The Speech Stimulus
Perceiving Phonemes
Top-Down Processing
Is Speech Special?
The Speech Stimulus
• Spoken words are composed of phonemes
• phoneme: smallest unit of speech that affects
the meaning of a word
The Speech Stimulus: Articulation
• vocal tract open or closed: vowels are
produced with the vocal tract open
• voicing: vibrating of vocal cords
• place of articulation: point at which airstream
is blocked
• manner of articulation: degree to which
airstream is blocked
Analyzing The Speech Stimulus
• sound spectrogram - graph showing
intensities of various frequencies over time
• formants - bands of high intensity frequencies
in vowel sounds
• formant transitions - shifts preceding or
following formants; carry information about
Perceiving Phonemes
• variability problem: acoustic properties of
phonemes change depending on
– context (coarticulation)
– speech rate
– speaker
– loudness
Perceiving Phonemes
• segmentation problem: there is often no clear
separation between one phoneme and
Perceiving Phonemes
• categorical perception: phonemes are
perceived as categories; many different
sounds can be perceived as within the same
phonemic category
Top-Down Processing of Speech
• phonemic restoration effect: listeners
perceive a phoneme even when it is deleted
and replaced by noise
• demonstrates effect of context on speech
Top-Down Processing of Speech
• McGurk Effect: phoneme perception is
affected by visual cues about lip movements
• demonstrates effect of visual information on
speech perception
Top-Down Processing of Speech
• The problems for speech perception are
general perceptual problems, such as
• Similar top-down processing strategies might
be used (e.g., Gestalt heuristics)
Top-Down Processing of Speech
• indexical characteristics - characteristics of a
person’s voice that carry information about
– Age
– Gender
– Emotion
– Region
– Intent
Is Speech Special?
• According to the “Speech is Special”
hypothesis, we have a speech module:
specialized abilities for perceiving speech
• Motor theory: the speech module contains
knowledge of how phonemes are produced;
we use this knowledge for speech perception
Is Speech Special?
• Left hemisphere specialized for language
– Broca’s area - damage causes Broca’s Aphasia
– Wernicke’s area - damage causes Wernicke’s
• Categorical perception of phonemes
Is Speech Special?
• Although locations in the left hemisphere are
important for speech, specific brain areas are
important for other perceptual abilities as well
• Categorical perception of phonemes does not
require a speech module because:
– Infants show categorical perception of phonemes
– Quail and chinchillas also show it
– Non-speech sounds can be perceived categorically