GENERALIZATION IN HUMAN CATEGORY LEARNING - A CONNECTIONIST ACCOUNT OF DIFFERENCES IN GRADIENT AFTER DISCRIMINATIVE AND NON-DISCRIMINATIVE TRAINING

Two experiments are reported that investigate the difference in gradie nt of generalization observed between one-category (non-discriminative ) and two-category (discriminative) training. Extrapolating from the r esults of a number of animal learning studies, it was predicted that t he gradient should be steeper under discriminative training. The first experiment confirms this basic prediction for the stimuli used, which were novel, prototype-structured, and constructed from 12 symbols pos itioned on a grid. An explanation for the effect, based on the Rescorl a-Wagner theory of Pavlovian conditioning (Rescorla & Wagner, 1972), i s that under non-discriminative training ''incidental stimuli'' have s ignificant control over responding, whereas under discriminative train ing they do not. Incidental stimuli are those aspects of the stimulus, or the surrounding context, that are not differentially reinforced un der discriminative training. This explanation leads to the prediction that a comparable effect of blocked versus intermixed discriminative t raining should also be found. This prediction is disconfirmed by the s econd experiment. An alternative model, still based on the Rescorla-Wa gner theory but with the addition of a decision mechanism comprising a threshold unit and a competitive network system, is proposed, and its ability to predict both the choice probabilities and the pattern of r esponse times found is evaluated via simulation.