Lecture Lexical Semantics CS 4705 What is lexical

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Lecture Lexical Semantics CS 4705

Lecture Lexical Semantics CS 4705

What is lexical semantics? • • • Meaning of Words Lexical Relations Word. Net

What is lexical semantics? • • • Meaning of Words Lexical Relations Word. Net Thematic Roles Selectional Restrictions Conceptual Dependency

What is a word? • Lexeme: an entry in the lexicon that includes –

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 (‘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

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? • • • General semantic interpretation

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 lexeme) – They

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

• Polysemy: – Distinct but related meanings – idea bank, sperm bank, blood 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

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

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? – – 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:

Hyponomy • 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. Word. Net (George

Semantic Networks • Used to represent lexical relationships – e. g. Word. Net (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, . . .

– – – Is a rock edible? What are the parts of a human

– – – 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)

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) ^ 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 –

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 ?

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

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

Next time • Word sense disambiguation and information retrieval • Chapter 17