Lecture Lexical Semantics CS 4705

What is lexical semantics? Meaning of Words Lexical Relations WordNet Thematic Roles Selectional Restrictions Conceptual Dependency

What is a word? Lexeme: an entry in the lexicon that includes an orthographic representation a phonological form a symbolic meaning representation or sense Dictionary entries: Red (‘red) n: the color of blood or a ruby Blood (‘bluhd) n: the red liquid that circulates in the heart, arteries and veins of animals Word Sense Disambiguation For any given lexeme, how can its sense be reliably distinguished? Lex. Rel. III: Metaphor, Metonymy What is metaphor? That doesn't scare Digital. What is metonymy? GM killed the Fiero. Extension of existing sense to a new meaning. Lexical Relations IV: Synonomy What is synonomy? Substitutability. How big is that plane? How large is that plane? Compare: A big fat apple ?A large fat apple A big sister ?A large sister Influences on substitutability: subtle shades of meaning differences polysemy register collocational constraints Lexical Relations V: Hyponomy What is hyponomy? General: hyponym Specific: hypernym Example: ``car'' is a hyponym of ``vehicle'' and ``vehicle'' is a hypernym of ``car.'' Test: ``That is a car'' implies ``That is a vehicle'' What is ontology? What is taxonomy? What is object hierarchy? %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Semantic Networks Used to represent relationships between words Example: WordNet - created by George Miller's team at Princeton http://www.cogsci.princeton.edu/$\sim$wn WordNet (1.6) WordNet is the most widely used hierarchically organized lexical database for English -- Fellbaum (1998). \vspace{.1in \epsfxsize=1\textwidth \fig{\file{figures{fig16.01.ps Format of WordNet Entries WordNet sense entries consist of a set of synonyms, a dictionary-style definition (or gloss), and some examples of uses. \fig{\file{figures{fig16.02.ps Sense Distribution for WordNet Verbs \fig{\file{figures{fig16.03.ps Lexical (N) Relations in WordNet \fig{\file{figures{fig16.04.ps Verb Relations in WordNet \fig{\file{figures{fig16.05.ps Adj. and Adv. Relations in WordNet \fig{\file{figures{fig16.06.ps Synsets in WordNet WordNet is organized around the notion of synset. { chump, fish, fool, gull, mark, patsy, fall guy, sucker, schlemiel, shlemiel, soft touch, mug } Important: It is this exact synset that makes up one of the sense for each of the entries listed in the synset. Theoretically, each synset can be viewed as a concept in a taxonomy -- like the concepts described in Chapter 14. Hyponomy in WordNet fig16.07.ps Internal Structure of Words What are the meaning components underlying word sense? Thematic Roles (theta-roles) What is a thematic role? E w,x,y,z Giving (x) ^ Giver(w,x) ^ Givee(z, x) ^ Given(y,x) $ E w,x,z Breaking (x) ^ Breaker(w,x) ^ Broken(z,x) Generic Thematic Roles fig16.08.ps Examples of Thematic Roles fig16.09.ps Early Theories of Thematic Roles 1967-1968: "The beginning of Lexical Semantics" (Fillmore; Gruber; Jackendoff (based on Gruber)) Two fundamentally different approaches to linguistics Gruber/Jackendoff: account for semantics and use grammar derived to say something about syntax Fillmore: account for syntax and use that to describe semantics Thematic Level Why posit a thematic level distinct from that of syntactic subcategorization? capture similarity between different (but related) uses of same lexical item) obviate need for subcategorization frames: mapping from syntax to lexical semantics Selectional Restrictions What are selectional restrictions? Recall the "Godzilla" example. Selectional Restriction Implementation A WordNet approach: hamburgers are edible fig16.10.ps Primitive Decomposition Jim killed his philodendron Jim did something to cause his philodendron to become not alive Schank's Primitives Conceptual Dependency fig16.11.ps Pred. Independence vs. Dependence Predicate-Independent single set of roles is chosen independent of the type of predicates involved (no reference to type of predicates) Schank Predicate-Dependent roles identified by particular positions arguments occupy with respect to primitive predicates Decomposition vs. Non-Decomposition Decomposition / Compositional Approach (Schank, Jackendoff) vs. Non-decomposition / Noncomposition Approach (Fillmore) Within compositional approaches: exhaustive (Schank) vs. nonexhaustive (Jackendoff) Schank: Motivation Underlying Motivation: "Strong AI" Focus: understanding. Argues that the representation is reversible. Rejects syntax during analysis. Allows it during generation. Attempts to come up with well-defined system of rules and conceptualizations. Inferences, expectation, syntax, conversational norms, real world. Conceptual Structure (CD): Language-independent conceptual level. Schank: Kill vs. Die Schank: Problem 1 "John caused Mary to die" vs. "John killed Mary" Identically substitutable? Flaw of all compositional approaches of this nature. Schank: Problem 2 The decompositions are very complex. Too specific Why are these conceptualizations so radically distinct from the syntactic realization? Talks CD from NL understanding point of view -what about generation? The NLP Bottleneck Acquisition of Computational Lexicons For Next Time Chapter 18

