Acquiring Receptive Morphology: A Connectionist Model

Acquiring Receptive Morphology: A Connectionist Model Michael Gasser Computer Science and Linguistics Departments Indiana University Abstract This paper describes a modular connectionist model of the acquisition of receptive inflectional morphology. The model takes inputs in the form of phones one at a time and outputs the associated roots and in- flections. Simulations using artificial language stimuli demonstrate the capacity of the model to learn suffix- ation, prefixation, infixation, circumfixation, mutation, template, and deletion rules. Separate network mod- ules responsible for syllables enable to the network to learn simple reduplication rules as well. The model also embodies constraints against association-line crossing. Introduction For many natural languages, a major problem for a language learner, whether human or machine, is the system of bound morphology of the language, which may carry much of the functional load of the grammar. While the acquisition of morphology has sometimes been seen as the problem of learning how to transform one linguistic form into another form, e.g., by [Plunkett and Marchman, 1991] and [Rumelhart and McClelland, 1986], from the learner's perspective, the problem is one of learning how forms map onto meanings. Most work which has viewed the acquisition of morphology in this way, e.g., [Cottrell and Plunkett, 1991], has taken tile perspective of production. But a human language learner almost certainly learns to understand polymor- phemic words before learning to produce them, and pro- duction may need to build on perception [Gasser, 1993]. Thus it seems reasonable to begin with a model of the acquisition of receptive morphology. In this paper, I will deal with that component of re- ceptive morphology which takes sequences of phones, each expressed as a vector of phonetic features, and identifies them as particular morphemes. This process ignores the segmentation of words into phone sequences, the morphological structure of words, and the the se- mantics of morphemes. I will refer to this task as root and inflection identification. It is assumed that children learn to identify roots and inflections through the pre- sentation of paired forms and sets of morpheme mean- ings. They show evidence of generalization when they are able to identify the root and inflection of a novel combination of familiar morphemes. At a minimum, a model of the acquisition of this ca- pacity should succeed on the full range of morphological rule types attested in the world's languages, it should embody known constraints on what sorts of rules are possible in human language, and it should bear a re- lationship to the production of morphologically com- plex words. This paper describes a psychologically motivated connectionist model (Modular Connection- ist Network for the Acquisition of Morphology, MC- NAM) which shows evidence of acquiring all of the basic rule types and which also experiences relative difficulty learning rules which seem not to be possible. In another paper [Gasser, 1992], I show how the representations that develop during the learning of root and inflection identification can support word production. Although still tentative in several respects, MCNAM appears to be the first …

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