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In a two‐key pigeon chamber, variable‐interval reinforcement was scheduled for a specified number of pecks, emitted either on a single key or in a particular sequence on the two keys. Although the distribution of pecks between the two keys was affected by whether pecks were required on one or on both keys, the total pecks emitted was not; the change from a one‐key to a two‐key requirement simply moved some pecks from one key to the other. Thus, each peck preceding the one that produced the reinforcer contributed independently to the subsequent rate of responding; the contribution of a particular peck in the sequence was determined by the time between its emission and the delivery of the reinforcer (delay of reinforcement), and was identified by the proportion of pecks moved from one key to the other when the response requirement at that point in the sequence was moved from one key to the other.

Key pecking in the pigeon was maintained under chained schedules in which the completion of one schedule component initiated the next component, and food was presented upon completion of a sequence of components. Under the chained schedules studied, a particular key color appeared during more than one component, and different key colors appeared during the other components. When seven 1‐min fixed‐interval components comprised a chained schedule and the response key was the same color during the first, third, fifth, and terminal components, patterns of positively accelerated responding were maintained during all but the first two components of each sequence. In general, response rates were always lowest during the first one or two components and highest during the terminal component when as few as three and as many as eight components comprised a schedule. Increasing the number of components from three to eight showed that response rate during a component increased when it was no longer one of the initial two components of the schedule, even though its temporal relation to food presentation had not changed. Finally, when seven components comprised a schedule and the response key was one color during the first, third, and fifth and a different color during the last component, response rates were low during the first five components and high during the last two components preceding food presentation.

Two White Carneaux hen pigeons were exposed to a 60‐sec random‐interval baseline procedure. Six different exteroceptive stimuli were successively correlated, within a single session, with blocks of 10 reinforcement presentations. Following this training, a non‐contingent reinforcement procedure was instated with inter‐reinforcement intervals of 5, 15, 30, 60, 120, and 240 sec. Within a single session, each non‐contingent frequency was correlated with one of the previously presented discriminative stimuli. After an initial increase in the rate of responding as the result of a high density of non‐contingent reinforcements, the rate declined as exposure to each non‐contingent frequency was prolonged.

While rats were responding in a single‐lever apparatus to avoid electric shock, a signal was presented and followed by a 5‐min timeout period when all shocks were omitted. For the response‐dependent member of each yoked pair, the first response 60 sec after onset of the pre‐timeout signal terminated the signal and initiated timeout. The other, yoked animal was exposed to the same sequence except that signal termination and timeout onset were response independent. Under the response‐dependent condition, response rates in the presence of the signal increased relative to baseline rates. Rate increases also occurred when timeout was response independent, but were of lesser magnitude and reliability. Subsequent reversal of the yoking arrangement produced stable and equivalent rate increases under both conditions. Other findings were that increased rates in the presence of the signal diminished when timeout was omitted but were maintained for a time on an avoidance‐extinction baseline. In general, the results supported the conclusion of previous experiments that timeout from avoidance can serve as a positive reinforcer. The finding that response‐independent presentation of timeout produced rate increases, particularly after a history with response‐dependent timeout, was interpreted in terms of adventitious reinforcement of previously established behavior.

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Two rhesus monkeys responded on a fixed‐ratio schedule in Stimulus 1 (blue light) to avoid the onset of Stimulus 2 (green light). Failure to avoid Stimulus 2 required a second fixed‐ratio performance to avoid Stimulus 3 (red light) in the presence of which unavoidable shock occurred. Relative frequencies of avoidance performance in the blue light and in the green light were inversely related to the ratio requirement under each stimulus condition. Both differential response‐cost and avoidance‐failure probability factors were related to the observed changes.

