PERCEPTION ACQUISITION AS THE CAUSES FOR TRANSITION PATTERNS IN PHONOLOGICAL EVOLUTION AU, CHING-PONG Dynamique Du Langage, UMR 5596 CNRS, Université Lyon 2, ISH 14 avenue Berthelot 69363 Lyon Cedex 07, France A computational model linking up developmental properties and sound changes was built, in order to seek for possible solutions for some controversial issues about the implementation of sound changes (Au, 2005). In the model, there is a population of agents. Each of them has a cognitive structure with four internal subsystems (perception, decoding, coding and production). The subsystems of the agents develop individually during development. The formation of perceptual categories is driven by statistical distributions of sounds that the new- born agents have listened to (e.g. Maye et al, 2002). A self-organizing map is used to simulate the category formation (Guenther et al, 1996). In the simulation results of the model, two seemingly contradicting hypotheses on sound change transitions Neogrammarian regularity (lexically regular; Osthoff et al, 1878) and lexical diffusion (lexically irregular; Wang, 1969), can both be observed under different conditions. During a shift, the pronunciations of the lexical items change regularly as described in the Neogrammarian hypothesis; during a merger, the spoken forms display a regular pattern at the beginning, and then become irregular lexically as described in lexical diffusion. These conditions are primarily matched with the empirical data supporting the two opposing hypotheses. With further investigation on the subsystems of the agents, the consistency of perceptual responses among agents was found to be the causes of different transition patterns. At the later stage of a merger of two sounds, when two groups of words become acoustically close, the perceptual responses of individual agents become inconsistent throughout the population due to the statistically determined nature of perceptual development. The locations and the sharpness of boundaries between two categories vary and some agents may even have only one category across the acoustic range of the two original sounds. As the word pronunciations are learnt through self-listening, the spoken forms of various words are scattered along the acoustic range of the two original sounds. This is the basis of the irregularity; but when a perceptual category is still far enough from the neighboring categories, the category formed of each agent is similar and stable as in the shifts or the beginning stages of mergers. All spoken forms of the words in the same group are picked within the same small phonetic range bounded by the perceptual category. The category location in the acoustic domain may differ slightly from generation to generation. When the acoustic differences accumulate, it appears that the spoken forms under the same perceptual category change simultaneously and gradually in the same direction as described in the Neogrammarian hypothesis. In conclusion, the model here provides a more precise description on how the phonological systems evolve from time to time. If the present model is able to describe the reality appropriately, it can be potentially extended into a model that provides insights for the emergence of phonological systems. References: Au, Ching-Pong (2005), Acquisition and Evolution of Phonological Systems. PhD Dissertation. City University of Hong Kong. Guenther F. H. and Gjaja M. N. (1996), The Perceptual Magnet Effect as an Emergent Property of Neural Map Formation. Journal of the Acoustical Society of America, 100, pp. 1111-1121. Maye, J., Werker, J.F., & Gerken, L. (2002), Infant Sensitivity to Distributional Information Can Effect Phonetic Discrimination. Cognition, 82(3), B101- B111. Osthoff, H. and Brugmann, K. (1878), Morphologische Untersuchungen auf dem Gebiete der indo-germanischen Sprachen, Vorwort I. iii-xx. (English Translation in Lehmann 1967) Wang, W. S-Y. (1969), Competing Changes as a Cause of Residue. Language. 45:9-25.