1. Lecture 26 Lexical Semantics
October 6, 2005
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2. Meaning The meaning of a text or discourse
Traditionally, meaning in language has been studied from
three perspectives
The meaning of a text or discourse
The meanings of individual sentences or utterances
The meanings of individual words
We started in the middle, now we’ll look at the meanings of
individual words.
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3. Word Meaning We didn’t assume much about the meaning of words when
we talked about sentence meanings
Verbs provided a template-like predicate argument structure
Nouns were practically meaningless constants
There has be more to it than that
The internal structure of words that determines
where they can go and what they can do (syntagmatic)
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4. What’s a word? Words?: Types, tokens, stems, roots, inflected forms?
Lexeme –
An entry in a lexicon consisting of a pairing of a form with
a single meaning representation
Lexicon - A collection of lexemes
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5. Lexical Semantics
The linguistic study of systematic meaning related structure of
lexemes is called Lexical Semantics.
A lexeme is an individual entry in the lexicon.
A lexicon is meaning structure holding meaning relations of lexemes.
A lexeme may have different meanings. A lexeme’s meaning component is known as one of its senses.
Different senses of the lexeme duck.
an animal, to lower the head, ...
Different senses of the lexeme yüz
face, to swim, to skin, the front of something, hundred, ...
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6. Outline: Lexical Semantics
Concepts about word meaning including:
Homonymy, Polysemy, Synonymy
Thematic roles
Two computational areas
Enabling resource:
WordNet
Enabling technology:
Word sense disambiguation
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7. Relations Among Lexemes and Their Senses
Homonymy
Polysemy
Snonymy
Hyponymy
Hypernym
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8. Homonymy Homonymy is a relation holds between words have same form
with unrelated meanings.
Bank -- financial institution, river bank
For this, we should create two senses of the lexeme bank.
Bat -- (wooden stick-like thing) vs (flying scary mammal thing)
The problematic part of understanding homonymy isn’t with the forms,
it’s the meanings.
Causes ambiguity.
Nothing particularly important would happen to anything else in English
if we used a different word for the little flying mammal things
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9. Homonymy causes problems for NLP applications
Text-to-Speech
Same orthographic form but different phonological form
bass vs bass
Information retrieval
Different meanings same orthographic form
QUERY: bat care
Machine Translation
Speech recognition
Why?
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10. Polysemy Polysemy is the phenomenon of multiple related meanings
in a same lexeme.
Bank -- financial institution, blood bank
these senses are related. Are we going to create a single sense or two different senses?
While some banks furnish sperm only to married women, others are less restrictive
Serve
Which flights serve breakfast?
Does America West serve Philadelphia?
Does United serve breakfast and San Jose?
Most non-rare words have multiple meanings
The number of meanings is related to its frequency
Verbs tend more to polysemy
Distinguishing polysemy from homonymy isn’t always easy
(or necessary)
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11. Synonymy Synonymy is the phenomenon of two different lexemes having
the same meaning.
Big and large
In fact, one of the senses of two lexemes are same.
There aren’t any true synonyms.
Two lexemes are synonyms if they can be successfully substituted
for each other in all situations
What does successfully mean?
Preserves the meaning
But may not preserve the acceptability based on notions of politeness, slang, ...
Example - Big and large?
That’s my big sister a big plane
That’s my large sister a large plane
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12. Hyponymy and Hypernym
Hyponymy: one lexeme denotes a subclass of the other lexeme.
The more specific lexeme is a hyponymy of the more general lexeme.
The more general lexeme is a hypernym of the more specific lexeme.
A hyponymy relation can be asserted between two lexemes when the meanings of the lexemes entail a subset relation
Since dogs are canids
Dog is a hyponym of canid and
Canid is a hypernym of dog
Car is a hyponymy of vehicle, vehicle is a hypernym of car.
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13. Metaphor and Metonymy Specific types of polysemy Metaphor: Metonymy
Germany will pull Slovenia out of its economic slump.
I spent 2 hours on that homework.
Metonymy
The White House announced yesterday.
This chapter talks about part-of-speech tagging
Bank (building) and bank (financial institution)
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14. Antonyms
Words that are opposites with respect to one feature of their meaning
Otherwise, they are very similar!
Dark light
Boy girl
Hot cold
Up down
In out
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15. Ontology
The term ontology refers to a set of distinct objects resulting from analysis of a domain.
A taxonomy is a particular arrangements of the elements of an ontology into a tree-like class inclusion structure.
A lexicon holds different senses of lexemes together with other relations among lexemes.
