Lecture
Lexical Semantics
CS 4705
What is lexical semantics?
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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
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
Homonomy
• 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?
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General semantic interpretation
Machine translation
Spelling correction
Speech recognition
Text to speech
Information retrieval
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.
• What’s the difference between polysemy and
homonymy?
• 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
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?
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
Synonomy
• Substitutability: different lexemes with the same
meaning
– How big is that plane?
– How large is that plane?
– How big are you? Big brother is watching.
• What influences substitutability?
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Polysemy (large vs. old sense)
register: He’s really cheap/?parsimonious.
collocational constraint: roast beef, ?baked beef
convention: economy fare/?price
Hyponomy
• General: hypernym (super…ordinate)
– dog is a hypernym of poodle
• Specific: hyponym (under..neath)
– poodle is a hyponym of dog
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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)
– 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 --> 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?
Thematic Roles
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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) ^ Broken(z,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
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
Next time
• Word sense disambiguation and information
retrieval
• Chapter 17