November 20, 2003 Chapter 16 Lexical Semantics

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November 20, 2003Chapter 16Lexical Semantics
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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

eats
Lexeme
eat
eaten
eat
ate
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Lexical Relations

• Homonymy
• Polysemy
• Synonymy
• Hyponymy

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Homonymy

• A relation that holds between two lexemes that
  have the same form with unrelated meanings
• Homophones
• Homographs
• found

sloping mound
Lexeme
bank
Financial institution
Lexeme
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Homonymy causes problems

• Spelling correction
• Confusables your vs. youre
• Speech recognition
• Homophones and pure homonyms
• Text-to-speech
• Homographs conduct
• Information retrieval
• Homographs and pure homonyms

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Polysemy

• The phenomenon where a single lexeme has multiple
  related meanings

Biological repository
bank
Lexeme
Financial institution
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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?

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Synonymy

• A relation that holds between two lexemes with
  the same sense

older
big
lexeme
Positive size
large
lexeme
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hyponymy

• A relation that hold between two lexemes where
  one denotes a subclass of the other

vehicle
vehicle
hypernym
car
car
hyponym

• ontology
• taxonomy
• object hierarchy

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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
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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

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Internal Structure of Words

• Thematic roles
• Selectional restrictions
• Primitive decomposition
• Semantic fields

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Thematic Roles

• Deep roles
• Houstons 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)

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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

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Thematic Roles

• Thematic roles can be used as a shallow semantic
  language
• Can be used to determine surface realization
• AGENT gt INSTRUMENT gt 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.

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Thematic Roles

• Doesnt 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

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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

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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 cant 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.

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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.

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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

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Metaphor

• Using metaphor, we refer to, and reason about,
  concepts using terminology appropriate to
  completely different kinds of concepts.
• CORPORATION AS PERSON
• That doesnt 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.

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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

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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.

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Summary

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