Pigeons made observing responses for stimuli signalling either a fixed-interval 30-sec schedule or a fixed-ratio x schedule, where x was either 20, 30, 100, 140, or 200 and the schedules alternated at random after reinforcement. If observing responses did not occur, food-producing responses occurred to a stimulus common to both reinforcement schedules. When the fixed-interval schedule was paired with a low-value fixed ratio, i.e., 20 or 30, the presentation of the stimulus reliably signalling the fixed-ratio schedule reinforced observing behavior, but the presentation of the stimulus reliably signalling the fixed-interval schedule did not. The converse was the case when the fixed-interval schedule was paired with a large-valued fixed ratio, i.e., 100, 140, or 200. The results demonstrated that the occasional presentation of the stimulus signalling the shorter interreinforcement interval was necessary for the maintenance of observing behavior. The reinforcement relationship was a function of the schedule context and was reversed by changing the context. Taken together, the results show that the establishment and measurement of conditioned reinforcement is dependent upon the context or environment in which stimuli reliably correlated with differential events occur.A conditioned reinforcer is a stimulus that has become effective as a reinforcer because it has signalled the availability of a primary reinforcer, i.e., a biologically important stimulus, or another conditioned reinforcer. A stimulus acquires conditioned reinforcing strength because of a specified experimental history of conditioning; in other words, through association with a primary reinforcer by a pairing or chaining operation.

Pigeons made observing responses for stimuli signalling the availability of either 10-sec or 2-sec access to grain on fixed-interval 1-min schedules. If observing responses did not occur, food-producing responses occurred to a stimulus common to both reinforcement magnitudes. When the stimuli remained on for the duration of the components and signalled differential reinforcement magnitudes, observing responses were maintained; however, when the stimuli remained on for 10 sec, observing responses decreased markedly. In addition, it was shown that the occasional presentation of the stimulus signalling 10-sec access to grain was necessary for the maintenance of observing behavior. A control condition demonstrated that when all the available stimuli signalled 6-sec access to grain, observing responses declined. Taken together, the results demonstrated that the occasional presentation of the stimulus that remained on for the duration of the component and signalled the larger reinforcement magnitude was necessary for the maintenance of observing behavior.

Pigeons received food for responding on a fixed-interval 32-sec schedule divided into three equal parts, each correlated with a distinctive, response-independent, visual stimulus. Response rate was very low during the first two thirds of the interval but high during the terminal third. When a response-dependent brief stimulus correlated with the terminal third was arranged for each response in the presence of the stimuli correlated with the first two thirds, response rate was enhanced, especially in the middle third. However, response rate was suppressed when each response in the presence of the stimulus correlated with the final third produced a brief stimulus correlated with the initial third. A similar suppressive effect occurred when each response produced a brief stimulus correlated with the middle third. Response suppression decreased over successive responsedependent brief-initial-stimulus manipulations. The results were interpreted in terms of reinforcement, punishment, and discriminative stimulus control by visual stimuli correlated with parts of a fixed-interval schedule.Control of fixed-interval (FI) behavior by exteroceptive, discriminative stimuli can be demonstrated with either a continuous or discontinuous visual clock which demarcates the passage of time during an interval. For example, in experiments by Ferster and Skinner (1957), a small slit of light grew continuously and linearly throughout a lO-min interval, reaching its maximum size at reinforcement. FolIowing reinforcement, the clock reset to an initial value and started recycling. Ferster and Skinner showed that the pause after reinforcement lengthened and the terminal response rate increased as control by the clock developed.More recent studies of Fl clock schedules (Farmer

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The probability of observing-key responses for pigeons trained to observe during a mixed-fixed ratio 50 fixed-interval 2-min. schedule of reinforcement decreased sharply when only the mixed schedule or the corresponding multiple schedule was programmed. First session observing-key response probability values were greater in the mixed-schedule condition, suggesting that the mixed-schedule stimulus set the occasion for pecking the observing-key following reinforcement offset. In the mixed-schedule condition, the results for one bird suggested that stimuli associated with the early portion of each component controlled a relatively high response-rate; and, when the component was fixed-interval, stimuli arising from the organism's behavior controlled a pause which followed more than 50 responses. This post-response pause contributed to the determination of a relatively low response-rate in the fixed-interval component of the mixed-schedule.