Structure and Intonation in Spoken Language Undestanding

The structure imposed upon spoken sentences by intonation seems frequently to be orthogohal to their traditional surface-syntactic structure. However, the notion of "intonational structure" as formulated by Pierrehumbert, Selkirk, and others, can be subsumed under a rather different notion of syntactic surface structure that emerges from a theory of grammar based on a "Combinatory" extension to Categorial Gram, mar. Interpretations of constituents at this level are in tam directly related to "information structure", or discourse-related notions of "theme", "rheme", "focus" and "presupposition". Some simplifications appear to follow for the problem of integrating syntax and other high-level modules in spoken language systems. One quite normal prosody (13, below) for an answer to the following question (a) intuitively impotes the intonational structure indicated by the brackets (stress, marked in this case by raised pitch, is indicated by capitals): (1) a. I know that Alice prefers velveL But what does MAry prefer? b. ( M A r y prefers) (CORduroy). Such a grouping is orthogonal to the traditional syntactic structure of the sentence. Intonational structure nevertheless remains strongly constrained by meaning. For example, contours imposing bracketings like the following are not allowed: (2) #(Three cats)(in ten prefer corduroy) *I am grateful to Steven Bird, Julia Hirschberg, Aravind Joshi, Mitch Marcus, Janet Pierrehumben, and Bonnie Lynn Webber for comments and advice. They are not to blame for any errors in the translation of their advice into the present form. The research was supposed by DARPA grant no. N0014-85-K0018, and ARO grant no. DAAL03-89-C003 l. 9 Halliday [6] observed that this constraint, which Selkirk [14] has called the "Sense Unit Condition", seems to follow from the function of phrasal intonation, which is to convey what will here be called "information structure" that is, distinctions of focus, presupposition, and propositional attitude towards enfloes in the discourse model. These discourse entities are more diverse than mere nounphrase or propositional referents, but they do not include such nonconcepts as "in ten prefer corduroy." Among the categories that they do include are what Wilson and Sperber and E. Prince [13] have termed "open propositions". One way of introducing an open proposition into the discourse context is by asking a Wh-question. For example, the question in (1), What does Mary prefer? introduces an open proposition. As Jackendoff [7] pointed out, it is natural to think of this open proposition as a functional abstraction, and to express it as follows, using the notation of the A-calculus: (3) Ax [(prefer' x) mary'] (Primes indicate semantic interpretations whose detailed nature is of no direct concern here.) When this function or concept is supplied with an argument corduroy', it reduces to give a proposition, with the same function argument relations as the canonical sentence: (4) (prefer' corduroy') mary' It is the presence of the above open proposition rather than some other that makes the intonation contour in (1)b felicitous. ( l~at is not to say that its presence uniquely determines this response, nor that its explicit mention is necessary for interpreting the response.) These observations have led linguists such as Selkirk to postulate a level of "intonational structure", independent of syntactic structure and related to information structure. The theory that results can be viewed as in Figure 1: directionality of their arguments and the type of their result: LF:Argument Structure I Surface Structure ~.____q LF:Information Structure I I