Transcript March 2: Building a Lexicon

Building a Lexicon
Statistical learning & recognizing words
Overview
• More on segmentation: the role of statistics
• Using different cues to segmentation:
Transitional probabilities vs. Stress cues
• Once words are pulled out and remembered,
is lexical access mature?
• What are some steps that occur?
Word Segmentation
• In writing, blank spaces between words, but not in
speech
Once you know words, you can hear them
But how do babies pull them out? Only some in isolation!
What have babies learned by 810 months?
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Phonetic categories (position specific!)
Phonotactics (consonant sequences allowed)
Stress patterns (English predominantly Sw)
Ability to use distributional information
– A particular word in different contexts might pop
out, e.g.
– “the milk”, “drink your milk”, “do you want some
milk”
• Can babies use these to help segment words?
Word Segmentation
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At 7.5 months only segment Sw words
Succeed on words like DOCtor and HAMlet
But fail on words like guiTAR and deVICE
For WS words, seem to pull out only the
strong syllable
• Indeed, if the Strong syllable is always
followed by a particular function word, will
consider it part of the word
• E.g. pulled out “TAR is”
Transitional Probabilities
• Why would infants pull out ‘Tar is”
Different types of statistics
• Frequency
– the occurrence of an item, high vs low
– Sw words are higher in frequency
• Distributional probabilities
– the distribution of occurrences, bimodal vs. monomodal
• Transitional probabilities
– the probability that one item will follow (or be followed
by) another
Transitional probabilities and
segmentation
• What are transitional probabilities?
“ABCABCABC” “ABCBABCBA”
121424214 etc.
• The problem:
Listeningtoenglishwhenyoudon’tknowwhereonew
ordendsandthenextbeginsfeelsalittlelikethis.
Listening to English when you don’t know where
one word ends and the next begins feels a little
like this.
Transitional probabilities and
segmentation
• A sensitivity to transitional probabilities could be
useful:
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Lookattheprettybaby.
Whatalittlebaby!
Mybabygirliscrying.
Ilikemybagelwithcreamcheeseandlox.
• Probability of Ba+by is 3/4
• If you’re trying to figure out the “Units” in speech,
this might be a useful property
Transitional probabilities and
segmentation
• Presented infants with a 2min sequence of
syllables with no pauses:
• Bidakupadotigolabubidakugolabu...
• Test: golabu (word) vs tigola (partword)
1.0
0.33
Infants looked longer when the partwords were
played showing that they discriminated between
the two test items.
Saffran et al., 1996
Transitional probabilities and
language learning
• Infants can use the transition probabilities
between segments to pull out candidate words
from continuous English speech.
• At 8 months
• Maybe segmented words are part of prelexical
representations
Which comes first?
• So – we’ve learned that infants can use
stress cues to segment words
• And, they can use transitional probabilities
to segment words
• Heard speech has both cues
• We never produce monotone utterances like
Bidakupadotigolabubidakugolabu
• It seems, that if we combine Stress cues and
predictive transitional probabilities segmentation
might be even easier
• But how could you learn stress cues if you didn’t
already know some words?
• Johnson & Jusczyk (2001) suggest that infants
learn English stress frequencies from hearing
words in isolation
• Is this necessary? Not if infants use TPs first.
• Thiessen & Saffran (Dev. Psych, 2003) tested this
question
• They use the Saffran, et. al., type task, but added
Stress cues
• Compared infants of 6-7 and 8-9 months on their
ability to learn S-W vs. W-S words
• At 6-7 months, if the TP predicted a word, the
infants were able to segment it whether it was SW
or WS
• At 8-9 months, they only succeeded on SW words
• To use the Saffran, et. al. example
• An infant of 6-7 months could learn either:
– bidakupadotiGOlabubidakuGOlabu OR
– bidakupadotigoLAbubidakugoLAbu
• But an infant of 8-9 months showed better
learning of:
– bidakupadotiGOlabubidakuGOlabu
* This has been simplified from the way the study
was actually done
• Infants can use many different types of
statistics to learn about words
• These include frequency, distributional
probabilities, and transitional probabilities
• Different types of statistics might be
important at different points in development
for helping infants along the way
Is learning transitional
probabilities specific for learning
language?
• Tone sequences
AFB, F#A#D, EGD#, CG#C#
AFBF#A#DAFB EGD#CG#C#
• Test: AFB (“word”) vs. D#CG# (“partword”)
• Infants of 8 months looked longer when the
partwords were played, showing that infants
discriminated between the two test items.
Saffran et al., 1999
How specific is this type of
learning for language?
• Visual sequences
• Presented 1 object at a time, no pauses,
objects were in pairs
Kirkham et al. 2001
How specific is this type of
learning for language?
• Test:
(real pair) vs. (novel pair)
• Infants as young as 2 months looked longer
at the “novel pairs”
Kirkham et al. 2001
Are humans the only species
sensitive to statistics?
• Birds are sensitive to distributions of vowels: can
discriminate vowels when they are presented in a
bimodal distribution
Kluender et al., 1998
Cotton-top Tamarins (monkeys) heard the
continuous nonsense words of Saffran et al., and
responded the same way as infants, by looking
longer at the part-words
Hauser et al., 2001
Summary: statistical learning
• Human infants (and adults) can do it
• Humans can do it for language, but also for
tones and for visual images!
• Birds and monkeys can do it too.
• Statistical learning seems to be a very basic
learning mechanism that human infants use
to help “bootstrap” into language.
But is that all word learning is?
