This dissertation consists of three studies that discover and explore the novel copular function of a sign, SELF, which has cascading implications on our understanding of ASL nonverbal predication. First, a corpus study consisting of three epochs of historical ASL encompassing 150 years demonstrate SELF’s participation in a grammaticalization cycle that led to its primary function as a copula (Sampson & Mayberry 2022). In contrast to previous diachronic iterations, SELF in contemporary ASL appears most frequently in between the subject noun and an adjectival or nominal predicate in expressions of identity (A = B) and class membership (A is a member of set B), which are hallmarks of copulas. This finding is significant because it contradicts claims that there are no overt copulas in ASL. Second, an experimental study validated the finding that SELF functions as a copula in contrast to other potentially felicitous strategies in ASL nonverbal predication. To test the hypothesis, the distinction between individual-level (IL) predicates, which denote spatiotemporally immutable inherent features of an entity (e.g., deaf, plumber), and stage-level (SL) predicates, which denote spatiotemporally mutable features of an entity (e.g., busy, sit; Carlson 1977, Kratzer 1995), were compared with SELF and two other potential copular items: the covert null element (∅) and IX, a pronominal point. The Likert scale results show that SELF was preferred with IL nominal predicates, IX was preferred with SL verbal predicates, and there was no clear preference for the null element. Due to a conflation between the semantic and syntactic properties of the predicates used in the study, a third study was carried out with only adjectival predicates, to control syntactic category while varying semantic properties: SELF and IX were compared with adjectival predicates of either IL or SL properties. SELF was again preferred with IL adjectives and there was no preference for the sign IX in relation to IL or SL adjectives. These results attest to the markedness of SELF which, coupled with ASL consultants’ intuitions, suggest that SELF carries grammatical modality in marking assertions of truth and raises new important questions about copulas and nonverbal predication in sign languages.

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.

This dissertation investigated how the number systems of sign languages (SLs) of deaf people are similar to and different from the number gestures of hearing cultures and the number systems of spoken languages. Number representation in SLs was investigated here in its typological, morphological, and processing aspects. Additionally, the thesis explored the effects of early language deprivation on the acquisition of number, since it is common in the deaf community (90% of deaf children are born in hearing families, do not learn SL from their parents).Culturally specific number gestures are often used by language speakers to indicate number. While SL numeral systems share the same articulatory and perceptual systems, SLs are linguistic systems, unlike number gestures. In the first study of the dissertation, a corpus of numeral systems in 82 SLs was analyzed to investigate how they are similar to and different from spoken languages number systems and number gestures, and to discover the modality-specific, systematic properties of SL numeral systems, their use of iconicity, and the global distribution of one- and two-handed numeral systems. The second study investigated how the iconic two-handed numeral system in Russian SL interacts with the non-concatenative morphology of the language discovered some new phonological constraints in sign language. The number systems of SLs are linguistic, but also visual – a property they share with Arabic digits. Using a Number Stroop Test, the third study revealed that the processing of number lexemes in American SL (ASL) is similar to that of spoken language number lexemes rather than Arabic digits. Both number formats are affected by early language deprivation in terms of processing speed. However, automatic magnitude representation still can be formed despite delayed language exposure. Finally, the linguistic representation of number extends beyond the number systems to grammatical number marking. The last study investigated the impact of early language deprivation on the acquisition of plural classifier constructions, which are a complex morphological means of number marking in ASL. It revealed that late first language learners prefer morphologically simpler strategies over classifiers and make errors specific to their language acquisition background.

Kenyan Sign Language (KSL) is a thriving national sign language used by tens of thousands of signers in Kenya, and which emerged out of two deaf schools in western Kenya in the early 1960s. In this thesis, I provide a thorough description and analysis of the basic phonological components of the KSL lexicon used in the southwestern region of Kenya (formerly south Nyanza Province).

This phonological grammar of (SoNy)KSL makes contributions in three domains. In the descriptive domain, it provides a thorough report of the basic units in the main phonological parameters; i.e., Handshape (Ch. 4), Location (Ch. 5), and Movement (Ch 6, 7), as well as the evidence for the distinctiveness of each unit. The description for Movement and Location are particularly noteworthy because those parameters have received less attention in sign linguistics in general compared to Handshape.

In the methodological domain, the grammar is based on a KSL Lexical Database built for this project, in which over 50 phonetic characteristics of 1,880 non-compound signs were coded. This database is currently one of only a few such richly coded lexical databases of sign languages. In addition, this grammar employs a rigorous approach to determining lexical contrast, which has yielded a separate dataset of 461 minimal pairs (Ch. 3). This dataset is unique in sign linguistics and reveals patterns of lexical contrast that were not previously known—and which have generated new hypotheses about how lexical contrast may be constrained by degrees of visual similarity.

Finally, this thesis makes a theoretical contribution by comparing how different models of sign phonology can account for sign types in KSL. By evaluating the explanatory power of the main theories of sign phonology on the basis of specific descriptive data, this thesis gives unique insights into the theoretical validity of these models. It also proposes modifications in some cases, especially with regard to how the Dependency Model (DPM) can account for the representation of movement features and their relationship to the timing tier. In addition, a new movement feature, [dispersed], is described and its implementation worked out in the DPM.

Understanding how producers and comprehenders converge on the same discourse entities is a central task in psycholinguistics. For example, how does a language producer decide to use a pronoun (‘she’) instead of a noun in a sentence such as ‘the cook hates the teacher, because she is grumpy’? Similarly, how does the comprehender determine which of the previously mentioned entities such an ambiguous pronoun refers to? Previous work on spoken language referential cohesion has been concerned with questions such as how the accessibility of the referent in the discourse context influences choice of referring expression, whether pronouns prefer antecedents with a particular grammatical role, and how verb semantics influences pronoun resolution. Research on sign language linguistics, however, has focused primarily on the spatial co-reference made possible by the use of space and the body.

This dissertation investigates how factors specific to and independent of the articulatory modality jointly influence referential cohesion in American Sign Language (ASL). The first study (Chapter 2) examines the referential hierarchy proposed for spoken languages and finds that it largely applies to reference tracking in ASL. The second study (Chapter 3) focuses on whether the articulatory modality influences how a specific kind of verb semantics, known as implicit causality, is distributed across verbs in ASL. Results indicate a distribution that is language-specific rather than modality-specific. Finally, the third study (Chapter 4) examines how spatial co-reference, grammatical role, and implicit causality biases influence pronoun production and interpretation in ASL. The results suggest a much greater role for the modality-independent factors than had previously been assumed.

Together, the studies in this dissertation indicate that referential cohesion is remarkably similar across modalities. This has important theoretical implications in the domains of psycholinguistic theory and sign language linguistics, as well as practical implications for the education of Deaf signing children.