November 20 2003 Chapter 16 Lexical Semantics Words

November 20, 2003 Chapter 16 Lexical Semantics

Words have structured meanings • Lexeme – a pairing of a form with a sense • Orthographic form – the way the lexeme looks on the page • Phonological form – the way the lexeme sounds • Lexicon – finite list of lexemes eaten eat ate Lexeme eat

Lexical Relations • Homonymy • Polysemy • Synonymy • Hyponymy

Homonymy A relation that holds between two lexemes that have the same form with unrelated meanings • Homophones • Homographs • “found” Lexeme sloping mound bank Lexeme Financial institution

Homonymy causes problems • Spelling correction – Confusables – your vs. you’re • Speech recognition – Homophones and pure homonyms • Text-to-speech – Homographs – conduct • Information retrieval – Homographs and pure homonyms

Polysemy The phenomenon where a single lexeme has multiple related meanings Biological repository bank Lexeme Financial institution

Polysemy • How many senses does a word have? – Zeugma: Does Midwest Express serve breakfast and Philadelphia – Kim has an uncle and so does Sandy – Kim has a bat and so does Sandy • How are they related? • How can you tell which sense should be attributed to a given word?

Synonymy A relation that holds between two lexemes with the same sense older big lexeme Positive size large lexeme

hyponymy A relation that hold between two lexemes where one denotes a subclass of the other vehicle car • ontology • taxonomy • object hierarchy hypernym vehicle hyponym car

Word. Net • A large electronic database of lexical relations • A web-based interface • Sets of lexical entries corresponding to unique orthographic forms, accompanied by sets of senses associated with each form Category Unique Forms Number of Senses Noun 94474 116317 Verb 10319 22066 Adjective 20170 29881 Adverb 4546 5677

Word. Net synset • Synonymy is organized around the notion of a synset • {chump, fish, fool, gull, mark, patsy, fall guy, sucker, schlemiel, soft touch, mug} • The synset is the sense associated with the Word. Net entry. • The semantic relations are relations between sysnets

Internal Structure of Words • Thematic roles • Selectional restrictions • Primitive decomposition • Semantic fields

Thematic Roles • Deep roles: Houston’s Billy Hatcher broke a bat e, x, y Isa(e, Breaking) Breaker(e, Billy. Hatcher) Broken. Thing(e, y) Isa(y, Baseball. Bat) He opened a door e, x, y Isa(e, Opening) Opener(e, he) Opened. Thing(e, y) Isa(y, Door)

Thematic Roles • Breaker and Opener are agents • Broken. Thing and Opened. Thing are themes • Some other commonly used thematic roles – Experiencer – Force – Result – Content – Instrument – Beneficiary – Source – Goal

Thematic Roles • Thematic roles can be used as a shallow semantic language • Can be used to determine surface realization: AGENT > INSTRUMENT > THEME The highest thing in the hierarchy will typically be the subject; the lowest thing will be the object. • Linking theory looks at the mapping between conceptual structure and grammatical function.

Thematic Roles • Doesn’t work for all verbs: donate, return, transfer • It only helps for NP and PP arguments of verbs • Differing perspective – Amie bought the sandwich from Benson for three dollars – Benson sold Amie the sandwich for three dollars – Amie paid Benson three dollars for the sandwich

Selectional Restrictions The senses of lexemes enforce selectional restrictions on their arguments • Which airlines serve Denver – The Served. Thing is a geographical location • Which airlines serve breakfast – The Served. Thing is a meal This helps tell which sense of a lexeme is intended in a given context

Selectional Restrictions Selectional restrictions occur at varying levels of specificity In rehearsal, I often ask the musicians to imagine a tennis game. They tell of jumping over beds they can’t imagine clearing while awake. I cannot even imagine what this lady does all day. Atlantis lifted Galileo from the launch pad. Mr. Kruger lifted the fish from the water. To diagonalize a matrix is to find its eigenvalues.

Representing Selectional Restrictions The semantic contribution of a verb like eat e, x, y Eating(e) Agent(e, x) Theme(e, y) Isa(y, Edible. Thing) The phrase ate a hamburger would get something like e, x, y Eating(e) Agent(e, x) Theme(e, y) Isa(y, Edible. Thing) Isa(y, hamburger) Instead of using logical concepts, we can just use Word. Net synsets: { food, nutrient } { hamburger, beefburger } is a hyponym of this.

Creativity and the Lexicon We can use more word meanings that can be explicitly listed in the lexicon. There are productive processes for creating new senses from those explicitly listed, including • Metaphor • Metonymy

Metaphor Using metaphor, we refer to, and reason about, concepts using terminology appropriate to completely different kinds of concepts. CORPORATION AS PERSON • That doesn’t scare Digital, which has grown to be the worlds second-largest… • Triton Group Ltd. , a company it helped resuscitate, has begun acquiring Fuqua shares • But if it changed its mind, however, it would do so for investment reasons, the filing said.

Metonymy The use of one concept to refer to another concept closely related to it. PRODUCT FOR PROCESS • GM killed the Fiero because it had dedicated a full-scale factory to building the plastic bodied car… AUTHOR FOR WORKS • He likes Shakespeare PLACE FOR INSTITUTION • The White House had no comment

Approaches to Metaphor and Metonymy • Convention-based approaches hard-wire metaphors like CORPORATION AS PERSON and metonyms like PRODUCT FOR PROCESS • Reasoning-based approaches treat this as a problem for general, not necessarily linguistic, reasoning, such as analogical reasoning.

Summary • Lexeme • Lexical Relations – Homonymy – Polysemy – Synonymy – Hyponymy • Word. Net • Thematic Roles • Selectional Restrictions • Metaphor & Metonymy