Cortical auditory evoked potential correlates of categorical perception of voice-onset time

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Cortical auditory evoked potential correlates of
categorical perception of voice-onset time

• By Anu Sharma and
• Michael F. Dorman

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Previous research

• Listeners are better able to discriminate
  between sounds which have been identified as
  belonging to different phonetic categories than
  between sounds which have been identified as
  belonging to the same phonetic category.

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Previous research

• Experiments on the encoding of VOT by fibers in
  the auditory nerve on chinchillas have shown that
  the neural encoding of acoustic features was more
  salient in the middle of the VOT continuum.
• This corresponds to the uncertainty point for
  human listeners.

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Purpose of present study

• Examine neural correlates of VOT perception at
  the level of the auditory cortex (in humans).
• Test how the MMN of listeners will differ across
  and within categories of sound pairs.

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Behavioral experimentSubjects

• 16 adults aged between 20-30
• All were native speakers of English
• They had no history of speech or hearing
  disorders.

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Behavioral experimentStimuli

• A continuum of nine tokens varying from /da/ to
  /ta/.
• VOT varied from 0 ms to 80 ms (10 ms interval)

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Behavioral experimentProcedures

• Identification task classify sounds as /ta/ or
  /da/
• Participants had an initial practice.
• After they heard each sounds ten times in a
  random order.

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Behavioral experimentProcedures

• AX discrimination task across-category pairs
  (30-50ms) and within-category pairs (60-80ms).
• 50 pairs of each kind were used.
• ISI 500 ms
• Subjects had to answer if the stimuli were same
  or different.
• Again they had a practice trial.

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Behavioral experimentResults
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Behavioral experimentResults
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Behavioral experimentResults

• Results of the behavioral experiments are
  consistent with results from previous research.

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Electrophysiologic experimentSubjects

• 9 participants (same as for behavioral
  experiment) aged between 22-26.
• Participants were asked to watch a movie and to
  ignore the stimuli they heard through their right
  ear.

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Electrophysiologic experimentStimuli (MMN)

• Same stimuli as for the behavioral experiment.
• One repetitive stimulus and occasionally one
  target stimulus that is different.
• Across-category pair
• 30ms (standard) 50ms (deviant)
• Within category pair
• 60ms (standard) 80ms (deviant)

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MMN

• MMN is a response to stimulus change and occurs
  only when the deviant stimulus is presented in
  the context of a sequence of standard stimuli.

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Results
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Results
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Results

• Results from the electrophysiologic experiment
  complement well the results of the discrimination
  task. When participants say they hear different
  sounds, an apparent difference occurs in the
  brain wave.