Sharon Peperkamp

The acquisition of allophonic rules: Statistical learning with linguistic constraints

Phonological rules relate surface phonetic word forms to abstract underlying forms that are stored in the lexicon. Infants must thus acquire these rules in order to infer the abstract representation of words. We implement a statistical learning algorithm for the acquisition of one type of rule, namely allophony, which introduces context-sensitive phonetic variants of phonemes. This algorithm is based on the observation that different realizations of a single phoneme typically do not appear in the same contexts (ideally, they have complementary distributions). In particular, it measures the discrepancies in context probabilities for each pair of phonetic segments. In Experiment 1, we test the algorithms performances on a pseudo-language and show that it is robust to statistical noise due to sampling and coding errors, and to non-systematic rule application. In Experiment 2, we show that a natural corpus of semiphonetically transcribed child-directed speech in French presents a very large number of near-complementary distributions that do not correspond to existing allophonic rules. These spurious allophonic rules can be eliminated by a linguistically motivated filtering mechanism based on a phonetic representation of segments. We discuss the role of a priori linguistic knowledge in the statistical learning of phonology.

Discovering words in the continuous speech stream: the role of prosody

Abstract Finding words in sentences is made difficult by the absence of obvious acoustic markers at word boundaries, such as silent pauses. Recent experimental evidence suggests that both adults and infants are able to use prosodic boundary cues on-line to constrain lexical access. French adults performing a word detection task were slowed down by local lexical ambiguities within phonological phrases but not across a phonological phrase boundary (Christophe, Peperkamp, Pallier, Block, & Mehler, J. Mem. Language (in revision)). Thirteen-month-old American infants who were trained to turn their heads upon hearing a bisyllabic word, such as ‘paper’, in a variant of the conditioned head-turning paradigm, responded more often to sentences that contained the target word than to sentences containing both its syllables separated by a phonological phrase boundary (Gout, Christophe, & Morgan, J. Mem. Language (in revision)). Taken together, these results suggest that both French adults and 13-month-old American infants perceive phonological phrase boundaries as natural word boundaries, and that they do not attempt lexical access on pairs of syllables which span such a boundary. We discuss the potential generalization of these results to other languages, the universality of prosodic boundary cues as well as their use in on-line syntactic analysis and syntax acquisition.

Limits on Bilingualism Revisited: Stress Deafness in Simultaneous French-Spanish Bilinguals.

We probed simultaneous French-Spanish bilinguals for the perception of Spanish lexical stress using three tasks, two short-term memory encoding tasks and a speeded lexical decision. In all three tasks, the performance of the group of simultaneous bilinguals was intermediate between that of native speakers of Spanish on the one hand and French late learners of Spanish on the other hand. Using a composite stress deafness index measure computed over the results of the three tasks, we found that the performance of the simultaneous bilinguals is best fitted by a bimodal distribution that corresponds to a mixture of the performance distributions of the two control groups. Correlation analyses showed that the variables explaining language dominance are linked to early language exposure. These findings are discussed in light of theories of language processing in bilinguals.

Language-specific stress perception by 9-month-old French and Spanish infants

During the first year of life, infants begin to have difficulties perceiving non-native vowel and consonant contrasts, thus adapting their perception to the phonetic categories of the target language. In this paper, we examine the perception of a non-segmental feature, i.e. stress. Previous research with adults has shown that speakers of French (a language with fixed stress) have great difficulties in perceiving stress contrasts (Dupoux, Pallier, Sebastián & Mehler, 1997), whereas speakers of Spanish (a language with lexically contrastive stress) perceive these contrasts as accurately as segmental contrasts. We show that language-specific differences in the perception of stress likewise arise during the first year of life. Specifically, 9-month-old Spanish infants successfully distinguish between stress-initial and stress-final pseudo-words, while French infants of this age show no sign of discrimination. In a second experiment using multiple tokens of a single pseudo-word, French infants of the same age successfully discriminate between the two stress patterns, showing that they are able to perceive the acoustic correlates of stress. Their failure to discriminate stress patterns in the first experiment thus reflects an inability to process stress at an abstract, phonological level.