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Peak Shift on an Artificial Dimension

Blough (1975) proposed an elemental model of generalization and discrimination phenomena in which stimuli from physical dimensions, such as wavelength, are conceptualized as overlapping sets of elements. In order to test predictions generated by this model, we constructed an artificial “dimension” of stimuli as a series of overlapping sets of arbitrary “icons” (small, unrelated shapes). In Experiment 1, we trained pigeons to discriminate two neighbouring stimuli from this artificial dimension and then assessed their responses to other stimuli from the dimension. The results of these tests typified those obtained with stimuli from genuine physical dimensions-that is, we obtained positive and negative peak shifts. In Experiment 2, human subjects were given a similar discrimination task and produced an analogous set of results to those of the pigeons. These results support Blough's elemental theory and are not readily accommodated by Pearce's (1987) configural theory. In Experiment 3, both humans and pigeons were trained and tested with stimuli from a real physical dimension (luminance). The pigeons’ results were again consistent with Blough's analysis, but those of the humans suggested the use of more sophisticated strategies that are unavailable to pigeons and that lie outside the scope of elemental models of discrimination learning.