Exploring the sensitive period for speech perception

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Exploring the sensitive period for speech
perception

• Jason D. Zevin
• Postdoctoral Associate
• Sackler Institute for Developmental Psychobiology
• Weill-Cornell Medical College

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Sensitive period effects in speech
perception/production
The later one starts learning a language, the
harder it is.
Flege, Yeni-Komshian Liu, 1999, JML
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How do developing systems become resistant to
change?

• Is there a hard maturational limit on plasticity?
• Or is there a more dynamic process of
  entrenchment
• Neural networks constructed to perform Task A
• Interferes with learning Task B
• Birdsong
• Reading
• Speech perception
• Hypothesis the timecourse of the sensitive
  period will be related to the developmental
  trajectory of speech perception.

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Plan of Attack

• Study Age of Acquisition effects
  cross-sectionally
• Study children at the same ages
• Develop methods
• developmentally feasible
• provide comparable data for different
  ages/experience

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Task Free fMRI Measure Dishabituation

• Subject lies still and watches a DVD with no
  soundtrack
• Listens to trains of stimuli presented in silent
  intervals between functional scans

Deviant
Standard
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Initial results

Speech vs silence contrast (collapsing across
standard and deviant trials
Bilateral superior temporal gyrus responds to
speech (relative to silent baseline).
Zevin McCandliss (2005, BBF)
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Initial results

Deviant vs. standard contrast.
A region of posterior superior temporal gyrus
(border with supramarginal gyrus) responds
specifically to phonetic change.
Zevin McCandliss (2005, BBF)
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How specific is this response?

• Using natural speech means there are
  extra-phonetic cues to stimulus change.
• How do we know this region is specialized for
  speech?
• Follow-up study with synthesized stimuli
• between and within category deviants
• Controlling for size of acoustic change

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Synthetic Stimuli
A subtle change in the third formant changes the
percept from /ga/ to /da/
Between-category change
The same physical change, but now all stimuli are
perceived as /ga/
Within-category change
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This response is pretty specific!

• This region responds more strongly to
  between-category deviants than within-category
  deviants.

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Application to second language learning

• Preliminary study of Native Japanese speakers
• /ra/ - /la/ not present in Japanese
• /da/ - /ga/ is the same across languages
• Comparing responses across phonetic contrasts
  gives us a measure of how native-like speech
  perception is

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Data from native Japanese speakers

• Our old friend in posterior superior temporal
  gyrus responds specifically to phonetic change
• responds to /da/ - /ga/ stimuli common to
  Japanese and English
• No differential response to the /ra/ - /la/
  stimuli present in English only.

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Age-limited plasticity
The dishabituation response to /la/ - /ra/
stimuli depends on Age of Acquisition
Early learners show a stronger (more native-like)
response.
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The Future

• Fully cross-sectional study of Age of Acquisition
  effect
• Data from children
• looking for patterns of
• lateralization
• focalization
• Specialization
• Mechanistic account of STG/SMGs role in speech
  perception

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Thanks

• The Sackler family
• Weill-Cornell Sackler Group, esp. Bruce
  McCandliss
• Marc Joanisse, U Western Ontario
  (between/within collaboration)
• NIH/NIDCD