LIN6932

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LIN6932

• Lecture 10
• Computational Lexical Semantics
• Hana Filip

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Meaning

• Traditionally, meaning in language has been
  studied from three perspectives
• The meanings of individual words
• How those meanings combine to make meanings for
  individual sentences or utterances (compositional
  semantics)
• How those meanings combine to make meanings for a
  text or discourse (discourse semantics)
• We are going to focus today on word meaning, also
  called lexical semantics.

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Outline Comp Lexical Semantics

• Intro to Lexical Semantics
• Homonymy, Polysemy, Synonymy
• Online resources WordNet
• Computational Lexical Semantics
• Word Sense Disambiguation
• Supervised
• Semi-supervised
• Word Similarity
• Thesaurus-based
• Distributional

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Preliminaries

• Whats a word?
• Notions weve used so far Types, tokens, stems,
  roots, inflected forms, etc...
• Lexeme An entry in a lexicon consisting of a
  pairing of a form with a single meaning
  representation

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Lexeme

• 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 Relationships between word
meanings

• Homonymy
• Polysemy
• Synonymy
• Hyponymy
• Hypernymy
• Meronymy
• Antonymy

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Homonymy

• A relation that holds between two lexemes that
  have the same form (phonological, orthographic or
  both) with unrelated meanings
• Homophones
• right - write
• piece - peace
• Homographs
• bass (fish) - bass (guitar)
• Homographs (and also homophones)
• Bat (wooden stick-like thing)
• Bat (flying scary mammal thing)
• Bank (financial institution)
• Bank (riverside)

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Homonymy

• A relation that holds between two lexemes that
  have the same form (phonological, orthographic or
  both) with unrelated meanings
• Example BANK

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 (same orthographic form but different
  phonological form)
• bass vs bass
• Information retrieval
• Homographs and pure homonyms (different meanings
  but same orthographic form)
• QUERY bat care

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Polysemy

• a single lexeme has multiple related meanings
• While some banks furnish sperm only to married
  women, others are less restrictive.
• I withdrew the money from the bank.
• The bank is constructed from red brick.

Biological repository
bank
Lexeme
Financial institution
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Polysemy

• A single lexeme with multiple related meanings
  (bank the building, bank the financial
  institution)
• Most common words have multiple meanings
• The number of meanings is related to its
  frequency
• Verbs tend more to polysemy

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Specific types of polysemy Metaphor and Metonymy

• Metaphor
• Sally is cold as ice.
• The relationship is on the rocks.
• Metonymy
• The White House announced its decision yesterday.
• Shakespeare takes up 5 feet of my bookshelves.

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Metaphor

• Metaphors help us understand one domain of
  experience (typically some abstract domain) in
  terms of another (typically a concrete domain).
• Sally is a block of ice
• is easy to understand but
• Sally is a table
• is odd. The reason is that
• we systematically and habitually understand the
  domain of interpersonal relationships in terms of
  temperature.

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Metaphor

• Time understood in terms of space
• The end of semester is coming upon us.

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Metonymy

• The use of one concept (domain) to refer to
  another concept (domain) whereby both belong to
  the same encompassing experiential
  (superordinate) domain
• PRODUCER FOR PRODUCT
• Ill have an Anchorsteam.
• AUTHOR FOR WORKS
• He likes Shakespeare.
• PLACE FOR INSTITUTION (people who constitute that
  institution)
• The White House had no comment.
• OBJECT USED FOR THE USER
• The gun he hired wanted fifty grand.
• PART FOR WHOLE
• We dont hire skinheads.

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How do we know when a word has more than one
sense?

• airline examples
• Which flights serve breakfast?
• Does Delta serve Philadelphia?
• The zeugma test/effect
• ?Does United serve breakfast and San Jose?
• Distinct meanings of an ambiguous lexeme
  (homophone,
• homograph) can give rise to the zeugma effect
  - oddity

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How do we know when a word has more than one
sense?

• The zeugma test/effect
• ?Does Delta serve breakfast and San Jose?
• Coordination construction with ellipsis
• Does Delta serve breakfast and ltdoes Delta
  servegt San Jose?
• Syntactic constraint on the coordinands (units)
  in a coordination construction they are
  typically of the same syntactic category e.g.,
  V, VP, clause, etc.
• Semantic constraint If a part of the second
  coordinand is elided, it must be interpreted in a
  parallel (ideally identical) fashion with the
  first coordinand.
• Hence, due to the parallelism, the choice of a
  reading for serve in the first
• coordinand determines the reading of the
  understood serve in the second elided
  constituent.
• The choice of a distinct meaning of the
  ambiguous lexeme serve gives
• rise to a zeugma effect in the above example

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How do we know when a word has more than one
sense?

• Another example
• ?John took some money out of his savings and then
  a vacation.
• The corresponding (understood) not elided
  coordination construction
• ?John took1 some money out of his savings and
  then he took1 a vacation.

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How do we know when a word has more than one
sense?

• Yet another example Zeugma test and VP anaphora
• (1) John saw a mole and so did Bill.
• The corresponding (understood) coordination
  construction
• (2) John saw a mole and Bill saw a mole.
• Due to the parallelism, the choice of a reading
  for a mole in the first coordinand determines
  its reading in the second VP in (2), and also in
  the corresponding VP anaphor so did in (1)

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How do we know when a word has more than one
sense?

• Distinct meanings of a polysemous lexeme
  (homophone, homograph) do NOT give rise to zeugma
  effects - oddity
• Pick up the glass1, and pour it1 into the
  pitcher.
• (example from Green 1989, p. 48)
• Here, different meanings of a single
  polysemous lexeme glass (container, content)
  are used in different clauses without oddity

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Synonymy

• Two lexemes are synonyms if they can be
  successfully substituted for each other in all
  situations without changing the content of a
  whole sentence (preservation of truth conditions)
• Some words that have the same meaning in some
  contexts.
• filbert hazelnut
• youth adolescent
• big large
• automobile car
• couch sofa
• soda pop
• There are probably no exact synonyms
• What are some differences between big and
  large?

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Synonymy

• How big is that plane? a big brother
• How large is that plane? a large brother
• A big fat apple
• ?A large fat apple

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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Hyponymy

• What is hyponymy?
• Not symmetric
• Example car is a hyponym of vehicle and vehicle
  is a hypernym of car
• Test That is a car implies That is a vehicle
  (but not vice versa)
• What is an ontology?
• Ex CAR1 is an object of type car
• What is a taxonomy?
• Ex car is a kind of vehicle. CAR1 is an object
  of type car
• What is an object hierarchy?

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Resources

• There are lots of lexical resources available
  these days
• Word lists
• On-line dictionaries
• Corpora
• The most ambitious one is WordNet
• A database of lexical relations for English
• Versions for other languages are under development

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WordNet Relations
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WordNet Hierarchies