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The Role of Response-Reward Correlation in Stimulus-Response Overshadowing

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.