• The “word” learning we have talked about so far
this week and last, is really just about learning
familiar word forms
• We need word forms
• But we need to learn word meanings as well
• Indeed, a full lexical entry has:
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Word form
Word meaning (intension: definition)
Word meaning (extension: other objects it can apply to)
Grammatical class
The mature “lexicon” and
language processing
• Your lexicon has all this information about
every word you hear
• So, when you hear a sentence like,
– “Get the leash so we can take the dog to the park”
• You’ll recognize the highly predictable word
“dog” at the very beginning of the “d”
• The semantic context has been set up, you
know a noun must come after “the”, and those
two facts make you expect “dog”
• This allows you to synthesize, process more
efficiently, and look forward for what might
be coming next in the sentence
• Imagine if I had said:
– “Get the leash so we can take the drapes down”
• Fernald, et. al. (1998) ask if infants lexicons are
this mature from the beginning.
• They find they are not – but that they mature
quickly
– there are rapid gains in the speed of word recognition
between 15 and 24 months of age
– At 15 months, infants have to hear the whole word and
have a chance to see both objects
– By 24 months, they, like adults, are making a guess on
the basis of the beginning sounds of the word
More on accessing the word form…
• So, by 24 months, infants need only part of the sound
to access the whole word in the lexicon
• But this is just about accessing word form
• How about when infants start learning the meaning
of words as well?
• To explore this, we tested infants at the very
beginning stages of word meaning – an associative
understanding of word-object linkages
• (not nec the same thing as referential understanding)
• We asked how easy it is for infants to access the full
detail in the lexicon
Word-learning: Switch Design
(Werker, Cohen, Lloyd, Casasola, & Stager, DevPsych, 1998)
Baby in the Switch Task
Infants succeed by 14 months
Werker, Cohen, Lloyd, Casasola, & Stager, Dev. Psych. 1998
Looking Time During Test Phase
12
Looking Time (s)
10
8
Same
6
Switch
4
2
0
8
10 to 12
Age in Months
14
Learning Similar Words
Stager & Werker, Nature 1997
Early Word Learning
Results: Similar sounding words
• Infants of 14-months failed to learn similar words, even
though they can discriminate the words
• This suggested that at the early stages of word learning,
when the task is challenging for the infant, they do not
have the attentional resources available to attend to the
fine phonetic detail in words
• To confirm this, we tested older infants who are more
accomplished word learners (with Fennell, Corcoran, &
Stager, Infancy, 2002)
• To rule out the possibility that perhaps the objects were
too much alike, we used less similar objects
Testing infants on minimal pair
words
Habituation Phase
“bih”
Test Phase
Same
Switch
“dih”
“bih”
“dih”
Infants fail until 17 months
Stager & Werker, Nature, 1997
Werker, Fennell, Corcoran, & Stager, Infancy, 2002
-Unless they know the words well (Fennell & Werker,
Lang. & Speech, 2003; Swingley & Aslin, 2002)
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Looking Time (s)
14 tested in a preference test with both objects as
-Or
12
reminders
(with Fennell, Swingley, & Yoshida)
10
8
-And
see Julia Wales & George Hollich (CDS, 2003)Same
6
Switch
4
2
0
14
17
Age in Months
20
Infants fail until 17 months
Stager & Werker, Nature, 1997
Werker, Fennell, Corcoran, & Stager, Infancy, 2002
16
Looking Time (s)
14
12
10
Same
8
6
Switch
4
2
0
14
17
Age in Months
20
Minimal pair word learning
(Werker, Cohen, et al, 1998; Stager & Werker, 1997; Werker, et. al., 2001)
Explanation to date
(Stager & Werker, 1997; Fennell & Werker, in press; Werker &
Fennell, in press)
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Word-object linking is initially difficult for infants
May pick up detail while listening to words
Cannot learn linkage and utilize full phonetic detail
The task of learning the associative link is so difficult
that without the object there, they can’t remember the
precise details of the word form even though it is
likely stored in the lexicon
• As become more accomplished word learners, detail
can be used
Swingley & Aslin (2001, 2002)
do find evidence of use of phonetic detail
Where’s the Baby? OR Where’s the Vaby?
2 DVs: Latency to look away from mismatch (14 & 18-22 mos)
Total looking time to the match (14 mos)
How to explain different results?
• The two choice looking procedure reveals
remind the child of what the link is
– Can measure on-line processing via latency
– All information available for recognition memory
to operate in overall LT to match
Experiment 2: Combined Method
(w/Dan Swingley, Katie Yoshida & Chris Fennell)
• Teach infants 2 new minimal pair words via
habituation design
• Test using side by side S&A design
• 2 DVs
– latency to look toward match
– Total looking time to match
Habituation Phase
“bih”
“dih”
Test Phase (8 trials)
“bih”
• When tested this way, when both the object
and the word are presented, the infant has
more cues to remind them, and is better able
to access the full detail in the lexicon
To think about….
• Infants can use a variety of statistics to help
them learn about words
– Frequency
– Transitional probabilities
– Distributional probabilities
• Indeed, at the earliest stages of word
learning, associative statistics are helpful
• Do you think statistical & associative
learning can explain language?
Problems for statistics….
• How do infants know what to calculate their
statistics across?
• If they can detect transitional probabilities in tones
and visual images as well as syllables, why don’t
they ever connect syllables to tones?
• How do they know to listen for syllables to begin
with?
• NGG might suggest that just as the child is a filter
for motherese, the child’s language learning biases
act as a filter for which statistical regularities to
pay attention to, and when