Visual stimulus compounding with pigeons

Eight pigeons responded to successively presented color and line stimuli correlated with different likelihoods of reinforcement (100, 50 and 10%). An intertriai interval was included during training for half of the birds. In Experiment 1, tests with superimposed compounds resulted in variable responding, but overall, rates of responding were intermediate to the rates controlled by the individual component stimuli. In Experiments 2a and 2b, tests with compounds of spatially separated stimuli yielded rates of responding that were intermediate to, and in some cases lower than, responding controlled by the individual component stimuli. These results suggest limitations to the generality of additive models of stimulus compounding when visual stimulus combinations are employed.