| Summary: | Pigeons were trained to discriminate between four keys, one of which provided food in the mornings, another which provided food in the afternoons, and two which never provided food. Three experiments were performed to determine if pigeons could track food availability over a 24-hr period. In Experiment 1, food was available at one place (pecking key) in the mornings and at a different place in the afternoons. Although the length of time between sessions was much greater than in previous studies (e.g., Wilkie & Willson, 1992), all subjects appeared to demonstrate time-place associative learning. In order to rule out the possibility of an alternation strategy, in Experiment 2 only morning or only afternoon sessions were given. Subjects maintained well above chance performance, showing that they were not simply alternating between the two rewarded locations. In Experiment 3, the length of time between morning and afternoon sessions was varied. The results indicated an increase in errors as the inter-session interval decreased, which provides further support for a timing as opposed to an alternation mechanism. Experiment 4 was designed to investigate mechanisms underlying the timing behaviour. Lights-on time was shifted back by 6 hr and no decrease in performance was found during the first session following this phase shift. This finding rules out an interval timer and suggests that a circadian type of timing mechanism, with a self-sustaining oscillator, mediates time-place learning over a period of 24 hr. Further support for this notion was found in Experiment 5 in which subjects were tested in constant dim light. In that experiment subjects' continued correct responding provides additional support for a self-sustaining circadian timer.
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