Recent experimental work offers evidence that infants become aware of suffixes at a remarkably early age, as early as 6 months for the English suffix -s. Here, we seek to understand this ability though the strategy of computational modeling. We evaluate a set of distributional learning models for their ability to mimic the observed acquisition order for various suffixes when trained on a corpus of child-directed speech. Our best-performing model first segments utterances of the corpus into candidate words, thus populating (...) a proto-lexicon. It then searches the proto-lexicon to discover affixes, making use of two distributional heuristics that we call Terminus Frequency and Parse Reliability. With suitable parameter settings, this model is able to mimic the order of acquisition of several suffixes, as established in experimental work. In contrast, models that attempt to spot affixes within utterances, without reference to words, consistently fail. Specifically, they fail to match acquisition order, and they extract implausible pseudo-affixes from single words of high token frequency, as in [pi-] from peekaboo. Our modeling results thus suggest that affix learning proceeds hierarchically, with word discovery providing the essential basis for affix discovery. (shrink)

Phonemes have variant pronunciations depending on context. For instance, in American English, the [t] in pat [pæt] and the [d] in pad [pæd] are both realized with a tap [ɾ] when the –ing suffix is attached, [pæɾɪŋ]. We show that despite greater distributional and acoustic support for the [t]-tap alternation, 12-month-olds successfully relate taps to stems with a perceptually-similar final [d], not the dissimilar final-[t]. Thus, distributional learning of phonological alternations is constrained by infants' preference for the alternation of perceptually-similar (...) segments. Further, the ability to relate variant surface forms emerges between 8- and 12-months. Our findings of biased learning provide further empirical support for a role for perceptual similarity in the acquisition of linguistically-relevant categories. We discuss the implications of our findings for phonological theory, language acquisition and models of the mental lexicon. (shrink)