1. Lexical Semantics Chapter 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
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)
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