Pigeons' key pecks turned off a key light, which also went off independently of their pecks. The pigeons were able to correctly discriminate the cause of the stimulus change, although their attributions were strongly affected by the amount and the delay of reward given for correct responses. Their discrimination was based on the asynchrony between a response and the change in the key light. A simple detection model that combined detectability and motivational factors provided a good description of the data. It was shown that the discriminative criteria did not change with changes in distribution of noise events in an optimal fashion and that the pigeons therefore were not ideal detectors. In the final experiment, the pigeons were asked to discriminate the cause of a key light change, of a hopper illumination, and of a feeding. Performance decreased with each condition and with the duration of the last two events. It was noted that the memory trace for a stimulus change decays at the same rate as the primary reinforcement gradient but that it decays faster when the delay is filled with an event such as reinforcement. The possibility that the effects of reinforcement may be blocked by reinforcement is briefly discussed. Discriminative stimuli are denned in terms of their impact on current behavior, but they also affect subsequent behavior. Analysis of memory in animals has been stimulated by theories of learning that posit an important role for it and other cognitive processes (e.g., Grant, Brewster, & StierhofF, 1983; Shimp, 1976a; Wagner, Rudy, & Whitlow, 1973). The stimuli to be remembered may be not only lights and tones but also reinforcement schedules (Lattal, 1975; Rilling & McDiarmid, 1965), the animal's own behavior (Reynolds, 1966; Shimp, 1983), and even the animal's characterization of a particular behavior-reinforcement contingency (see e.g., Commons & Nevin, 1981). The techniques used in these studies permit evaluation of sensitivity to events in a motivational context different from the ones in which they occurred and thus allow the measurement of detectability separate from motivational biases. An instance of this approach that provides the prototype for the research to be reported