Taaltheorie en Taalverwerking BSc Artificial Intelligence Raquel Fernández Institute for Logic, Language, and Computation Winter 2012, lecture 5a Raquel Fernández TtTv 2012 - lecture 5a 1 / 21
Plan for this Week Topic: lexical semantics Today: • Word senses • Lexical relations between senses • Word Sense Disambiguation (WSD) Next lecture: • Word similarity • Distributional semantics models Raquel Fernández TtTv 2012 - lecture 5a 2 / 21
Compositional vs. Lexical Semantics Last week we looked into compositional semantics: how the meaning of sentences (expressed as FOL formulas) can be compositionally built up from the meanings of their constituents. S λ u . ( u @ vincent ) @ λ x . love ( x , mia ) � λ x . love ( x , mia ) @ vincent ⇒ love ( vincent , mia ) NP VP λ v . [ λ x . [ v @ λ y . love ( x , y )]] @ λ u . ( u @ mia ) N � λ x . [ λ u . ( u @ mia )@ λ y . love ( x , y ) ] λ u . ( u @ vincent ) � λ x . [ λ y . love ( x , y )@ mia ] Vincent � λ x . love ( x , mia ) V N λ v . [ λ x . [ v @ λ y . love ( x , y )]] λ u . ( u @ mia ) loves Mia Raquel Fernández TtTv 2012 - lecture 5a 3 / 21
Compositional vs. Lexical Semantics The compositional approach does not focus on the meaning of words. Words are considered basic expressions associated with an entity, a property, or a relation in the world (a FOL model). Polder is expensive Every pineapple is sweet Mia loves Vincent Expensive ( Polder ) ∀ x [ Pineapple ( x ) → Sweet ( x )] Love ( Mia , Vincent ) Raquel Fernández TtTv 2012 - lecture 5a 4 / 21
Compositional vs. Lexical Semantics The compositional approach does not focus on the meaning of words. Words are considered basic expressions associated with an entity, a property, or a relation in the world (a FOL model). Polder is expensive Every pineapple is sweet Mia loves Vincent Expensive ( Polder ) ∀ x [ Pineapple ( x ) → Sweet ( x )] Love ( Mia , Vincent ) This is a rather crude representation of word meaning: what do words like expensive or pineapple actually mean? Raquel Fernández TtTv 2012 - lecture 5a 4 / 21
Compositional vs. Lexical Semantics The compositional approach does not focus on the meaning of words. Words are considered basic expressions associated with an entity, a property, or a relation in the world (a FOL model). Polder is expensive Every pineapple is sweet Mia loves Vincent Expensive ( Polder ) ∀ x [ Pineapple ( x ) → Sweet ( x )] Love ( Mia , Vincent ) This is a rather crude representation of word meaning: what do words like expensive or pineapple actually mean? • Lexical semantics is the sub-field of linguistics that deals with word meanings. • It is related to lexicography: a discipline dedicated to the design and compilation of dictionaries. Raquel Fernández TtTv 2012 - lecture 5a 4 / 21
Word Forms and Word Senses The main aspect that makes lexical semantics a challenging problem is that the relation between word form and word meaning is not one-to-one: • Several words can have the same meaning → synonymy ∗ ‘buy’ / ‘purchase’ ∗ ‘car’ / ‘automobile’ Raquel Fernández TtTv 2012 - lecture 5a 5 / 21
Word Forms and Word Senses The main aspect that makes lexical semantics a challenging problem is that the relation between word form and word meaning is not one-to-one: • Several words can have the same meaning → synonymy ∗ ‘buy’ / ‘purchase’ ∗ ‘car’ / ‘automobile’ • One word can mean different things → lexical ambiguity ∗ ‘bank’ 1 : the slope of land adjoining a body of water ∗ ‘bank’ 2 : a business establishment in which money is kept Raquel Fernández TtTv 2012 - lecture 5a 5 / 21
Word Forms and Word Senses The main aspect that makes lexical semantics a challenging problem is that the relation between word form and word meaning is not one-to-one: • Several words can have the same meaning → synonymy ∗ ‘buy’ / ‘purchase’ ∗ ‘car’ / ‘automobile’ • One word can mean different things → lexical ambiguity ∗ ‘bank’ 1 : the slope of land adjoining a body of water ∗ ‘bank’ 2 : a business establishment in which money is kept Note that when we talk about word forms, we refer to lemmas (stems or roots). Word senses are the meanings associated with lemmas. Raquel Fernández TtTv 2012 - lecture 5a 5 / 21
Lexical Ambiguity: One Form, Several Senses Raquel Fernández TtTv 2012 - lecture 5a 6 / 21
