Four groups of four pigeons each were studied on two different multiple schedules. The cues correlated with the schedule components were localized on the response key for two groups and were not localized for the others. Two groups worked on multiple schedules with variable interval 15-sec in both components, and variable interval 15-sec in one component and extinction in the other. The other two groups had identical procedures except that food was presented on a response-independent variable-time schedule. Variable-interval birds with localized stimuli showed marked behavioral contrast; variable-interval birds with non-localized stimuli showed no behavioral contrast. Variable-time birds with key-light stimuli acquired high rates of autopecking, which changed as treatment changed in a manner that paralleled rate changes, resulting in behavioral contrast for variable-interval birds. Variable-time birds with non-localized stimuli key pecked only at a low rate. The findings indicate that behavioral contrast in pigeons may result from the autopecking that is obtained with stimulus-contingent food presentation
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