Transcript Lecture 10: Language November 26, 1999

Language
Overview:
I. What is Linguistics?
II. Physiological Foundations
III. Aspects of Language
I. What is Linguistics?
- Linguistics as the scientific study of language
attempts to answers the question – what is language?
and how is it represented in the mind?
- Language is a system that uses physical symbols
(sound, mark on paper) to express meaning:
How is it that a particular sequence of sounds/written symbols
gives rise to a meaningful interpretation in the mind?
Language is:
1. Communicative: Permits communication between
individuals
2. Arbitrary: The relationship between language symbols
and their meaning is arbitrary
3. Structured: The pattern of language symbols is not
arbitrary:
“the boy ran from the dog” vs “the dog ran from the dog”
4. Generative: Basic units of language (words) can be used
to build limitless number of meanings. A sentence that has
never seen before can be understood with no problem.
5. Complex: There are an infinitely large set of possible
expressions, only a small portion of which are meaningful.
“The dog irritated Mary” (yes)
“Mary irritated the dog” (yes)
“Dog the Mary irritated” (no)
“Irritated the dog Mary” (no)
…..
For a given list of 10 words, the number of possible
sentences is 10! (= 3,628,800!!)
6. Innate:
The number of all possible 10-word sentences is
(105)(105) …(105) = 1050!!!
Yet the language is learned effortlessly by all normal
humans before they reach the age of five, with virtually
no direct instruction and with little or no systematic
presentation of relevant data.
Any child has the capacity to acquire any natural
language regardless of genetic heritage.
Notion of linguistic universals: Genetically determined,
hard-wired predisposition of linguistic features common
to all languages (e.g., visual & auditory perception)
Computability of Language
- Is language ( production & comprehension) a
NP hard problem?
- Re Searl’s Chinese Room Argument, it may not
actually possible to build the look-up table because
doing so would be NP-hard.
II. Physiological Foundations
1. Language Areas
2. Varieties of Aphasia
1. Language Areas
Lateralization of language function
Left brain: 98% of right-handed people have almost all of
the language functions represented in the lefthemisphere of the neocortex
Right-hemisphere processes language functions
regarding metaphorical or artistic meaning of language
(e.g., humor)
In left-handed people, language function is represented
predominantly in the right-hemisphere of the
neocortex
Broca’s Area
Production of spoken language
(Motor programs for controlling
speech sounds)
Wernicke’s Area
Comprehension of language
(Interpretation of spoken and written words)
Arcuate fasciculus
Connection between Broca’s and Wernicke’s areas
Visual Cortex
Processing of written language
Angula Gyrus
Connection among Broca’s, Wernicke’s and visual
cortex
Mortor Cortex
Making of speech sounds (i.e. controlling of vocal
muscles)
***Web site***
2. Varieties of Aphasia
Broca's aphasia:
- Damage to Broca’s area
- Prevent a person from producing speech.
- Nonfluent, telegraphic speech
- Words are not properly formed (more like a telegram)
(e.g.)
“I’m a sig … no… man … uh, well, … again.”
“Well..mess..uh..sgga..diz..es..”
- Person can understand language
Wernicke's aphasia:
- Damage to Wernicke’s area
- Loss of the ability to understand language
- Fluent but unintelligent speech
- Can form words properly but the words that are put
together make no sense
(e.g.)
“I go to a dog of cookies in TV”
“I cooked a radio for my mother on the door”
Conduction aphasia:
- Damage to Arcuate fasciculus
- Fluent speech/good comprehension, but unable to
repeat what is head or read
Acquired alexia:
- Damaged connection between visual cortex and
Wernicke’s area
- Inability to read, but can see words
Agraphia:
- Inability to write words
- damages to where??
Dyslexia:
- loss & deficits of reading skills, spelling and
recognizing word sounds
III. Aspects of Language
1. Phonetics, Syntax & Semantics
2. Speech Perception
3. Language Comprehension
- Lexical Processing
- Sentence Processing
4. Language Acquisition
1. Phonetics, Syntax &
Semantics
Phonetics: Sound structure of language
Syntax: Grammatical structure of language
Semantics: Meaning structure of language
PHONETICS
When language is spoken, words are represented
as sequences of discrete sounds or utterances
-- an acoustic signal produced in the upper
respiratory and vocal tracts.
One-to-one relationship between an abstract
symbol and the corresponding phoneme.
Bet <--> /bet/
SYNTAX
From a statistical viewpoint, language consists of a set
of arbitrary but distinct symbols that have high order
statistical dependence (I.e., hierarchical constraints;
no dependence, no language).
‘t’ ‘n’ (high freq), ‘x’ ‘z’ (low freq)
‘think’ ‘thank’ vs ‘thunk’ ‘thnki’
‘%^#*’ ‘&#@!!’ vs ‘%#*^’ ‘!#$@^&’
“I think of you” vs “I of you think”
Syntax is the study of sentence structure. It attempts to
describe what is grammatical in a particular language
in terms of rules, that detail an underlying structure
and a transformational process.
(e.g.)
- Subject-verb-object sentence order:
“John hit the ball”
- Transformational process:
“The ball was hit by the john”
- Ungrammatical:
“The ball hit by the john was”
Question: What makes a sentence grammatical, and
what makes it ungrammatical?
