This study examined how two models of timing, scalar expectancy theory (SET) and learning to time (LeT), conceptualize the learning process in temporal tasks, and then reports two experiments to test these conceptualizations. Pigeons responded on a two-alternative free-operant psychophysical procedure in which responses on the left key were reinforceable during the first two, but not the last two, quarters of a 60-s trial, and responses on the right key were reinforceable during the last two, but not the first two, quarters of the trial. In Experiment 1 three groups of birds experienced a difference in reinforcement rates between the two keys only at the end segments of the trial (i.e., between the first and fourth quarters), only around the middle segments of the trial (i.e., between the second and third quarters), or in both end and middle segments. In Condition 1 the difference in reinforcement rate favored the left key; in Condition 2 it favored the right key. When the reinforcement rates differed in the end segments of the trial, the psychometric function-the proportion of right responses across the trial-did not shift across conditions; when it occurred around the middle of the trial or in both end and middle segments, the psychometric function shifted across conditions. Experiment 2 showed that the psychometric function shifts even when the overall reinforcement rate for the two keys is equal, provided the rates differ around the middle of the trial. This pattern of shifts of the psychometric function is inconsistent with SET. In contrast, LeT provided a good quantitative fit to the data.Key words: models of timing, learning to time, psychometric function, temporal bisection, key peck, pigeonThe scalar property has loomed large in recent studies, theories, and discussions of how animals time events. Indeed it has overshadowed the analysis of other equally critical aspects of timing. One of these aspects is how animals learn to regulate their behavior in time, or, equivalently, how current theories conceptualize the learning process in timebased tasks. The goal of the present study was to contrast the learning conceptualizations of two models of timing: scalar expectancy theory (SET; e.g., Gibbon, 1977 and a derivative of Killeen and Fetterman's (1988) behavioral theory of timing (BeT; see also Killeen, 1991) called learning to time (LeT; Machado, 1997; see also Machado & Cevik, 1998;Machado & Keen, 1999). We start with a description of the two models, paying particular attention to learning-related assumpThis research was supported by a FIRST award grant from the National Institute of Mental Health to the first author. We thank students Mark Cannon, Erica Gerstman, and Wendy Tai for their help in conducting the experiments.Address correspondence to Armando Machado, Department of Psychology, Indiana University, 1101 East 10th Street, Bloomington, Indiana 47405 (E-mail: amachado@indiana.edu).tions, and then use the models to try to understand a puzzling result obtained with a free-operant psychophysical pro...