Introduction to Computational Linguistics

Dr. Radhika MamidiENG 270

Lecture 3

Semantic Analysis

• Meaning Representation in Lexicon using• Semantic Features• Case Frames

• Ambiguity Resolution by:• Selectional Restrictions• Semantic Filtering

We are using a different representation for semantics –from a computational perspective!

Defining words using:

Semantic Features: A semantic feature is a notational method which can be used to express the existence or non-existence of semantic properties by using plus and minus signs.

(+physical)(+living)(+human)(-male) (-adult)

These features help in permitted and non permitted combination of words. Eg: happy pencil, table eats, green ideas will not be permitted.

Defining NOUNS using semantic features:

Man is [+HUMAN], [+MALE], [+ADULT]Woman is [+HUMAN], [-MALE], [+ADULT]Boy is [+HUMAN], [+MALE], [-ADULT]Girl is [+HUMAN], [-MALE], [-ADULT]

Defining words using:

Case Frames for VERBSDefining words in terms of sentence contextsFormulated as case frames stating the cases for verbs Object Object

>------------’collide’----------------<

Agent Object >----------------’strike’------------------<

InstrumentRepresentation of case frames using predicate calculus

Verb is specified as predicate and the cases as its argumentscollide (Object1, Object2)strike (Agent, Object, Instrument)

sleep (Agent, Place)

Ambiguity Resolution

Selectional RestrictionsWord sense hierarchy based on subset relation helps in disambiguation. This relation allows restrictions in terms of broad classes of objects.Eg: The dishwasher read the article Dishwasher can be a ‘person’ or a ‘machine’

Article can be a ‘text’ or an ‘object’Constraints: for ‘read’- agent must be a person, theme must be text object

Allows now: boy, man, woman etc for agent and book, paper, journal etc for theme

So ‘dishwasher’ is a person and ‘article’ is a text.

Semantic FilteringEg: I wrote a letter to my aunt from London.

I saw the boy with a telescope.

PP (Prepositional Phrase) attachment ambiguity

Intuition for right reading is captured

Pragmatic Analysis

• Grice’s Conversational Implicatures• Speech Act Theory• Austin’s Felicity Conditions

Think if it is easy to make the computers understand the pragmatics of language!

Grice’s theory of implicatures

• Proposed by H.P.Grice (1975) • A theory of how people use the language.• Guidelines for effective & efficient use of

language in conversation.• Four Gricean Maxims

– Quality, Quantity, Relevance and Manner– Together describe the general principle of co-operative

interaction: “...make your contribution such as is required, at the stage at which it occurs, by the accepted purpose or direction of the talk exchange in which you are engaged.”

In short...

... in order to converse in a maximally efficient, rational, co-operative way, the participants should speak sincerely, relevantly and clearly, while providing sufficient information.

Counter-example 1 A: Is your father at home? B: Today is Monday.Counter-example 2 A: Thanks for calling me for dinner B: I am very sorry…..

Speech Act Theory

• Speech Acts– Locutionary, Illocutionary, Perlocutionary Acts– Indirect Speech Acts

• Utterances– Constatives – stating Eg: I eat apples. [You may not be eating when you utter this sentence]– Performatives – performing Eg: I apologise. [You are actually apologising when you utter this sentence]

Speech Acts - illustrationJohn to his cousin: “I will come to your wedding definitely.”

• Locutionary act: the utterance of a sentence with determinate sense and reference– Reference of ‘I’, ‘your’

• Illocutionary act: the making of a statement, offer, promise, etc. in uttering a sentence, by virtue of the conventional force associated with it ( or with its explicit performative paraphrase)– Austin calls this act as SPEECH ACT– Here, it is an act of promise.

• Perlocutionary act: the bringing about of effects on the audience by means of uttering the sentence, such effects being special to the circumstances of utterances– John’s cousin feels happy.

Indirect Speech Acts

• A class of utterances whose syntactic forms do not match their illocutionary force.

• The implicature arises when the sincerity conditions are not met.

