November 20, 2003Chapter 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

eats

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”

bank

slopingmound

Financialinstitution

Lexeme

Lexeme

• 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

The phenomenon where a single lexeme has multiple related meanings

bank

Biologicalrepository

Financialinstitution

Lexeme

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

big

large

Positivesize

older

lexeme

lexeme

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

• 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

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

• 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, 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

• 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 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 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 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, 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

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

• WordNet

• Thematic Roles

• Selectional Restrictions

• Metaphor & Metonymy