An analysis of the facilitative effect of similarity on second-order conditioning

Five experiments used autoshaping in pigeons to investigate the effect of stimulus similarity on second-order conditioning of one stimulus (S2) when it signals another, previously conditioned stimulus (S1). Experiment 1 found that the artificial induction of similarity between S2 and S1, by the addition of experimentally separable common elements, improved performance during second-order conditioning of S2. An analysis of these results is given in terms of stimulus similarity encouraging the selection of particular components of S1 for association with S2. That selection is described as a natural consequence of the temporal relations among components of S2 and S1 which their similarity ensures. The analysis is used to generate circumstances under which the normal facilitative effect of similarity could be reversed once observed (Experiment 2A and 2B) or prevented from developing initially (Experiments 3 and 4). These experiments support a particular account of how a qualitative relation can affect the course of conditioning. However, that account requires the introduction of no special principles of conditioning unique to the case of similarity.