Linear-chain Hypothesis
A linear-chain grammar proposes that grammatical
sentences are constructed word by word, by selecting
next word in a sentence based on the associations of
the rest of the words in the sentence, for example,
“The boy took his baseball bat and the hit the _____”
Grammatical yet unlikely formed by associations:
“Colorless green ideas sleep furiously” (Chomsky, 1957)
Phase-structure Grammar
Grammatical sentences are intrinsically nonlinear, and
therefore must be represented as hierarchies.
Phase markers:
Sentence = noun phrase (NP) + verb phrase (VP)
Noun phrase (NP) = determiner (Det) + noun (N)
Verb phrase (VP) = verb (V) + noun phrase (NP)
- Recursive (infinite embedding)
- Ambiguity resolution
(e.g.) “Tonight’s TV program discusses stress, exercise and sex
with Dick Cavett.”
SEMANTICS
Semantics is the study of meaning. It is concerned to
describing how we represent the meaning of a word in
the mind and how we use this representation in
constructing sentences.
A sentence is an expression of world knowledge and/or
personal belief that is true or can be true under certain
circumstances. Sentences are related as synonymous
or contradictory (I.e., similarity structure).
“John believes the earth is flat”
“John believes the earth is round”
“John think the earth is plane”
“John think the earth is a plane”
Mental Lexicon Hypothesis
It proposes that words are recognized through a matching
process in which an input word is compared with a
mental dictionary called a lexicon.
- Dictionary of sound/spelling-meaning pairs for all the
words we know:
(e.g.) Word “Black”:
- Spelling: Black
- Pronunciation: /blaek/
- Part of speech: adjective
- Meaning pointer: ->
Mental Lexicon
2. Speech Perception
The first step in comprehending spoken
language is to identify the words being
spoken, performed in multiple stages:
1. Phonemes are detected (/b/, /e/, /t/, /e/, /r/, )
2. Phonemes are combined into syllables (/be/ /ter/)
3. Syllables are combined into words (“better”)
4. Word meaning retrieved from memory
Two Characteristics of Speech Perception
1. Segmentation:
The listener must unpack a continuous stream of
acoustic signal into ordered segments of phonemes
(consonants & vowels).
Further complicated by the fact that information about
different phonetic segments is transmitted in parallel by
the same acoustic segment.
--- An daunting task
2. Context effects:
(a) correct mispronunciation
(spoken) ‘We had a lot of compsiny over the weekend’
(heard) ‘We had a lot of company over the weekend’
(b) phoneme filling-in
/?ash/ vs /?ask/
(spoken) /?ash/ --> (heard) /dash/
(spoken) /?ask/ --> (heard) /task/
/tash/, /dask/ - not found in English
Such context effects suggest a top-down, knowledgedriven, & memory-based process of speech
perception.
In sum, Segmentation & Context are two major bottle
necks that we must overcome in the development of
computer speech recognition.
3. Language Comprehension
In normal conversation we usually understand the
meanings of the utterances without conscious effort
as soon as we hear.
The fact that language comprehension proceeds
with such little difficulty is a tribute to the
sophistication and efficiency of the language
processing system--I.e., pre-hard wired.
Auditory & visual analysis:
Syntactic Translation:
Translate identified words into a syntactic representation,
that specifies a temporal sequence of semantic categories
and relations (e.g., V, S, N, Det, NP, etc.)
Mental Lexicon in Memory:
Hypothesized storage place in memory for all information
pertaining to words such as pronunciation, meaning,
spelling, grammatical class, etc.
Spreading Activation in Mental Lexicon Network
Lexical Access Mechanism:
Both meanings of an ambiguous word are initially
retrieved and then the meaningfully interpretable,
appropriate one is selected.
A: “I need a new sink in the kitchen”
B: “I was sinking deep in the swimming pool”
How did we learn of the mechanism ?
-- Through semantic priming experiments
Priming
TargetLexical decision time
Sink
Car
?%#$
Swim
Swim
Swim
600 ms
750 ms
800 ms
Sink
Car
?%#$
Kitchen
Kitchen
Kitchen
605 ms
760 ms
800 ms
GTL (Grand Theory of Language)
4. Language Acquisition
1. Language is universal across all human societies. That is, all
societies use language in similar ways.
2. Despite the apparent diversity of human languages, any language
can be learned by anybody.
3. Accordingly, languages must have some common underlying
structures.
Universal Grammar (UG; Chomsky, 1965):
A set of abstract innate (i.e., hard-wired in the brain) principles
that are universal to all natural languages. Each language is
nothing but a specific implementation of these principles (e.g.,
temperature by F, C or K)
Discussion of the Paper “Actions from thoughts” by M A Nicolelis
-
HBMIs:
- Type I (cochlear implant) vs Type 2 (brain-controlled robot arm)
-
Challenges of building HBMIs:
- Recording brain signals (EEG; multichannel single-cell recording)
- Processing the signals & generating control outputs (NNs; learning rules)
-
Example HBMIs:
- Epilepsy control
- Prosthetic robot arm
-
How about possible dangers and pitfalls?
-
Neuronal ‘vote’
‘Brain pacemaker’
‘Phantom limb’
‘Virtual arm’