Similarity avoidance and the OCP

It has long been known that verbal roots containing homorganic consonant pairs are rare in Arabic, motivating the existence of an OCP-Place constraint (Obligatory Contour Principle on place of articulation) in the phonological grammar. We explore this constraint using an on-line lexicon of Arabic roots. The strength of the constraint is quantified by the ratio of the observed number of examples of each consonant pair to the number that would be statistically expected under random combination of phonemes. We show that the strength of the effect over all pairs is a gradient function of the similarity of the consonants in the pair. A similarity metric based on natural classes is developed, which solves the formal difficulties of contrastive underspecification theory while preserving the insight that contrastiveness plays an important role in perceived similarity. This metric is applied in an explicit model of the gradient OCP constraint, which achieves a better fit to the regularities and sub-regularities of the Arabic verbal lexicon than any prior approach. Lastly, we review evidence for the psychological reality of the constraint, for its existence in related forms in other languages, and for its cognitive/phonetic foundations in the speech processing system. We argue that the total body of evidence supports a model in which phonetic and cognitive pressures incrementally affect the lexicon, and phonotactic constraints are abstractions over the lexicon of phonological forms.