Lexical Semantics Chapter 16 Lindsay Butler Ling 538 5 December 2006

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

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

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

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

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

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

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

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 …

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.

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

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

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)

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

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.

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

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

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