In the present study, we examined whether manipulating the association between discriminative stimulus and reinforcers, which is believed to be involved in the control of instrumental behavior, would affect the stimulus control of the discriminative stimulus. In Phase 1, subjects received instrumental training using discriminative stimulus X. In Phase 2, the contingency between the discriminative stimulus and the reinforcer was manipulated by pairing another stimulus Y (experimental group) or the discriminative stimulus X (control group) with the reinforcer. In the test, we predicted that the instrumental response to the presentation of stimulus X would be decreased in the experimental group, where the contingency of the discriminative stimulus X and the reinforcer was decreased. As a result, there was no difference in the discrimination rates of the two groups in the test, and the manipulation of the contingency of the discriminative stimulus and the reinforcer had no effect on stimulus control of the discriminative stimulus. The results of this experiment suggest that stimulus control by discriminative stimulus may be determined by a different structure (e.g., the relationship between response and reinforcer) from a direct association with the reinforcer.

The present study examined memory retention over 24 hours in rats’ performance in a radial maze. Crystal & Babb (2008) demonstrated that rats can retain spatial memory for up to 25 hours in a radial maze, but these findings had not been reexamined until now. Therefore, the primary objective of our study was to replicate the results reported by Crystal & Babb (2008). Each trial included a learning phase, during which subjects were required to sequentially visit four randomly selected arms, followed by a free-choice test that encompassed all eight arms after a retention interval. Although Crystal & Babb cleaned the arms of the maze with Nolvasan solution during the retention interval, they did not conduct further controls for potential odor cues left by markings to identify unvisited baited arms. Consequently, the second objective of our study was to eliminate the possibility of the results being influenced by such odor cues. To achieve this, we covered touchable parts of the maze, including the arms and the platform, and alternated these covers among the arms, while also rotating the cover on the platform during the retention interval. Performance significantly exceeded chance levels under the 24-hour retention interval, demonstrating that rats can retain spatial memory for 24 hours, thus replicating the findings of Crystal & Babb (2008) without the influence of odor cues marked in the maze.