Lexical Semantics Chapter 16

1. Lexical SemanticsChapter 16 Lindsay Butler Ling 538 5 December 2006

2. What is lexical semantics? • Systematic meaning-related structure • Lexeme – pairing of an orthographic or phonological representation with a meaning (Saussurian sign) • Lexicon – a list of lexemes (finite) • Sense – the meaning component of a lexeme • Lexemes are not analyzable units. They have internal structure that determines how they combine with other elements in the sentence • The lexicon is not simply a finite listing, but rather a “creative generator” of infinite meanings

3. Senses of lexemes • Homophony – words that have the same form but different meaning • bank = financial institution • bank = sloping mound • Homograph (orthographic) • bass = type of fish • bass = musical instrument • versus homophone (phonological) • would = auxiliary verb • wood = hard, fibrous substance

4. Senses of lexemes • Polysemy – a single lexeme with multiple related meanings • bank (financial institution) and bank (sloping mound) are not related (etymologically) • but, bank (financial institution) and… • blood bank (not financial, but same concept of holding a deposit, just of blood) • “You can bank on Mans” (not financial, but it has the sense of ‘security’) • Finding the right meaning is the task of word sense disambiguation

5. Senses of lexemes • Synonymy – different lexemes with the same meaning • Test of substitutability • Example: big and large

6. Senses of lexemes • Hyponymy – a class of synonymy – pairings of lexemes where one denotes a subclass of the other • Hyponym: the more general of the pair • Car is a hyponym of vehicle • Hypernym: the more specific of the pair • Set of hyponyms have proved useful approximations of ontologies, taxonomies, and object structures

7. WordNet • A database of lexical relations for English • http://wordnet.princeton.edu • Three databases for: nouns, verbs, adjectives and adverbs • Based on the concept of a synset synonymy:{chump, fish, fool, gull, mark, patsy, fall guy, sucker, schlemiel, shlemiel, soft touch, mug}= a person who is gullible and easy to take advantage of

8. Structure of lexemes • Thematic roles – a set of categories that characterize certain arguments of verbs into a shallow semantic language • Jon climbed the wall • Shannon washed his hands • Deep roles are specific to the event: climb, wash • Shallow roles reveal a commonality between climbing and washing: They have animate volitional actors that are causers of the event. Thus, they demonstrate the thematic role of agent

9. More thematic roles • Theme – participant most directly affected • Experiencer – simply, the experiencer • Force – non-volitional causer • Instrument – simply, an instrument used • Beneficiary – simply, the beneficiary • Source – origin of the object of a transfer • Goal – destination of the object of a transfer • …

10. FrameNet • Lexical resource for English thematic roles (Baker et al., 1998; Lowe et al., 1997) • http://framenet.icsi.berkeley.edu • More than 625 semantic frames.

11. Structure of lexemes • Selectional restrictions • Lexemes have restrictions on which concepts can perform certain thematic roles • Example: • I wanna eat someplace that’s close to campus • eat is intransitive and doesn’t select an object (or theme) • You don’t want to eat the someplace that’s close to campus • I wanna eat some really good Chinese food today • eat is transitive and does select an object (or theme) some really good Chinese food

12. Representing selectional restrictions • Using event-oriented semantics to capture selctional restrictions • Hyponomy relations in WordNet: Evidence that hamburgers are edible

13. Structure of lexemes • Primitive decomposition • Example (motivated by McCawley (1968): • Andrew killed his evil twin • Andrew caused his evil twin to become not alive • Though kill and cause to become not alive are not synonyms, they have the same meaning • Decomposing a predicate into a more complex set of predicates: DO, CAUSE, BECOME, NOT, ALIVE • Conceptual Dependency (Schank, 1972) (more decomposition) is the most widely used in NLP • 11 primitives such as: ATRANS (the abstract transfer of possession or control from one entity to another), PROPEL (the application of physical force to move an object)

14. Structure of lexemes • Semantic field • set of words from a single domain may be captured by a more integrated or holistic relationship among them • The semantic domains that FrameNet employs, such as HEALTH CARE, CHANCE, PERCEPTION, COMMUNICATION, TRANSACTION, TIME, SPACE, BODY, MOTION, etc., can be used to represent a semantic field

15. Creativity in the lexicon • Metaphor – We have in mind a certain concept or situation, but we use words and phrases that are relevant to totally different kinds of concepts • Conventional metaphor (one type) • Such as CORPORATION AS PERSON • Fuqua Industries, Inc. said Triton Group, Ltd., a company it helped resuscitate, has begun acquiring Fuqua shares • And Ford was hemorrhaging; its losses would hit $1.54 billion in 1980.

16. Creativity in the lexicon • Metonymy – We denote a concept by using a closely related concept • Example: PLACE FOR INSTITUTION • The White House had no comment • Example: AUTHOR FOR AUTHOR’S WORKS • He likes Shakespeare

17. Computational approaches • For metaphor and metonymy • Convention-based • apply language specific knowledge • Reasoning-based • not specifically language related but rather a general reasoning ability

18. Conclusions • Lexical semantics deals with the vast meaning and structure of words/lexemes • Words cannot be analyzed in isolation • They can have multiple meanings, selectional restrictions on what can co-occur with them, and can be decomposed • Databases to help deal with the complexity of sense and structure: WordNet and FrameNet • The lexicon, though a finite list of lexemes, has infinite generative power (creativity of language) • How do we deal with the vastness and creativity of language computationally?: Decomposition