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The work reported here experimentally investigates a striking generalization about vocabulary acquisition: Noun learning is superior to verb learning in the earliest moments of child language development. The dominant explanation of this phenomenon in the literature invokes differing conceptual requirements for items in these lexical categories: Verbs are cognitively more complex than nouns and so their acquisition must await certain mental developments in the infant. In the present work, we investigate an alternative hypothesis; namely, that it is the information requirements of verb learning, not the conceptual requirements, that crucially determine the acquisition order. Efficient verb learning requires access to structural features of the exposure language and thus cannot take place until a scaffolding of noun knowledge enables the acquisition of clause-level syntax. More generally, we experimentally investigate the hypothesis that vocabulary acquisition takes place via an incremental constraint-satisfaction procedure that bootstraps itself into successively more sophisticated linguistic representations which, in turn, enable new kinds of vocabulary learning. If the experimental subjects were young children, it would be difficult to distinguish between this information-centered hypothesis and the conceptual change hypothesis. Therefore the experimental learners are adults. The items to be “acquired” in the experiments were the 24 most frequent nouns and 24 most frequent verbs from a sample of maternal speech to 18-24-month old infants. The various experiments ask about the kinds of information that will support identification of these words as they occur in mother-to-child discourse. In Experiment 1, subjects were required to identify the words from observing several extralinguistic contexts for their use (silent videos in which mothers are seen uttering the “mystery word” several times to the infants, with each such use cued by a beep or a nonsense word). The findings under these conditions mimicked the known learning trajectory for infants at the inception of speech and comprehension: Nouns are learned far more efficiently than verbs. Experiment 2 showed that the Experiment 1 results are best understood as concreteness differences that are correlated with lexical class membership in the common useage of mothers to young children. Experiment 3 presented (different) subject groups with 24 verbs under varying information Conditions; namely: (1) extralinguistic information; (2) noun-co-occurrence information; (3) both (1) and (2); (4) syntactic-frame information but with nouns and verbs represented by nonsense words; (5) both (2) and (4); (6) both (1) and (5). Each Condition led to greater identification success than the preceding Condition. Moreover, not only the number but the type of verb that was efficiently learned was different under the different information conditions. We discuss these results as consistent with the incremental construction of a highly lexicalized grammar by cognitively and pragmatically sophisticated human infants, but inconsistent with a procedure in which lexical acquisition is independent of and antecedent to syntax acquisition.
Date Posted: 24 August 2006
University of Pennsylvania Institute for Research in Cognitive Science Technical Report No. IRCS-98-22.