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Models of interval timing typically include a response threshold to account for temporal production. The present study sought to evaluate the dependent concurrent fixed-interval fixed-interval schedule of reinforcement as a tool for selectively isolating the response threshold in rats, pigeons, and humans. In this task, reinforcement is available either at one location after a short delay or at another location at a longer delay. Because the reinforced location is not signaled, subjects normally respond on the first location and, if reinforcement is not delivered, then switch to the second location. The latency to switch between locations served as the primary dependent measure. After training rats, pigeons, and humans with equal reinforcement magnitudes in the short and long delays, the magnitude of reinforcement was increased threefold on the long-delay location. Consistent with model predictions, this biasing procedure decreased estimates of the response threshold of rats and pigeons, but also reduced temporal control in these species and increased response-threshold estimates in humans. Human and pigeon performance also suggested a magnitude-induced increase in the speed of the internal clock. Collectively, these results suggest that differences in reinforcement magnitude between response alternatives appear to modulate the response threshold, but not selectively, and may provide guidance for better isolating response-threshold effects in humans. [ABSTRACT FROM AUTHOR]