Rats pressed levers for food reward which was delivered, when appropriate, 0·4 s after the response. For one group, the delay interval was filled by a light cue; for the other group, the same number of lights was given but they were not correlated with food delivery. In Experiment I, all lever presses were reinforced and there were no differences in response rate between groups. In Experiments II and III, lever pressing was rewarded according to a VI and VR schedule respectively. Group differences were observed in Experiment II but they disappeared in Experiment III. The results of Experiments I and II show that a reward-related stimulus does not overshadow a lever response unless the stimulus is a better predictor of reward. Differences in salience or competition from sign-tracking behaviors were ruled out as causes of this phenomenon. Experiment III demonstrated, however, that a weak response-reward correlation is not a sufficient condition for the overshadowing effect. A fourth experiment replicated the results of Experiment III using naive animals. The results of these last two experiments are not consistent with an information theory approach unless (a) a response-units concept is adopted or (b) the cue involved in overshadowing is not the pre-food light but the end of a temporal interval, whose salience is enhanced by the light.

Previous research has shown that response rates on a variable interval (VI) schedule of reinforcement decrease if a brief response-produced signal is given prior to reward. One explanation is that the signal overshadows the response because it is a better predictor of reinforcement. The S-R overshadowing effect does not occur with variable ratio (VR) schedules, however. Tarpy, Lea, and Midgley (1983) explained this fact by suggesting that the signal functions to enhance the salience of the temporal interval offset on the VI schedule (a characteristic not possessed by VR schedules), which then overshadows the response. In this experiment, the salience of the temporal interval was enhanced in another way: signaled or unsignaled reward was provided to rats responding on either a VI or fixed interval (FI) reward schedule. As predicted, rates were lowest for animals receiving signaled reinforcement on an FI schedule and highest for those receiving unsignaled reinforcement on a VI schedule.

In two experiments, rats were trained to deposit ball bearings down a hole in the floor, using an algorithmic version of shaping. The experimenter coded responses expected to be precursors of the target response, ball bearing deposit; a computer program reinforced these responses, or not, according to an algorithm that mimicked the processes thought to occur in conventional shaping. In the first experiment, 8 of 10 rats were successfully shaped; in the second, 5 of 5 were successfully shaped, and the median number of sessions required was the same as for a control group trained using conventional shaping. In both experiments, "misbehavior," that is, excessive handling and chewing of the ball bearings, was observed, and when the algorithmic shaping procedure was used, misbehavior could be shown to occur in spite of reduced reinforcement for the responses involved.

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