• Examples:1. Can you pass the salt? = ‘‘please pass the salt’’2. Will you clean your room today? = ‘‘clean your room

today’’3. It’s cold in here. = ‘‘can you shut the windows’’4. You are standing on my foot. = ‘‘please move’’

Examples of performative verbs

• I apologise• I declare the games open• I hereby christen the ship H.M.S. Ulysses• I promise to get you the books tomorrow• I warn you that parking here is not allowed• I object• I sentence you to ten years of hard labour

Austin’s Felicity Conditions

Austin (1962) formulated conditions for the performatives to be successful:

A. (i) There must be a conventional procedure having a conventional effect

(ii) The circumstances and persons must be appropriate, as specified in the procedure

B. The procedure must be executed (i) correctly and (ii) completely C. Often, (i) the persons must have the requisite thoughts, feelings

and intentions, as specified in the procedure, and (ii) if consequent conduct is specified, then the relevant parties

must so do.

Examples• A marriage ceremony cannot be performed by

just anyone. “I hereby pronounce you husband and wife”• Saying ‘sorry’ without actually feeling

apologetic. “I apologise”• Warning by a small boy with a toy gun. “I warn you to drop your guns”• Declaration that the university will be closed

tomorrow by a student. “I declare tomorrow a holiday”

Discourse Knowledge

• Co-reference relations• Anaphoric forms• Local Discourse Context• Pronouns and Centering• Ellipsis• Cue Phrases

Discourse knowledge is essential in building Human-Computer interactive systems.

Co-reference relations

Co-referential forms cannot be interpreted semantically in their own right - they refer to something else for their interpretation.

Exophora: The reference is outside the text. Eg. ‘Look at that’. that =

Endophora: The reference is inside the text. Anaphoric: Look at the fish. It is blue.

Cataphoric: It is blue, the fish.

Types of Anaphoric form

(a) Repeated form: The Prime Minister met the cricketers. The Prime Minister wished them good luck.

(b) Partially repeated form: Prof Ruslan Mitkov met the students. Prof. Mitkov had something nice to say to them.

(c) Lexical Replacement: Mary’s daughter came first again. The child is good at studies.

(d) Pronominal form: Zoha said she would have to take Noor to the doctor.

(e) Substituted form: Mary has a birthday in May. Sue has one too.

(f) Ellided form: Anne is in London. So is Kate. And Linda.

Local Discourse Context

• Includes syntactic and semantic structures of preceding sentences.

• Useful for:– detecting antecedents of pronouns– interpreting sentences with VP ellipsis

• Discourse Entity list is its important aspect– to find likely antecedent of the pronounEg: John loves this house. It is big. [‘It’ John, house] John loves this house. He is funny. [‘He’ John, house]

Detecting antecedents of pronouns – 3 methods

• Ali slapped Sam. He was stunned.• Ali slapped Sam. He was scolded.

1. Expectation Matching - AnaphoraAli slapped SamExpectation1: Hurt & Theme: SamExpectation2: Wrong & Theme: AliExpectation3: Stunned & Theme: SamExpectation4: Scolded & Theme: Ali

2. Anaphora based on History Lists

Given the history list, recency constraint helps in resolving pronominal reference– check for the most recent local context that

matches all the constraints related to the pronoun– if no antecedent is in the current local context

then search at the next most recent local context in the history list

Eg: If the baby does not like the new toy, throw it away. If the baby does not like the new toy, give it the old one.

3. Pronouns and CenteringEg: Mary went to the shopping mall. She met Sally. She bought some

books.

Centering Constraints:a. If any object in the local context is referred to by a pronoun in the

current sentence, then the center of that sentence must also be pronominalised.

b. The center must be the most preferred discourse entity in the local context that is referred to by a pronoun.

c. Continuing with the same center from one sentence to the next is preferred over changing the center.

Eg: Mary went to the market. She met Sally. She bought some books. Sally bought some chocolates. She wanted to give them to her brother .

Ellipsis

• Syntactically incomplete sentences• Parts missing are retrieved from previous

major clause.Example: (Allen: 451)

Helen saw the movie. Mary did too.A correspondence between the two subject forms existThe subject from first sentence is abstracted and applied

to the new information to produce the semantic form of the elliptical clause.

Cue Phrases

Two classes– identifies semantic relationships between clauses or states used for continuation, reason, contrast, conclusion

eg. Jack went to the store. Sam stayed at home. (read with ‘and’, ‘because’, ‘but’, ‘so’ inserted in second sentence)

– indicates discourse structure (they indicate segment boundaries used to end the current topic, to end the discourse, start

digression eg. OK/fine, Bye/thanks, By the way

Exercise

• Give the semantic features for the following nouns: puppy, husband, book, dream

• Give the case frames for the following verbs: sleep, eat, hit, read, see• Give two examples for indirect speech acts.• Explain the speech acts for the utterance “I am so thirsty”• Read all the slides!