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16. Lexical Resourses There are lots of lexical resources available
Word lists
On-line dictionaries
Corpora
The most ambitious one is WordNet
A database of lexical relations for English
Versions for other languages are under development
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17. WordNet WordNet is widely used lexical database for English.
WebPage:
It holds:
The senses of the lexemes
holds relations among nouns such as hypernym, hyponym, MemberOf, ..
Holds relations among verbs such as hypernym, …
Relations are held for each different senses of a lexeme.
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18. WordNet
Demo:
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19. Format of Wordnet Entries
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20. WordNet Relations
Some of WordNet Relations (for nouns)
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21. WordNet Hierarchies Hyponymy chains for the senses of the lexeme bass
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22. WordNet - bass
The noun "bass" has 8 senses in WordNet. 1. bass -- (the lowest part of the musical range) 2. bass, bass part -- (the lowest part in polyphonic music) 3. bass, basso -- (an adult male singer with the lowest voice) 4. sea bass, bass -- (the lean flesh of a saltwater fish of the family Serranidae) 5. freshwater bass, bass -- (any of various North American freshwater fish with lean flesh (especially of the genus Micropterus)) 6. bass, bass voice, basso -- (the lowest adult male singing voice) 7. bass -- (the member with the lowest range of a family of musical instruments) 8. bass -- (nontechnical name for any of numerous edible marine and freshwater spiny-finned fishes)
The adjective "bass" has 1 sense in WordNet. 1. bass, deep -- (having or denoting a low vocal or instrumental range; "a deep voice"; "a bass voice is lower than a baritone voice"; "a bass clarinet")
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23. WordNet –bass Hyponyms
Results for "Hyponyms (...is a kind of this), full" search of noun "bass" 6 of 8 senses of bass                                                    Sense 2 bass, bass part -- (the lowest part in polyphonic music)        => ground bass -- (a short melody in the bass that is constantly repeated)        => thorough bass, basso continuo -- (a bass part written out in full and accompanied by figures for successive chords)        => figured bass -- (a bass part in which the notes have numbers under them to indicate the chords to be played) Sense 4 sea bass, bass -- (the lean flesh of a saltwater fish of the family Serranidae)        => striped bass, striper -- (caught along the Atlantic coast of the United States) Sense 5 freshwater bass, bass -- (any of various North American freshwater fish with lean flesh (especially of the genus Micropterus))        => largemouth bass -- (flesh of largemouth bass)        => smallmouth bass -- (flesh of smallmouth bass) Sense 6 bass, bass voice, basso -- (the lowest adult male singing voice)        => basso profundo -- (a very deep bass voice) Sense 7 bass -- (the member with the lowest range of a family of musical instruments)        => bass fiddle, bass viol, bull fiddle, double bass, contrabass, string bass -- (largest and lowest member of the violin family)        => bass guitar -- (the lowest six-stringed guitar)        => bass horn, sousaphone, tuba -- (the lowest brass wind instrument)            => euphonium -- (a bass horn (brass wind instrument) that is the tenor of the tuba family)            => helicon, bombardon -- (a tuba that coils over the shoulder of the musician)        => bombardon, bombard -- (a large shawm; the bass member of the shawm family) Sense 8 bass -- (nontechnical name for any of numerous edible marine and freshwater spiny-finned fishes)        => freshwater bass -- (North American food and game fish)
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24. WordNet – bass Synonyms
Results for "Synonyms, ordered by estimated frequency" search of noun "bass" 8 senses of bass                                                         Sense 1 bass -- (the lowest part of the musical range)        => low pitch, low frequency -- (a pitch that is perceived as below other pitches) Sense 2 bass, bass part -- (the lowest part in polyphonic music)        => part, voice -- (the melody carried by a particular voice or instrument in polyphonic music; "he tried to sing the tenor part") Sense 3 bass, basso -- (an adult male singer with the lowest voice)        => singer, vocalist, vocalizer, vocaliser -- (a person who sings) Sense 4 sea bass, bass -- (the lean flesh of a saltwater fish of the family Serranidae)        => saltwater fish -- (flesh of fish from the sea used as food) Sense 5 freshwater bass, bass -- (any of various North American freshwater fish with lean flesh (especially of the genus Micropterus))        => freshwater fish -- (flesh of fish from fresh water used as food) Sense 6 bass, bass voice, basso -- (the lowest adult male singing voice)        => singing voice -- (the musical quality of the voice while singing) Sense 7 bass -- (the member with the lowest range of a family of musical instruments)        => musical instrument, instrument -- (any of various devices or contrivances that can be used to produce musical tones or sounds) Sense 8 bass -- (nontechnical name for any of numerous edible marine and freshwater spiny-finned fishes)        => percoid fish, percoid, percoidean -- (any of numerous spiny-finned fishes of the order Perciformes)
WordNet – bass Synonyms
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25. Internal Structure of Words
Paradigmatic relations connect lexemes together in particular ways
but don’t say anything about what the meaning representation of
a particular lexeme should consist of.