Do dictionaries give us definitions? Right (‘rt) adj: located nearer the right hand esp. being on the right when facing the same direction as the observer Left (‘left) adj: located nearer to this side of the body than the right Do dictionaries give us definitions? Some are circular All are defined in terms of other lexemes You have to know something to learn something What can we learn from dictionaries? Relations between words: Oppositions, similarities, hierarchies

Homonomy Homonyms: Words with same form but different, unrelated meanings, or senses (multiple lexemes) A bank holds investments in a custodial account in the client’s name. As agriculture is burgeoning on the east bank, the river will shrink even more Word sense disambiguation: what clues? Similar phenomena homophones - read and red (different orth. form) homographs - bass and bass (different phon. form)

Ambiguity: Which applications will these cause problems for? General semantic interpretation Machine translation Spelling correction Speech recognition Text to speech Information retrieval

What is polysemy? Word with multiple but related meanings (same lexeme) They rarely serve red meat. He served as U.S. ambassador. He might have served his time in prison. What’s the difference between polysemy and homonymy? Homonymy: Distinct, unrelated meanings Different etymology? Coincidental similarity?

For either, practical task: Polysemy: Distinct but related meanings idea bank, sperm bank, blood bank, bank bank How different? Different subcategorization frames? Domain specificity? Zeugma: Can the two candidate senses be conjoined? ?He served his time and as ambassador to Norway. For either, practical task: What are its senses? (related or not) How are they related? (polysemy ‘easier’ here) How can we distinguish them?

Metaphor, Metonymy What is metaphor? What is metonymy? Father of the atom bomb. What is metonymy? GM killed the Fiero. The ham sandwich wants his check. Both extend existing sense to new meaning Metaphor: use completely different concept (but cf conventional metaphors like GM) Metonymy: use related concepts

Synonomy Substitutability: different lexemes with the same meaning How big is that plane? How large is that plane? How big are you? Big brother is watching. What influences substitutability? Polysemy (large vs. old sense) register: He’s really cheap/?parsimonious. collocational constraint: roast beef, ?baked beef convention: economy fare/?price

Hyponomy General: hypernym (super…ordinate) dog is a hypernym of poodle Specific: hyponym (under..neath) poodle is a hyponym of dog Test: That is a poodle implies that is a dog What is ontology? Object in some domain What is taxonomy? Structuring of those objects What is object hierarchy? Structured hierarchy that supports feature inheritance

Semantic Networks Used to represent lexical relationships e.g. WordNet (George Miller et al) http://www.cogsci.princeton.edu/~wn Most widely used hierarchically organized lexical database for English Synset: set of synonyms, a dictionary-style definition (or gloss), and some examples of uses --> a concept Databases for nouns, verbs, and modifiers Applications can traverse network to find synonyms, antonyms, hierarchies,...

Is a rock edible? What are the parts of a human body? What is a cheeseburger? What are its parts? What is the opposite of ambitious?

Thematic Roles E w,x,y,z Giving (x) ^ Giver(w,x) ^ Givee(z, x) ^ Given(y,x) E w,x,z Breaking (x) ^ Breaker(w,x) ^ Broken(z,x) A set of roles: agent, experiencer, force, theme, result, content, instrument, beneficiary, source, goal,... The dog ate the cheeseburger. What is cheeseburger? The sniper shot his victim with a rifle. What is rifle?

Early Theories of Thematic Roles 1967-1968: "The beginning of Lexical Semantics" (Fillmore; Gruber; Jackendoff (based on Gruber)) Two fundamentally different approaches to linguistics Gruber/Jackendoff: account for semantics and use grammar derived to say something about syntax Fillmore: account for syntax and use that to describe semantics

Why do we need a thematic level? We already have syntactic subcategorization Capture similarity between different (but related) uses of same lexical item) Avoid need for subcategorization frames: mapping from syntax to lexical semantics

What are Selectional Restrictions? George ate a cheeseburger/his lunch/dirt. Jim killed his philodendron ?His philodenron killed Jim. The flu killed Jim.

Schank's Conceptual Dependency Eleven predicate primitives represent all predicates Objects decomposed into primitive categories and modifiers But few predicates result in very complex representations of simple things Ex,y Atrans(x) ^ Actor(x,John) ^ Object(x,Book) ^ To(x,Mary) ^ Ptrans(y) ^ Actor(y,John) ^ Object(y,Book) ^ To(y,Mary) John caused Mary to die vs. John killed Mary

Next time Word sense disambiguation and information retrieval Chapter 17