The fixed-interval schedule of reinforcement is one of the more widely studied schedules in the experimental analysis of behavior and is also a common baseline for behavior pharmacology. Despite many intensive studies, the controlling variables and the pattern of behavior engendered are not well understood. The present study examined the microstructure and superstructure of the behavior engendered by a fixed-interval 5-and a fixed-interval 15-minute schedule of food reinforcement in the pigeon. Analysis of performance typical of fixed-interval responding indicated that the scalloped pattern does not result from smooth acceleration in responding, but, rather, from renewed pausing early in the interval. Individual interresponse-time (IRT) analyses provided no evidence of acceleration. There was a strong indication of alternation in shorter-longer IRTs, but these shorter-longer IRTs did not occur at random, reflecting instead a sequential dependency in successive IRTs. Furthermore, early in the interval there was a high relative frequency of short IRTs. Such a pattern of early pauses and short IRTs does not suggest behavior typical of reinforced responding as exemplified by the pattern found near the end of the interval. Thus, behavior from clearly scalloped performance can be classified into three states: postreinforcement pause, interim behavior, and terminal behavior. Key words; fixed-interval schedule of reinforcement, pauses, interresponise-time analysis, microanalysis, cumulative record, factor analysis, second-order deviations, pigeonsThe fixed-interval schedule of reinforcement is perhaps the simplest schedule of intermittent reinforcement to arrange: The first appropriate response occurring a fixed time after some stimulus event (typically the previous reinforcer) is reinforced. However, the simplicity of the arrangement belies the complexity of the behavior that results.The behavior engendered by fixed-interval schedules, over a wide range of temporal parameters, has been subjected to intensive study in the experimental analysis of behavior because it models what is called temporal con-

Previous studies of choice between two delayed reinforcers have indicated that the relative immediacy of the reinforcer is a major determinant of the relative frequency of responding. Parallel studies of choice between two interresponse times have found exceptions to this generality. The present study looked at the choice by pigeons between two delays, one of which was always four times longer than the other, but whose absolute durations were varied across conditions. The results indicated that choice is not uniquely determined by the relative immediacy of reinforcement, but that absolute delays are also involved. Models for concurrent chained schedules appear to be more applicable to the present data than the matching relation; however, these too failed to predict choice for long delays.Key words: choice, delay of reinforcement, matching law, IRTs, concurrent-chain schedules, pigeons When a reinforcer is delayed, generally it becomes less effective in strengthening or maintaining behavior. The effects of delayed reinforcement have been assessed through a variety of techniques. For example, delays have been studied with concurrent-schedule procedures where the effects due to one delay value can be assessed relative to another delay value. Chung (1965) arranged a two-key concurrentschedule-of-reinforcement procedure for pigeons under which responding on one key was immediately reinforced while responding on the other key was reinforced after a delay. The overall rates of reinforcement were equated by including blackouts on the immediate-reinforcement key. Chung found that the absolute and relative response rates on the delay key

After key pecking had been autoshaped, six pigeons were exposed to a condition in which the duration of grain availability at the end of an 8-second trial depended on the number of responses emitted during the trial (0.25-second access to grain per response). This procedure, called correlated reinforcement, alternated across conditions with the automaintenance baseline in which the 8-second trial terminated with a constant 2.5-second access to grain. Two control procedures were run; in both, the reinforcer durations were yoked to those obtained in the last correlated session. In the yoked control no responses were required, but in the single-response yoked control at least one response was required to receive the yoked duration. The correlated condition maintained response rates above those produced by the two control conditions. These results may be accounted for by differential reinforcement.

Male Long-Evans rats were injected with 0, 1, 3, or 6 mg/kg of cadmium chloride on the first day of life. Animals free of morphological stigmata at weaning were selected for study. Tissue concentrations of cadmium and operant behavior under various fixed-ratio (FR) schedules of reinforcement were evaluated when these rats were adults. Dose-related increases in cadmium were present in the brains, livers, and kidneys. Dose-related differences in behavior were most evident during the transition from fixed ratio 25 (FR 25 or 25 responses/reinforcer) to FR 75. An inverted U describes the relationship between response output during the transition to FR 75 and cadmium chloride dose response output increased at 3 mg/kg and decreased at 6 mg/kg. The rate decreases were not correlated with weight loss that appeared after some of the animals exposed to 6 mg/kg reached 60 days of age. Challenge doses of d-amphetamine revealed no interaction between neonatal exposure to cadmium and d-amphetamine. The occurrence of alterations in operant behavior in animals that appeared normal on a number of preweaning evaluations suggests that operant behavior in transition was sensitive to subtle effects not observed with other commonly used tests. The data provide evidence for delayed effects in the adult that are due to neonatal exposure to cadmium.

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