Lexical Ambiguity: One Form, Several Senses Homonymy: accidental ambiguity between unrelated senses (1) a. Mary walked along the bank of the river. b. ABN-AMRO is the richest bank in the city. (2) a. Nadia’s plane taxied to the terminal. b. The central data storage device is served by multiple terminals. c. He disliked the angular planes of his cheeks and jaw. Raquel Fernández TtTv 2012 - lecture 5a 6 / 21
Lexical Ambiguity: One Form, Several Senses Homonymy: accidental ambiguity between unrelated senses (1) a. Mary walked along the bank of the river. b. ABN-AMRO is the richest bank in the city. (2) a. Nadia’s plane taxied to the terminal. b. The central data storage device is served by multiple terminals. c. He disliked the angular planes of his cheeks and jaw. Polysemy: ambiguity between semantically related senses (3) a. The bank raised its interest rates yesterday. b. The store is next to the newly constructed bank. (4) a. John crawled through the window. b. The window is closed. (5) a. The lamb is running in the fild. b. John ate lamb for dinner. (6) a. John spilled coffee on the newspaper b. The newspaper fired its editor. Raquel Fernández TtTv 2012 - lecture 5a 6 / 21
Polysemy vs. Homonymy In dictionaries, it is common to group polysemous senses within one lexical entry and to include a different lexical entry for each homonymous sense or group of senses. http://www.dictionary.com/ Raquel Fernández TtTv 2012 - lecture 5a 7 / 21
Polysemy vs. Homonymy In dictionaries, it is common to group polysemous senses within one lexical entry and to include a different lexical entry for each homonymous sense or group of senses. http://www.dictionary.com/ The distinction between homonymy and polysemy is one of degree: there is no hard threshold for how related two senses must be to be considered polysemous. Raquel Fernández TtTv 2012 - lecture 5a 7 / 21
Homophones & Homographs Two other types of lexical ambiguity that cause problems when dealing with spoken language: Raquel Fernández TtTv 2012 - lecture 5a 8 / 21
Homophones & Homographs Two other types of lexical ambiguity that cause problems when dealing with spoken language: Homophones: one pronunciation, several forms and several senses. break / brake knows / nose to / too / two waste / waist They pose problems for any application that requires speech recognition. Raquel Fernández TtTv 2012 - lecture 5a 8 / 21
Homophones & Homographs Two other types of lexical ambiguity that cause problems when dealing with spoken language: Homophones: one pronunciation, several forms and several senses. break / brake knows / nose to / too / two waste / waist They pose problems for any application that requires speech recognition. Homographs: one form, several senses and several pronunciations. All candidates are present today. The boss will present the award at 10:00. They pose problems for any application that requires speech synthesis. Raquel Fernández TtTv 2012 - lecture 5a 8 / 21
Relations between Senses: Synonymy & Antonymy Besides ambiguity, lexical semantic theories are also interested in accounting for semantic relations that hold between senses. Raquel Fernández TtTv 2012 - lecture 5a 9 / 21
Relations between Senses: Synonymy & Antonymy Besides ambiguity, lexical semantic theories are also interested in accounting for semantic relations that hold between senses. • Synonymy: a relation of semantic identity (or near identity) between senses. aurora/dawn/sunrise whore/prostitute big/large Raquel Fernández TtTv 2012 - lecture 5a 9 / 21
Relations between Senses: Synonymy & Antonymy Besides ambiguity, lexical semantic theories are also interested in accounting for semantic relations that hold between senses. • Synonymy: a relation of semantic identity (or near identity) between senses. aurora/dawn/sunrise whore/prostitute big/large • Antonymy: relation of semantic oppositeness between senses. tall/short dead/alive up/down Raquel Fernández TtTv 2012 - lecture 5a 9 / 21
Relations between Senses: Synonymy & Antonymy Besides ambiguity, lexical semantic theories are also interested in accounting for semantic relations that hold between senses. • Synonymy: a relation of semantic identity (or near identity) between senses. aurora/dawn/sunrise whore/prostitute big/large • Antonymy: relation of semantic oppositeness between senses. tall/short dead/alive up/down • Note that antonyms have opposite but very similar meanings: automatically distinguishing synonyms from antonyms can be difficult. Raquel Fernández TtTv 2012 - lecture 5a 9 / 21
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