Thirteen pigeons were exposed to a variety of second‐order schedules in which responding under a component schedule was reinforced according to a schedule of reinforcement. Under different conditions, completion of each component resulted in either (1) the brief presentation of a stimulus also present during reinforcement (pairing operation), (2) the brief presentation of a stimulus not present during reinforcement (nonpairing operation), or (3) no brief stimulus presentation (tandem). Brief‐stimulus presentations engendered a pattern of responding within components similar to that engendered by food. Patterning was observed when fixed‐interval and fixed‐ratio components were maintained under fixed‐ and variable‐ratio and fixed‐ and variable‐interval schedules. There were no apparent differences in performance under pairing and nonpairing conditions in any study. The properties of the stimuli presented in brief‐stimulus operations produced different effects on response patterning. In one study, similar effects on performance were found whether brief‐stimulus presentations were response‐produced or delivered independently of responding. Response patterning did not occur when the component schedule under which a nonpaired stimulus was produced occurred independently of the food schedule. The results suggest a reevaluation of the role of conditioned reinforcement in second‐order schedule performance. The similarity of behavior under pairing and nonpairing operations is consistent with two hypotheses: (1) the major effect is due to the discriminative properties of the brief stimulus; (2) the scheduling operation under which the paired or nonpaired stimulus is presented can establish it as a reinforcer.

Key pecking by pigeons was reinforced with food under second‐order schedules with fixedratio units. A constant total number of key pecks was required for reinforcement under each condition, but the size and, inversely, number of fixed‐ratio components were varied. The total response requirement of 256 pecks was divided into fixed‐ratio units of 128, 64, 32, 8, and 2 responses. A brief stimulus, which always preceded food reinforcement, was presented upon completion of each fixed‐ratio unit. Under most conditions, the pattern of within‐unit responding was typical of that under simple fixed‐ratio schedules. Overall response rate was an inverted U‐shaped function of component size. That is, response rates were highest under moderate sized units (fixed ratio 128 and 64). This relationship is consistent with previous determinations of rate as a function of fixed‐ratio value for simple fixed‐ratio schedules.

The performance of pigeons was studied under a second‐order schedule composed of fixed‐interval components, each of which was associated with a different discriminative stimulus, the stimuli occurring in a fixed order. In one condition, food presentation followed the completion of the fourth component. This was designated a fixed‐ratio sequence schedule. In another condition, responses in the first component completed after a fixed time were reinforced. This was designated a fixed‐interval sequence schedule. Although the stimulus order and maximum reinforcement frequency were identical under the two schedules, considerably more responding occurred under the fixed‐interval sequence schedule in all components. Relatively few food presentations occurred after responding during any but the terminal components of the fixed‐interval sequence schedule, a feature independent of the parameter values investigated. In addition, while a pattern of increased responding between food presentations prevailed under both schedules, under the fixed‐interval sequence schedule the rate in the terminal component was frequently less than in the penultimate component. The fixed‐interval sequence schedule appeared to have several properties of simple fixed‐interval schedules.

Responding by one pigeon was reinforced with food on fixed‐interval schedules of 30, 60, and 300 sec duration. A second pigeon was studied under fixed‐interval durations of 60 and 300 sec. For both birds, the average post‐reinforcement pause was one‐half the duration of the fixed interval. Autocorrelation coefficients revealed first‐order sequential dependencies in series of post‐reinforcement pauses. On the 300‐sec fixed‐interval schedule, successive post‐reinforcement tended to alternate between long and short durations. At the shorter fixed‐interval durations there was less evidence of alternation sequences. A second experiment was conducted to determine if the time intervals between the first response after reinforcement and the next reinforcement (the work periods) were responsible for the alternation patterns in the series of post‐reinforcement pauses. To evaluate the role of the work period, several procedures were used to modify the work period from that obtained on the fixed‐interval 300‐sec schedule. Adding a schedule to the fixed‐interval schedule that set the minimum amount of time that could elapse between the first response after reinforcement and the next reinforcement eliminated the alternation pattern. Control schedules indicated that the elimination of alternation patterns resulted from constraints on the work period per se and not from confounded changes in the interreinforcement intervals.