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Lexical SemanticsChapter 16

Lindsay Butler

Ling 538

5 December 2006

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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

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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

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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

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Senses of lexemes

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

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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

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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

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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

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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• …

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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.

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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

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Representing selectional restrictions

• Using event-oriented semantics to capture selctional restrictions

• Hyponomy relations in WordNet: Evidence that hamburgers are edible

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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)

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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

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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.

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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

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Computational approaches

• For metaphor and metonymy– Convention-based

• apply language specific knowledge

– Reasoning-based• not specifically language related but rather a

general reasoning ability

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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