Various approaches have been followed to describe
the semantics of lexemes.
Thematic roles in predicate-bearing lexemes
Selection restrictions on thematic roles
Decompositional semantics of predicates
Feature-structures for nouns
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26. Thematic Roles
Thematic roles provide a shallow semantic language for characterizing certain arguments of verbs.
For example:
Ali broke the glass.
Veli opened the door.
Ali is Breaker and the glass is BrokenThing of Breaking event;
Veli is Opener and the door is OpenedThing of Opening event.
These are deep roles of arguments of events.
Both of these events have actors which are doer of a volitional event, and things affected by this action.
A thematic role is a way of expressing this kind of commonality.
AGENT and THEME are thematic roles.
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27. Some Thematic Roles
AGENT --The volitional causer of an event -- She broke the door
EXPERIENCER -- The experiencer of an event -- Ali has a headache.
FORCE - The non-volitional causer of an event --The wind blows debris.
THEME -- The participant most directly effected by an event --
She broke the door.
INSTRUMENT -- An instrument used in an event --
He opened it with a knife.
BENEFICIARY -- A beneficiary of an event -- I bought it for her.
SOURCE -- The origin of the object of a transfer event --
I flew from Rome.
GOAL -- The destination of the object of a transfer event --
I flew to Ankara.
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28. Thematic Roles (cont.) Takes some of the work away from the verbs.
It’s not the case that every verb is unique and has to completely specify how all of its arguments uniquely behave.
Provides a mechanism to organize semantic processing
It permits us to distinguish near surface-level semantics
from deeper semantics
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29. Linking AGENTS are often subjects
Thematic roles, syntactic categories and their positions in larger syntactic structures are all intertwined in complicated ways.
For example…
AGENTS are often subjects
In a VP->V NP NP rule, the first NP is often a GOAL
and the second a THEME
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30. More on Linking John opened the door AGENT THEME
The door was opened by John
THEME AGENT
The door opened
THEME
John opened the door with the key
AGENT THEME INSTRUMENT
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31. Deeper Semantics
He melted her reserve with a husky-voiced paean to her eyes.
If we label the constituents He and her reserve as the Melter and Melted, then those labels lose any meaning they might have had.
If we make them Agent and Theme then we don’t have the same problems
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32. Selectional Restrictions
A selectional restriction augments thematic roles by allowing lexemes to place certain semantic restrictions on the lexemes and phrases can accompany them in a sentence.
I want to eat someplace near Bilkent.
Now we can say that eat is a predicate that has an AGENT and a THEME
And that the AGENT must be capable of eating and the THEME must be capable of
being eaten
Each sense of a verb can be associated with selectional restrictions.
THY serves NewYork. -- direct object (theme) is a place
THY serves breakfast. -- direct object (theme) is a meal.
We may use these selectional restrictions to disambiguate a sentence.
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33. As Logical Statements For eat…
Eating(e) ^Agent(e,x)^ Theme(e,y)^Isa(y, Food)
(adding in all the right quantifiers and lambdas)
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34. WordNet
Use WordNet hyponyms (type) to encode the selection restrictions
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35. Specificity of Restrictions
What can you say about THEME in each with respect to the verb?
Some will be high up in the WordNet hierarchy, others not so high…
PROBLEMS
Unfortunately, verbs are polysemous and language is creative…
… ate glass on an empty stomach accompanied only by water
and tea
you can’t eat gold for lunch if you’re hungry
… get it to try to eat Afghanistan
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36. Discovering the Restrictions
Instead of hand-coding the restrictions for each verb,
can we discover a verb’s restrictions by using a corpus and WordNet?
Parse sentences and find heads
Label the thematic roles
Collect statistics on the co-occurrence of particular headwords with particular thematic roles
Use the WordNet hypernym structure to find the most meaningful level to use as a restriction
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37. Motivation
Find the lowest (most specific) common ancestor that covers a significant number of the examples
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38. Word-Sense Disambiguation
Word sense disambiguation refers to the process of selecting
the right sense for a word from among the senses that the word is known to have
Semantic selection restrictions can be used to disambiguate
Ambiguous arguments to unambiguous predicates
Ambiguous predicates with unambiguous arguments
Ambiguity all around
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39. Word-Sense Disambiguation
We can use selectional restrictions for disambiguation.
