Tests of the ratio rule in categorization

Many theories of learning and memory (e.g., connectionist, associative, rat ional, exemplar based) produce psychological magnitude terms as output (i.e ., numbers representing the momentary level of some subjective property). M any theories assume that these numbers may be translated into choice probab ilities via the ratio rule, also known as the choice axiom (Luce, 1959) or the constant-ratio rule (Clarke, 1957). We present two categorization exper iments employing artificial, visual, prototype-structured stimuli construct ed from 12 symbols positioned on a grid. The ratio rule is shown to be inco rrect for these experiments, given the assumption that the magnitude terms for each category are univariate functions of the number of category-approp riate symbols contained in the presented stimulus. A connectionist winner-t ake-all model of categorical decision (Wills & McLaren, 1997) is shown to a ccount for our data given the same assumption. The central feature underlyi ng the success of this model is the assumption that categorical decisions a re based on a Thurstonian choice process (Thurstone, 1927, Case V) whose no ise distribution is not double exponential in form.