Statistical Frequency in Word Segmentation

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Statistical Frequency in Word Segmentation
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Words dont come with nice clean boundaries
between them

• Where are the word boundaries?

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Question How do children work out where the word
boundaries are?
There are several potential clues

• Pauses (although this is dubious)

• Intonation (this too is dubious)

• Statistical regularities

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Statistical Regularities

• Words very rarely begin with dw,

• Words never begin with bn,

• Words never begin with lb,
• Etc.

• So if the child hears these sequences, the child
  hypothesizes the sequence occurred in the middle
  or at the end of the word.

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Statistical Regularities

• Voiceless stops that begin words are almost
  always aspirated,

• Voiced segments that end words are often
  de-voiced,

• Various other phonological processes may occur,
  e.g., word-final frication, etc.

• So these are phonological clues that may help
  segment the speech stream.

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Problem

• In order for children to be able to make use of
  these cues, they must be able to track the
  frequency of such items in the speech, otherwise
  it is a useless cue.

• So if the child is not able to track the
  frequency of bn at the beginning of words, what
  use is using this strategy?

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Statistical Tracking

• Very recent work suggests that children do in
  fact have the capacity to track statistical
  frequencies of certain elements in their
  environment.

• Major researchers Jenny Saffran (Wisconsin),
  Rebecca Gomez (Arizona), Elisa Newport
  (Rochester), Richard Aslin (Rochester), LouAnn
  Gerken (Arizona), Gary Marcus (NYU), etc.

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The Experiment - Overview

• Create a synthesized string of syllables that
  occur in a particular frequency (cant use
  English).

• Expose the children to this string of syllables
  for 20 minutes.

• Test children to see if they have a preference
  for the highly frequent syllable sets or the rare
  syllable sets.

• If children show a preference (no matter what
  direction that preference is in), then children
  are sensitive to frequencies of syllables in the
  input.

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Sample Stimulus

• Their language consisted of

• Four consonants (p,t,b,d)

• Three vowels (a, i, u)

• Which when combined created 12 syllables (pa, ti,
  bu, da, etc.).

• These then created six words

• babupu, bupada, dutaba, patubi, pidabu, and
  tutibu

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Transitional Probabilities

• The chances of a word containing bu are much
  greater than the chances of a word containing di.

• Transitional probabilities quantify this.

• The Transitional Probability of xy is
• xy
• x

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Transitional Probabilities

• So for the word babupu, the transitional
  probability of babu is calculated as follows

Frequency of babu / Frequency of ba ? 1/2 0.5
Frequency of bupu / Frequency of bu ? 1/4 0.25
Overall transitional probability of the word
babupu (0.50.25) / 2 0.375
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Whats the point?

• Transitional probability was manipulated so that
• The transitional probability was high within a
  word, but low across a word boundary. This is
  what a word IS in real life.

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• ba bu pu bu pa da du ta ba

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• 300 tokens of each of the six words were randomly
  concatenated.

• All word boundaries were removed

• This left 4536 continuous syllables, which were
  read by a speech synthesizer.

• Synthesizer produced a monotone of syllables at a
  rate of 216 syllables per minute.

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Procedures

• Subjects consisted of 24 undergraduate students.
• Subjects were told to listen to nonsense
  language.
• Task is to figure out where words begin/end.
• After 3 blocks of 7 minutes of exposure to the
  language, subjects were tested.

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Test Procedure

• Subjects heard two tri-syllabic strings, e.g.,

• bu-pa-da and pi-da-bu

Real word
Not a real word
Which sounds more like a word from this nonsense
language?
36 trials in the test.
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Results

• Mean score correct for all subjects was 27.2,
  where chance is 18. t-test shows this to be
  statistically significantly different from
  chance.
• Conclusion adults are able to recognize what is
  a word and what is not a word based purely on
  statistical frequency.

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• Additional finding

• the three words with the most common syllables in
  them were easiest to recognize.

• the three words with the least common syllables
  in them were hardest to recognize.

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But can kids do this too?

• Answer appears to be Yes.

• Saffran et al. (1996) used essentially the same
  stimuli on 8 month old children

• Used four strings of words instead of six.

• Children were exposed for only 2 minutes (not 21
  minutes)

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Methodology

• Head turning Procedure

Child
light
speakers
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Results

• Children looked statistically longer at the
  speaker from which novel words were being
  produced.
• Why is this? Why wouldnt they look longer at
  the speaker from which familiar words are being
  produced?

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Bottom Line

• Children have the ability to track transitional
  probabilities of sounds on the basis of very
  little exposure.
• This is therefore how words are parsed.

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Tool against Nativism?

• This has recently been the most prolific weapon
  against the idea that children use innate
  knowledge to acquire language.

• If children are using such sophisticated skills
  to segment words, why cant they use similar
  (non-linguistic) skills to learn syntax?

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But it isnt so simple

• Marcus et al. (1999) trained children on
  sentences of the following sort

• la ta la
• ga na ga
• da ba da

• x y x

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And tested them on

• wo fe wo
• gi tu gi
• po zi po

• Namely, words with
• new syllables, but
• the same structure (x-y-x)

And

• wo fe fe
• gi tu tu
• po zi zi

• Namely, words with
• new syllables, and
• new structure (x-y-y)

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Results

• Children appear to recognize the difference
  between these sets of stimuli
• ? Children are therefore tracking structure and
  not just simple statistics.

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Questions to ask yourself

• Why would statistical tracking be useful to
  linguists?
• As a tool to explain language acquisition.

• Does statistical tracking explain how children
  acquire language?

• No, only certain aspects of it.

• What aspects of language can we track?

?So far, it appears only phonologically related
things can be tracked like this (not
meaning-related things).
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Most Important Questions

• Is this useful for ALL languages on Earth?
• ? It appears that statistical tracking is only
  useful for auditory stimuli, not visualASL?

• Are humans the only creatures that can do this?
  (I hope so, otherwise other animals should have
  language too)

? No. Vervet and Tamarin monkeys have been shown
to have essentially the same abilities that
humans do.
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So what do we really know?

• Kids have spectacular abilities to track
  statistics.

• But so do adults (so why cant adults learn
  languages as well as kids?)

• But so do monkeys (so why cant monkeys learn
  language as well as humans?)

• This ability appears to be limited to statistics
  in auditory perception.