Relational learning in the rat.

The phenomenon of transposition revealed in experiments on discrimination has long provided one of the most crucial challenges to the neoPavlovian conception of cortical functioning.' The fact of transposition suggested the existence of a process of relational learning which did not seem understandable in terms of the independent acquisition of excitatory and inhibitory properties by isolated stimuli. Some years ago, however, Spence advanced an interpretation which met this challenge in a most ingenious manner.2 By making certain assumptions concerning the generalization of excitatory and inhibitory effects along relevant stimulus-dimensions, Spence finds it possible to explain, in essentially Pavlovian terms, not only the occurrence of transposition, but also its failure to occur under given experimental conditions. Even the fact that it is often more difficult for animals to learn to respond to stimuli of given characteristics, independently of context, rather than to a relation between pairs, can, paradoxically enough, be deduced from this theory which postulates no perception of relations by the animal.3 Recently, however, Lashley and Wade have vigorously criticized the conception of stimulus-generalization which is central to Spence's theory.4 Gradients of similarity, they maintain, do not come ready-made in the organism, but are established in the course of the animal's experience. Stimulus-dimensions are defined for the animal not by conditioning to a single stimulus but by training which provides opportunity to compare two or more stimuli-"either a direct comparison of sensory impressions