Fluency in a language requires understanding abstract relationships between types or classes of words - the syntax of language. The learning problem has seemed so overwhelming to some that - for a long time - the dominant view was that much of this structure was not or could not, in fact, be learned (e.g. Crain, 1992; Wexler, 1991). The object of my thesis work is to examine whether and under what conditions we can learn one particular aspect of language often assumed to be innate, namely phrase structure. In three experiments, I examine acquisition of category relationships (i.e. phrases) from distributional information in the context of two miniature artificial language paradigms - one auditory and one visual. In this set of studies, I find that learners are able to generalize on the basis of strong distributional cues to phrase information with the assistance of a non-distributional cue to category membership. While it was possible to learn some aspects of phrase structure from distributional information alone, in a large language the non-distributional cue appears to enable high-order abstract generalizations that depend on category membership and category relatedness. The third experiment creates a visual analogue to the auditory phrase structure learning paradigm. Learning outcomes in the visual system were commensurate with those from the auditory artificial language, suggesting the ability to learn higher-order relationships from distributional information is largely modality independent.

The aim of this research is to better understand why children consistently surpass adults in their ultimate attainment of language--the sensitive period for language acquisition. I propose the Nested Cognitive and Neural Asynchronies theory to explain this phenomenon. This theory first asserts that an expert neural system for language processing is built in a nested manner, with commitments made first to aspects of language that are learned early. Once built, this system--though expert in processing one's native language--is ineffective in processing other languages, particularly those that differ in these early‐learned aspects of language (such as sound structure). Second, asynchronies in the development of neural substrates supporting learning are thought to contribute to sensitive period phenomena. One consequence of the vast differences in the maturation of various substrates important for learning is that children end up with an abundance of implicit learning mechanisms relative to explicit. Since much of language learning is likely to occur via these implicit mechanisms, the result is that children might simply be better built to learn language.

The experiments reported here support this theory. The first study shows that languages that differ from one's native language in the most basic properties (i.e., sounds) are 1) more difficult to learn and 2) represented with distinct neural circuits. The second study demonstrates that the neural circuits recruited by children for a necessary component of explicit learning change substantially over the course of development, suggesting that the neural circuits deployed for learning in children differ greatly from those in adults. The third study shows that explicit learning can be detrimental for learning certain aspects of language. Moreover, when the primary substrate associated with this learning--the prefrontal cortex--is taken offline, this detriment of explicit learning is ameliorated, thereby showing that child-adult learning differences are related to neural maturation. Together, this collection of studies aims not to catalogue sensitive period phenomena in humans but rather to explain why the sensitive period for language exists.