Six-term transitive inference with pigeons: successive-pair training followed by mixed-pair training

In nonhuman animals, the transitive inference (TI) task typically involves training a series of four simultaneous discriminations involving, for example, arbitrary colors in which choice of one stimulus in each pair is reinforced [+] and choice of the other color is nonreinforced [-]. This can be represented as A+B-, B+C-, C+D-, D+E- and can be conceptualized as a series of linear relationships: A > B > C > D > E. After training, animals are tested on the untrained non-endpoint pair, BD. Preference for B over D is taken as evidence of TI and occurs because B is greater than D in the implied series. In the present study we trained pigeons using a novel training procedure-a hybrid of successive pair training (training one pair at a time) and mixed-pair training (training all pairs at once)-designed to overcome some of the limitations of earlier procedures. Using this hybrid procedure, we trained five premise pairs (A+B-, B+C-, C+D-, D+E-, and E+F-) which allowed us to test three untrained non-endpoint pairs (BD, CE, and BE). A significant TI effect was found for most subjects on at least two out of three test pairs. Different theories of TI are discussed. The results suggest that this hybrid training is an efficient procedure for establishing mixed-pair acquisition and a TI effect.