcement. From time to time, reinforcement would be available for a response, but in the presence of a particular stimulus, either red or green light on the key. Red or green was chosen irregularly from reinforcement to reinforcement, so that a proportion of the total number of reinforcements could be specified for each color. Experimental manipulations involved variations of (1) the proportions for each color, (2) changeover delay, or, alternatively, (3) a fixed-ratio changeover requirement. The main findings were: (1) relative overall rates of responding and relative times in the presence of a key color approximated the proportions of reinforcements obtained in the presence of that color, while relative local rates of responding changed little; (2) changeover rate decreased as the proportions diverged from 0.50; (3) relative overall rate of responding and relative time remained constant as the changeover delay was increased from 2 to 32 sec, with reinforcement proportions for red and green of 0.75 and 0.25, but they increased above 0.90 when a fixed-ratio changeover of 20 responses replaced the changeover delay; (4) changeover rate decreased as the delay or fixed-ratio was increased.Concurrent schedules specify that two (or more) reinforcement schedules function simultaneously. There are two (or more) operants, and reinforcement for each is scheduled independently. By one method, each operant and its schedule are assigned to an individual response key. By a second, all of the schedules are assigned to the same key (main key) and different exteroceptive stimuli are associated with each operant-schedule pair. Responses on a second key (changeover key) alternate the exteroceptive stimulus and the schedule in effect on the main key. Each schedule functions continuously, whether or not it is assigned to the main key at a given moment. The two procedures appear to be equivalent (Catania, 1966) Pliskoff, 1968;Catania, 1963Catania, , 1966Herrnstein, 1961

The pigeon and the rat partition total response output between both schedules of a concurrent variable-interval pair. The quantitative nature of a partition seems critically dependent on the relative rates with which the two schedules provide reinforcements for responding, in addition to the changeover delay. The manner in which the changeover delay controls the partition was studied by varying the duration of the changeover delay from 0 to 20 sec with each of two pairs of concurrent variable-interval schedules, viz., Conc VI 1.5-min VI 1.5-min and Conc VI 1-mimi VI 3-min. Rats served as the subjects and brain stimulation was employed as the reinforcer. When the schedules were Conc VI 1.5-min VI 1.5-min, relative response rate approximated 0.50 at all values of the changeover delay. When the schedules were Conc VI 1-min VI 3-min, relative response rate, computed with respect to the VI 1-min schedule, increased when the duration of the changeover delay increased. Changeover rate decreased when the duration of the changeover delay increased. The decrease was the same for both VI schedules of the Conc VI 1.5-min VI 1.5-min pair but was more rapid for the VI 3-min schedule of the Conc VI 1-min VI 3-min pair.A concurrent pair of variable-interval (VI) schedules of reinforcement involves the assignment of one schedule to each of two response keys. The VI schedules are independent, and the availability of reinforcement on one of the keys is unaffected by the availability of reinforcement on the other. However, reinforcements made available on a given key are produced by responses on that key only.The

Reinforcements were arranged independently of the pigeon's behavior by concurrent variable-interval schedules. The reinforcements arranged by one of the schedules occurred when the chamber was illuminated with amber light, and the reinforcements arranged by the other schedule occurred when the chamber was illuminated with blue light. Both schedules functioned concurrently, but reinforcers were delivered by each only in the presence of the appropriate stimulus condition. A response on a white key, the only key in the chamber, alternated the stimulus condition and the effective schedule. The results of this procedure were similar to those obtained with concurrent response-dependent variable-interval schedules of reinforcement. The proportion of the total session time spent in the presence of a schedule component approximated the proportion of the total number of reinforcements in the component. Changeover rate was a decreasing function of the changeover delay and of the difference between the relative rates of reinforcement for each pair of concurrent schedules.

Two variable-interval 3-min schedules functioned concurrently to arrange reinforcement of a pigeon's pecks on a single key, the main key. Each schedule was associated with a distinct color of the main key; a response on a second key alternated the color and schedule assignment of the main key. A changeover delay, a period of time following schedule and key-color alternation during which reinforcement of responding on the main key could not occur, was arranged with equal or with unequal durations for the two directions of alternation. Durations were varied from 0.33 sec to 27 sec, in addition to no delay. With equal delays for the two directions of alternation, the pigeon alternated the schedules less often the larger the delay duration. When the delays in the two directions of alternation were unequal, it could be shown that alternation of the schedules was reduced both by a delay just incurred by the last alternation and by a. delay to be incurred by the next. The latter delay was more potent in reducing the frequency of alternations.

The discriminative control over a spatial choice response exercised by prior behavior was studied using a procedure involving discrete exposures to a two-member chained schedule. The initial member (red key) was either a smaller or larger fixed ratio (Mix FR:FR), the completion of which produced, after a 1-sec delay, two white response keys. If the larger FR had been completed as the initial chain member, a single peck on the right white key was reinforced; after the smaller FR, a peck on the left white key was reinforced. Frequencies of unreinforced responses (S(Delta) responses) were determined with several pairs of red-key FRs: 95-5, 75-25, 65-35, 60-40, 58-42 and 50-50. The S(Delta) response frequencies were low through the FR pair 65-35; sharp increases were obtained with pairs 60-40 and 58-42. Later, curves analogous to stimulus generalization functions were obtained using a probe procedure. Finally, the delay interval between completion of a red-key FR and the white-key choice response was manipulated: results were variable, but S(Delta) response frequencies tended to increase with increasing delays.