Lexical Semantics

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Lecture

• Lexical Semantics

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What is lexical semantics?

• Meaning of Words
• Lexical Relations
• WordNet
• Thematic Roles
• Selectional Restrictions
• Conceptual Dependency

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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?

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• Right (rIt) 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

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Homonomy

• Homonyms Words with same form orth and pron --
  but different, unrelated meanings, or senses
  (multiple lexemes)
• A bank holds investments in a custodial account
  in the clients 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 (same pron/different
  orth)
• homographs - bass and bass (same orth/different
  pron)

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Ambiguity Which applications will these cause
problems for?

• A bass, the bank, read/red
• General semantic interpretation
• Machine translation
• Spelling correction
• Speech recognition
• Text to speech
• Information retrieval

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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.
• Whats the difference between polysemy and
  homonymy?
• Homonymy
• Distinct, unrelated meanings
• Different etymology? Coincidental similarity?

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• Polysemy
• Distinct but related meanings
• idea bank, sperm bank, blood bank, bank bank
• How different?
• Different subcategorization frames?
• Domain specificity?
• 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?

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Metaphor, Metonymy

• Metaphor?
• Father of the atom bomb.
• Metonymy?
• GM killed the Fiero.
• The ham sandwich wants his check.
• Both extend existing sense to new meaning
• Metaphor completely different concept
• Metonymy related concepts

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Synonomy

• Substitutability different lexemes, 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 Hes really cheap/?parsimonious.
• collocational constraint roast beef, ?baked beef
• convention economy fare/?price

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Hyponomy

• General hypernym (superordinate)
• 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

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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
  --gt a concept
• Databases for nouns, verbs, and modifiers
• Applications can traverse network to find
  synonyms, antonyms, hierarchies,...

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• 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?
• Why do we care?

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Thematic Roles

• E w,x,y,z Giving(x) Giver(w,x) Givee(z, x)
  Given(y,x)
• A set of roles for each event
• Agent volitional causer -- John hit Bill.
• Experiencer experiencer of event Bill got a
  headache.
• Force non-volitional causer The concrete block
  struck Bill on the head.
• Theme/patient most affected participant John
  hit Bill.
• Result end product Bill got a headache.
• Content proposition of propositional event
  Bill thought he should take up martial arts.

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• Instrument instrument used -- John hit Bill
  with a bat.
• Beneficiary qui bono John hit Bill to avenge
  his friend.
• Source origin of object of transfer event Bill
  fled from New York to Timbuktu.
• Goal destination of object -- Bill led from New
  York to Timbuktu.

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Thematic Roles and Selectional Restrictions

• Selectional restrictions semantic constraint
  that a word (lexeme) imposes on the concepts that
  go with it
• George hit Bill with
• .John/a gun/gusto.
• Jim killed his philodendron/a fly/Bill.
• ?His philodenron killed Jim.
• The flu/Misery killed Jim.

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Thematic Roles/Selectional Restrictions

• In practical use
• Given e.g. a verb
• What conceptual roles are likely to accompany it?
• What lexemes are likely to fill those roles?
• Assassinate
• Give
• Imagine
• Fall
• Serve

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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

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Next time

• Word sense disambiguation
• How do we decide what I went to the bank means?
• Chapter 17.1-2