Three models of conditional discrimination learning by pigeons are described: stimulus configuration learning, the multiple-rule model, and concept learning. A review of the literature reveals that true concept learning is not characteristic of the behavior of pigeons in matching-to-sample, oddity-from-sample, or symbolic matching studies. Instead, pigeons learn a set of sample-specific SD rules. Transfer of the discrimination to novel stimuli, at least along the hue dimension, is predicted by a "coding hypothesis", which holds that pigeons make a unique, but usually unobserved response, R,, to each sample, and that the comparison stimulus chosen depends on which R1 was emitted in the presence of the sample. Convincing evidence is found that pigeons do code sample hues, but there is little evidence that allows one to infer that the "coding event" must have behavioral properties. Parameters of the conditional discrimination paradigm are identified, and it is shown that by appropriate parametric manipulation, a variety of analogous tasks may be generated for both human and animal subjects. The tasks make possible the comparative study of complex learning, attention, memory, and information processing, with the added advantage that behavior processes may be compared systematically across tasks.

Pigeons had no greater difficulty learning a complex discrimination involving arbitrary among stimuli (symbolic matching) than one involving interrelations based on stimulus similarity (matching-to-sample). The relative rates of acquisitions of matching and symblic matching may be accounted for by the discriminability between sample stimuli and between comparison stimuli, with the former playing the more important role.