| Resumo: | In natural environments, most animals survive because they learn to respond appropriately to cues that signal the availability of food, a mate, or a predator. Sometimes there is more than one cue signaling the same outcome, and oftentimes these cues can change and become less reliable. In the laboratory, discrimination reversal tasks are good tests of behavioral adaptability to regularly changing environments. In this series of studies, we explore the determinants and the dynamics of behavior when time and the outcome of the previous response simultaneously signal the availability of a potential reinforcer. Hence, we analyzed the performance of rats and pigeons in different versions of the midsession reversal task. The traditional task consists of a simple simultaneous discrimination where responses to one stimulus are reinforced and responses to the other stimulus are not and, once throughout the session, contingencies reverse and the previously reinforced stimulus is now extinguished and vice versa. We used this task because it allows the independent manipulation of time and response outcomes as cues for future reinforcement: We manipulated the reliability of the outcomes by providing either continuous or partial reinforcement for each response alternative and manipulated the reliability of time by fixing the moment of reversal or making it unpredictable. Results suggest that behavioral control alternates between outcomes and time according to the relative reliability of each cue. Simple mathematical model simulations show that outcomes and time may jointly determine behavior, and that momentary reinforcement rate may determine their relative influence. We offer a general account of how animals may adapt to regularly changing environments.
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