In a series of 3 experiments, a total of 12 pigeons were trained to discriminate between arrays containing equal numbers of 2 different elements as stimulus-positive, and arrays that contained more elements of one kind than the other as stimulus-negative. Ss were then tested with the full range of the proportions of the 2 elements. This resulted in behavioral contrast and peak shift, as the Ss responded more to arrays containing more positive than negative elements than they did to the positive training arrays. Findings were obtained with elements that differed in color (blue vs red dots) and with elements that differed in orientation (horizontal vs vertical rectangles). Results indicate that the stimulus control exerted by the derived dimension of relative numerosity involved the same processes as the fundamental dimensions that characterize simple stimulus elements.
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