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Background: Stimuli perfectly correlated with impending access to food have been shown to both attenuate response rates and to prevent response acquisition when they occur during delays to reinforcement. One explanation of these findings is that the stimulus better predicts food than the operant response itself, and therefore, 'blocks' learning of the response-reinforcer association. That such stimuli can abolish operant learning implies a breakdown in an organism's ability to detect causality between its own behavior and effects on the environment. Method: Two response acquisition experiments in which a stimulus preceded food delivery were conducted. In one experiment, an attempt was made to replicate the prevention of response acquisition using a non-resetting delay procedure that parallels those that result in overshadowing. In a second experiment, stimulus-food pretraining was given to provide a better parallel to typical respondent-conditioning blocking procedures. Results: Under neither circumstance was response acquisition prevented. Discussion: The generality and robustness of blocking the response-reinforcer association in operant response acquisition is questioned. [ABSTRACT FROM AUTHOR]

Can rats discriminate between short (i.e., 10 s) and long (i.e., 50 s) intervals of opportunity to run in running wheels? Eight female Long- Evans rats were exposed to 50 opportunities to run in a wheel. Following each interval, rats made a response to identify the interval. A correct response produced 0.1 ml of 15% sucrose solution, while an incorrect response caused the chamber lights to flash on and off for 5 s. Five of 8 rats met the criterion of 5 successive sessions with 80% correct responses for both durations. Furthermore, the criterion of 80% correct responses in a session was met more often with the 10-s than with the 50-s duration. Rats can discriminate between opportunities to run for different durations, but the results cast doubt on the assumption that rats are timing the intervals. [ABSTRACT FROM AUTHOR]