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 (‘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
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
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 (same pron/different orth)
– homographs - bass and bass (same orth/different pron)
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
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?
• 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
• 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
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?
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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?
• Why do we care?
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.
– 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.
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.
Thematic Roles/Selectional Restrictions
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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
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
– How do we decide what I went to the bank means?
• Chapter 17.1-2