Experiments with pigeons and rats showed that: (1) When a brief blackout was presented in lieu of reinforcement at the end of 25% of intervals on a fixed-interval 2-min schedule, response rate was reliably and persistently higher during the following 2-min intervals (omission effect). This effect was largely due to a decrease in time to first response after reinforcement omission. (2) When blackout duration was varied, within sessions, over the range 2 to 32 sec, time to first response was inversely related to the duration of the preceding blackout, for pigeons, and for rats during the first few sessions after the transition from FI 2-min to FT 2-min with reinforcement omission. Post-blackout pause was independent of blackout duration for rats at asymptote. These results were interpreted in terms of differential depressive effects of reinforcement and blackout on subsequent responding.

Pigeons were exposed to four cycles per session of a schedule in which the duration of successive interreinforcement intervals differed by t-sec. A cycle was composed of seven increasing and seven decreasing intervals, from 2t to 8t sec in length. In Exp. 1, postreinforcement pause tracked interval duration on five cyclic schedules, with values of t ranging from 2 to 40 sec. Tracking was better at shorter t values, and when discriminative stimuli signalled increasing and decreasing parts of the cycle. Pooled data for the whole experiment showed postreinforcement pause to bear a power function relationship to interval length, with a smaller exponent than the comparable function for fixedinterval schedules. Tests in a second experiment showed that pigeons trained on an arithmetic progression could also track schedules in which successive intervals followed either a logarithmic or a geometric progression, although tracking was more stable in the logarithmic case.Cyclic-interval schedules of reinforcement (Staddon, 1964(Staddon, , 1967) are a form of mixed reinforcement schedule in which the same sequence of different interreinforcement intervals is presented in each experimental session. The simplest form of cyclic schedule involves intervals of only two durations. Staddon (1967) examined a two-valued schedule in which a cycle was made up of twelve 1-min intervals followed by four 3-min intervals. He found that post-reinforcement pause remained approximately constant across the four cycles making up a session. Pause duration was short, and seemed more appropriate to the 1-min than to the 3-min intervals. Other similar two-valued schedules have been studied (Kello, 1969;Innis and Staddon, 1970). In terms of overall response rate, the behavior generated by all these cyclic schedules has been similar-a higher rate during the longer intervals than during the sequence of twelve 1-min intervals. A more detailed examination of response rate across successive intervals in a cycle, however, reveals some unexpected differences. When the long inter-'This paper is based on a dissertation submitted to Duke University in partial fulfillment of the requirements for the Ph.D. degree. The research was supported by grants from the National Science Foundation, and the National Institute of Mental Health, USPHS, to Duke University (J. E. R. S., Principal Investigator).2Reprints inay be obtained from J. E. R. Staddon, Dept. of Psychology, Duke University, Durham, N. C. 27706.vals were 2 min, rate declined across successive 2-min intervals; when they were 3 min, it remained constant; when they were 6 min, it increased. Corresponding changes in postreinforcement pause were also recorded; during the second and subsequent of a series of 2-min intervals pauses became longer, while during the second of two 6-min intervals pause decreased, and if more than two such intervals were scheduled it remained low.In situations where successive interreinforcement intervals change progressively, rather than abruptly, changes in postreinforcement pause ...

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There are similarities between pigeons' behavior under interval reinforcement schedules and the behavior of rats in runways. A free-operant experiment analogous in certain respects to the double-runway procedure produced large "frustration effects" in pigeons, lending support to this comparison.A previous experiment (Staddon, 1964) showed that exposure of pigeons to a reinforcement schedule in which the frequency of reinforcement varied cyclically as a function of time, induced a periodicity in their responding matching that of the schedule, but outof phase with it; i.e., the birds' maximum rate of responding coincided with reinforcement rate minima. This phase lag seems to be independent of absolute reinforcement rate, period of cycle and amplitude of cycle over a wide range.There are similarities between this finding and the results of classical frustrative nonreward experiments (e.g., Amsel & Roussel, 1952;Bower, 1962). In the Amsel experiment rats ran to obtain food in a goal box at the end of a runway; after a fixed delay they were then allowed to run to a second goal box at the end of a second runway. When food was available in the second goal box on all trials, but in the first on only 50% of trials, running speed in the second runway was reliably higher following unrewarded trials in the first runway. This elevation in the response measure is the "frustration effect" (FE).The parallel between cyclic schedules and the Amsel procedure is apparent if we consider the simplest cyclic situation, in which the schedule provides a short fixedinterval followed by a long fixed-interval followed by a short fixed-interval and so on in a repetitive sequence. Under these conditions the animal will respond most rapidly during the long fixed-intervals (i.e., when reinforcement rate is least) (cf., Skinner, 1938, p. 272). Ignoring for the moment the stimulus difference between the two runways, the two procedures may be compared thus: when reward is available in the first goal box, running in each runway corresponds to a short fixed-interval; when reward is only available in the second goalbox, running in runway 1 followed by running in runway 2 corresponds to a long fixed-interval. Hence the elevation in running speed in runway 2 following nonreward is analogous to the higher response rates found in the longer of the two fixed-intervals in the cyclic procedure.In an attempt to explore this analogy a free-operant experiment similar in certain respects to the Amsel & Roussel procedure has been conducted. Instead of run-

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