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 g radient of generalization observed between one-category (non-d iscrim inative) and two-category (discrim inative) training. Extrapolating from the resu lts of a number of animal lear ning studies, it was predicted that the g radient should be steeper under discrim inative training. The ® rst experiment con ® rms this basic prediction for the stimuli used, which were novel, prototype-structured, and constructed from 12 symbols positioned on a g rid. An explanation for the effect, based on the Rescorla ± Wagner theory of Pavlovian conditioning (Rescorla & Wagner, 1972), is that under non-discrim inative train ing ``incidental stimuli’ ’ have signi ® cant control over responding, whereas under discrim inative train ing they do not. Incidental stimuli are those aspects of the stimulus, or the surrounding context, that are not differentia lly reinforced under discrim inative train ing. This explanation leads to the prediction that a comparable effec t of blocked versus interm ixed discrim inative train ing should also be found. This prediction is discon ® rmed by the second experiment. An alternative model, still based on the Rescorla ± Wagner theory but w ith the add ition 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 response times found is evaluated via simulation.