He cooked simple dishes.
He broke the dishes.
But sometimes, selectional restrictions will not be enough to disambiguate.
What kind of dishes do you recommend? -- we cannot know what sense is used.
There can be two lexemes (or more) with multiple senses.
They serve vegetarian dishes.
Selectional restrictions may block the finding of meaning.
If you want to kill Turkey, eat its banks.
Kafayı yedim.
These situations leave the system with no possible meanings, and they can indicate a metaphor.
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40. WSD and Selection Restrictions
Ambiguous arguments
Prepare a dish
Wash a dish
Ambiguous predicates
Serve Denver
Serve breakfast
Both
Serves vegetarian dishes
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41. WSD and Selection Restrictions
This approach is complementary to the compositional analysis approach.
You need a parse tree and some form of predicate-argument analysis derived from
The tree and its attachments
All the word senses coming up from the lexemes at the leaves of the tree
Ill-formed analyses are eliminated by noting any selection restriction violations
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42. Problems
As we saw last time, selection restrictions are violated all the time.
This doesn’t mean that the sentences are ill-formed or preferred less than others.
This approach needs some way of categorizing and dealing with
the various ways that restrictions can be violated
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43. WSD Tags A dictionary sense? What’s a tag?
For example, for WordNet an instance of “bass” in a text has 8 possible tags or labels (bass1 through bass8).
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44. WordNet Bass The noun ``bass'' has 8 senses in WordNet
bass - (the lowest part of the musical range)
bass, bass part - (the lowest part in polyphonic music)
bass, basso - (an adult male singer with the lowest voice)
sea bass, bass - (flesh of lean-fleshed saltwater fish of the family Serranidae)
freshwater bass, bass - (any of various North American lean-fleshed freshwater fishes especially of the genus Micropterus)
bass, bass voice, basso - (the lowest adult male singing voice)
bass - (the member with the lowest range of a family of musical instruments)
bass -(nontechnical name for any of numerous edible marine and
freshwater spiny-finned fishes)
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45. Representations Vectors of sets of feature/value pairs
Most supervised ML approaches require a very simple representation for the input training data.
Vectors of sets of feature/value pairs
I.e. files of comma-separated values
So our first task is to extract training data from a corpus with respect to a particular instance of a target word
This typically consists of a characterization of the window of text surrounding the target
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46. Representations
This is where ML and NLP intersect
If you stick to trivial surface features that are easy to extract from a text, then most of the work is in the ML system
If you decide to use features that require more analysis (say parse trees) then the ML part may be doing less work (relatively) if these features are truly informative
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47. Surface Representations
Collocational and co-occurrence information
Collocational
Encode features about the words that appear in specific positions to the right and left of the target word
Often limited to the words themselves as well as they’re part of speech
Co-occurrence
Features characterizing the words that occur anywhere in the window regardless of position
Typically limited to frequency counts
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48. Collocational [guitar, NN, and, CJC, player, NN, stand, VVB]
Position-specific information about the words in the window
guitar and bass player stand
[guitar, NN, and, CJC, player, NN, stand, VVB]
In other words, a vector consisting of
[position n word, position n part-of-speech…]
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49. Co-occurrence
Information about the words that occur within the window.
First derive a set of terms to place in the vector.
Then note how often each of those terms occurs in a given window.
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50. Classifiers Naïve Bayes (the right thing to try first) Decision lists
Once we cast the WSD problem as a classification problem, then all sorts of techniques are possible
Naïve Bayes (the right thing to try first)
Decision lists
Decision trees
Neural nets
Support vector machines
Nearest neighbor methods…
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51. Classifiers
The choice of technique, in part, depends on the set of features that have been used
Some techniques work better/worse with features with numerical values
Some techniques work better/worse with features that have large numbers of possible values
For example, the feature the word to the left has a fairly large number of possible values
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52. Statistical Word-Sense Disambiguation
Where s is a vector of senses,
V is the vector representation of the input
By Bayesian rule
By making independence assumption of
meanings. This means that the result is
the product of the probabilities of its
individual features given that its sense
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53. Problems One for each ambiguous word in the language
Given these general ML approaches, how many classifiers do I need to perform WSD robustly
One for each ambiguous word in the language
How do you decide what set of tags/labels/senses to use for a given word?
Depends on the application
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