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Lexical Semantics

      CS 4705
            What is lexical semantics?

•   Meaning of Words
•   Lexical Relations
•   WordNet
•   Thematic Roles
•   Selectional Restrictions
•   Conceptual Dependency
                    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
   – Right (‘rt) adj: located nearer the right hand esp.
     being on the right when facing the same direction as the
   – 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

• Homonyms: Words with same form but different,
  unrelated meanings, or senses (multiple lexemes)
   – A bank holds investments in a custodial account in the
     client’s 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 (different orth. form)
   – homographs - bass and bass (different phon. form)
    Ambiguity: Which applications will these
             cause problems for?
•   General semantic interpretation
•   Machine translation
•   Spelling correction
•   Speech recognition
•   Text to speech
•   Information retrieval
                 What is polysemy?

• Word with multiple but related meanings (same
   – They rarely serve red meat.
   – He served as U.S. ambassador.
   – He might have served his time in prison.
• What’s the difference between polysemy and
• Homonymy:
   – Distinct, unrelated meanings
   – Different etymology? Coincidental similarity?
• Polysemy:
   – Distinct but related meanings
   – idea bank, sperm bank, blood bank, bank bank
   – How different?
       • Different subcategorization frames?
       • Domain specificity?
       • Zeugma: Can the two candidate senses be
       ?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?
               Metaphor, Metonymy

• What is metaphor?
   – Father of the atom bomb.
• What is metonymy?
   – GM killed the Fiero.
   – The ham sandwich wants his check.
• Both extend existing sense to new meaning
   – Metaphor: use completely different concept (but cf
     conventional metaphors like GM)
   – Metonymy: use related concepts

• Substitutability: different lexemes with the same
   – 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: He’s really cheap/?parsimonious.
   –   collocational constraint: roast beef, ?baked beef
   –   convention: economy fare/?price

• General: hypernym (super…ordinate)
    – 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
                 Semantic Networks

• Used to represent lexical relationships
   – e.g. WordNet (George Miller et al)
   – Most widely used hierarchically organized lexical
     database for English
   – Synset: set of synonyms, a dictionary-style definition
     (or gloss), and some examples of uses --> a concept
   – Databases for nouns, verbs, and modifiers
• Applications can traverse network to find
  synonyms, antonyms, hierarchies,...
–   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?
                    Thematic Roles
• E w,x,y,z Giving (x) ^ Giver(w,x) ^ Givee(z, x)
• ^ Given(y,x)
• E w,x,z Breaking (x) ^ Breaker(w,x) ^
• A set of roles:
   – agent, experiencer, force, theme, result, content,
     instrument, beneficiary, source, goal,...
   The dog ate the cheeseburger.
   What is cheeseburger?
   The sniper shot his victim with a rifle.
   What is rifle?
       Why do we need a thematic level?

• We already have syntactic subcategorization
   – Capture similarity between different (but related) uses
     of same lexical item)
   – Avoid need for subcategorization frames: mapping
     from syntax to lexical semantics
      What are Selectional Restrictions?

George ate a cheeseburger/his lunch/dirt.
Jim killed his philodendron
?His philodenron killed Jim.
The flu killed Jim.
       Schank's Conceptual Dependency

• Eleven predicate primitives represent all
• Objects decomposed into primitive categories and
• 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
                   Next time

• Word sense disambiguation and information
• Chapter 17