UC San Diego

Deaf individuals are more likely to experience impoverished language during early life. Delayed sign language onset often leads to later language deficits, especially at the morpho-syntactical level. As early experience plays a crucial role in postnatal brain development, the developmental and processing difficulties may reflect altered brain development due to lacking sufficient early language. Examining the behavioral and neural outcomes in this population increases our understanding of the mechanisms of first language development.

In this dissertation, I focused on one basic syntactic cue – basic word order in simple transitive sentences. Pinning down the developmental, processing, and anatomical characteristics of native and late signers of ASL with respect to simple transitive structures is key to our understanding of the morpho-syntactic difficulties shown by this population. Simple transitive clauses represent the earliest of hierarchical structures, a hallmark of human language capacity. The current dissertation thus sheds light on the role of early language on the emergence of this core linguistic structure.

I examined the early syntactic development of American Sign Language among deaf individuals with an extremely late sign language onset, combining observations from three perspectives: longitudinal development, sentence processing strategies, and brain language pathways. Chapter 3 presents a longitudinal study of 4 deaf late signers on their word order development. The results suggest a similar developmental trajectory regardless of first language onset, but the process is prolonged for late signers, and only limited to the early stages. Chapter 4 uses a sentence-picture verification experiment to examine whether deaf late signers robustly rely on word order to comprehend simple Subject-Verb-Object sentences. The results show that, unlike native signers and second language signers who consistently rely on word order, deaf late signers prefer event plausibility over word order. Chapter 5 presents a study on the connectivity patterns of major language pathways in the brain using diffusion tensor imaging, and finds less robust connectivity in left arcuate fasciculus, a pathway crucial for syntactic processing. Together, these findings suggest profound effects of impoverished early language on early syntactic and brain development and is suggestive of links between early language and brain development.