Rick Dale & Morten H. Christiansen
Cornell University
Recent psycholinguistic research has revealed an interaction of phonological and prosodic information with syntactic regularity in sentence processing [e.g., 1]. This interaction takes on different shapes cross-linguistically, resulting in a variety of phonological and prosodic "cue constellations" [2]. A current hypothesis in developmental psycholinguistics is that these cue constellations contribute importantly to early acquisition of syntax by "bootstrapping" the child into the target language. An outstanding question in this research concerns how these cue constellations emerge, and what possible shapes they assume. We offer computational simulations of this emergence that illuminate how phonological and prosodic information comes to subserve language acquisition, and efficiently coordinate sentence processing.
A phrase-structure grammar template was used to generate sentences along with a set of associated cues for the training of simple recurrent networks (SRNs). The template incorporated a number of parameters that could be "mutated" across generations of networks. These parameters included (1) head ordering of phrase-structure rules (right-headed vs. left-headed rules); (2) places at which pauses could delimit structure (sentence vs. noun-phrase pauses); and (3) lexical cues (units devoted to words in the grammar). For each generation of networks, the template instantiation for which the SRNs acquired the most linguistic structure was selected as the winning grammar, and had some of its parameters mutated. This process was continued over many generations, allowing the cue constellations for winning grammars to evolve.
Our results offer clues about the emergence of cue constellations. First, in all simulations performance improved across generations. Second, the lexical level cue fluctuated significantly more in lexically simple languages, and became consistent with growing complexity (i.e., number of lexical items in the grammar template). It seems, therefore, that certain cues are more consistently exploited as languages become more complex. Third, pauses were consistently located regardless of lexical complexity (for example, at syntactic boundaries like noun phrases and verb phrases). This suggests that cues relevant to syntactic phrases are important for emerging languages with even impoverished vocabulary.
In conclusion, we argue that cue constellations adapt to service growing lexical and syntactic structure. Fueled by constraints on learning, cues facilitate the acquisition of complex linguistic structure, and inevitably contribute to coordinating sentence processing. These simulations serve as a first step towards understanding the principles that may underlie the emergence of phonological and prosodic cues, and how these emerging cue constellations service language acquisition and adult sentence processing.
References
[1] Kjelgaard, M. & Speer, S. (1999). Prosodic facilitation and interference in the resolution of temporary syntactic closure ambiguity. Journal of Memory and Language, 40, 143-194.
[2] Fisher, C. & Tokura, H. (1996). Acoustic cues to grammatical structure in infant-directed speech: Cross-linguistic evidence. Child Development, 67, 31923218.