The syntax–phonology interface refers to the way syntax and phonology are interconnected. Although syntax and phonology constitute different language domains, it seems undisputed that they relate to each other in nontrivial ways. There are different theories about the syntax–phonology interface. They differ in how far each domain is seen as relevant to generalizations in the other domain, and in the types of information from each domain that are available to the other.
Some theories see the interface as unlimited in the direction and types of syntax–phonology connections, with syntax impacting on phonology and phonology impacting on syntax. Other theories constrain mutual interaction to a set of specific syntactic phenomena (i.e., discourse-related) that may be influenced by a limited set of phonological phenomena (namely, heaviness and rhythm). In most theories, there is an asymmetrical relationship: specific types of syntactic information are available to phonology, whereas syntax is phonology-free.
The role that syntax plays in phonology, as well as the types of syntactic information that are relevant to phonology, is also a matter of debate. At one extreme, Direct Reference Theories claim that phonological phenomena, such as external sandhi processes, refer directly to syntactic information. However, approaches arguing for a direct influence of syntax differ on the types of syntactic information needed to account for phonological phenomena, from syntactic heads and structural configurations (like c-command and government) to feature checking relationships and phase units. The precise syntactic information that is relevant to phonology may depend on (the particular version of) the theory of syntax assumed to account for syntax–phonology mapping. At the other extreme, Prosodic Hierarchy Theories propose that syntactic and phonological representations are fundamentally distinct and that the output of the syntax–phonology interface is prosodic structure. Under this view, phonological phenomena refer to the phonological domains defined in prosodic structure. The structure of phonological domains is built from the interaction of a limited set of syntactic information with phonological principles related to constituent size, weight, and eurhythmic effects, among others. The kind of syntactic information used in the computation of prosodic structure distinguishes between different Prosodic Hierarchy Theories: the relation-based approach makes reference to notions like head-complement, modifier-head relations, and syntactic branching, while the end-based approach focuses on edges of syntactic heads and maximal projections. Common to both approaches is the distinction between lexical and functional categories, with the latter being invisible to the syntax–phonology mapping. Besides accounting for external sandhi phenomena, prosodic structure interacts with other phonological representations, such as metrical structure and intonational structure.
As shown by the theoretical diversity, the study of the syntax–phonology interface raises many fundamental questions. A systematic comparison among proposals with reference to empirical evidence is lacking. In addition, findings from language acquisition and development and language processing constitute novel sources of evidence that need to be taken into account. The syntax–phonology interface thus remains a challenging research field in the years to come.
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