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November 20, 2003Chapter 16Lexical Semantics: 

November 20, 2003 Chapter 16 Lexical Semantics

Words have structured meanings: 

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 eats ate Lexeme eat

Lexical Relations: 

Lexical Relations Homonymy Polysemy Synonymy Hyponymy

Homonymy: 

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

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 Homonymy causes problems

Polysemy: 

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

Polysemy: 

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: 

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

hyponymy: 

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

WordNet: 

WordNet 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

WordNet synset: 

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

Internal Structure of Words: 

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

Thematic Roles: 

Thematic Roles Deep roles: Houston’s Billy Hatcher broke a bat e, x, y Isa(e, Breaking)  Breaker(e, BillyHatcher)  BrokenThing(e, y)  Isa(y, BaseballBat) He opened a door e, x, y Isa(e, Opening)  Opener(e, he)  OpenedThing(e, y)  Isa(y, Door)

Thematic Roles: 

Thematic Roles Breaker and Opener are agents BrokenThing and OpenedThing are themes Some other commonly used thematic roles Experiencer Force Result Content Instrument Beneficiary Source Goal

Thematic Roles: 

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: 

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: 

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

Selectional Restrictions: 

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: 

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

Creativity and the Lexicon: 

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: 

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: 

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: 

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: 

Summary Lexeme Lexical Relations Homonymy Polysemy Synonymy Hyponymy WordNet Thematic Roles Selectional Restrictions Metaphor & Metonymy