1
Speech Perception: Non-Invariance, Segmentation, Categorical Perception
Issues in Speech Perception
1.
2.
3.
4.
Nature of speech (Why is speech perception difficult?)
Categorical Perception
Top-down vs. Bottom-up Processing (pre-lexical vs. post-lexical)
Context effect in speech perception
Basic Terms/Concepts
. Phonetics: The study of speech sounds; how they are produced in the vocal tract
(articulatory phonetics), their physical properties (acoustic phonetics), and how they are
perceived (auditory phonetics); roughly speaking, the study of human language sound
Terms:
 Articulator
 Consonants
o manner of articulation: how the airstream is modified by the vocal tract to
produce the sound where in the vocal tract the sound is made
o place of articulation
o voicing (openness of vocal folds)
 Vowels
o Tongue Height
o Tongue Advancement (or Tongue Backness)
2. Phonology: The study of the sound system of a language; roughly speaking, the study
of mental representation of language sound, (i.e. phonemes)
Terms:
 Phones: Speech Sounds
 Phonemes: A class of speech sounds identified by a native speaker as the same
sound; a mental entity or category related to various allophones by phonological
rules.
o You can test Phonemes with Minimal Pairs.
o Minimal Pairs: Two (or more) words that differ only by a single sound in the
same position and have different meanings
 Allophones: A set of nondistinctive realizations of the same phoneme
/t/
[t]
stop
[th]
top
[?]
kitten
[ſ]
little
2
Nature of speech
1. Non-discreteness of speech signal (linearity issue): There is no sharp physical break
between adjacent sounds in a syllable (e.g. b + a + d  bad)
2. Segmentation problem (there are no consistent gaps between words in connected
speech as there are in written language
Input
She’ll officially
She’s a must to avoid
By loose analogy
The parade was illegal
into opposing camps
Is he really?
I can’t fit any more on
in closing
the effective firing rate









Error
Sheila Fishley
She’s a muscular boy.
By Luce and Allergy
The parade was an eagle
into a posing camp
Israeli?
I can’t fit any, moron
enclosing
the effect of ...
3. Invariance problem (coarticulation, allophonic variation, diversity in speakers,
speed of speech): phoneme’s acoustic properties change across contexts: i.e. they
are NOT invariant; e.g. context dependence in consonant (/di/ vs. /du/)
4. Normalization issue (articulator variation, variations in accent, loudness & pitch,
and rate)
Characteristics of Human Speech Perception
1. Categorical perception: A special pattern of results in identifying and
discriminating speech stimuli that differ systematically along a phonetic continuum.
In categorical perception, listeners are only able to discriminate between stimuli that
are identified as members of two phoneme categories. this pattern of response is
suggestive of perceptual discontinuity across a continuously varying physical
dimension.
a. /p/ vs. /b/ distinction by Voice Onset Time ( 0 – 30 ms: /b/, 30 – 60 ms: /p/)
b. Voice Onset Time (VOT): The interval of time between the release of air
pressure in the production of word-initial stops and the onset of vocal cord
vibration associated with the voicing of the following vowel
c. Excellent discrimination across phoneme boundaries, but poor
discrimination within a category (correct discrimination shows its peak at
the boundary of two categories)
2. Example of categorical perception
3
3. Prototype (Goodness-of-fit): single idealized representation (version) of a
phoneme vs. storage of a whole range of exemplars
Context effect in Speech perception
1. Bottom-up processing: a listener’s perceptual analysis of the physical sound
pattern of speech “upward” from the level of the recognition of phonemes to
eventually comprehension of sentence meaning (auditory analysis  phonetic
analysis  phonological  semantic  syntactic level levels of analysis
2. Top-down processing: The use of prior knowledge or linguistic expectations to
facilitate word recognition and rapid sentence comprehension (semantics and syntax
 perception of syllables and words)
3. Phoneme Monitoring Task: press button when a target sound is heard
a. prelexical vs. postlexical code
b. word vs. non-word
c. probable word vs. less probable word
a. phoneme identification vs. word recognition
4. Phoneme Restoration: participants report that the deleted phoneme is perceptually
restored even they know it is missing (locating the cough in the speech, comparison
between replacing a phoneme vs. adding a noise (result: no detection suggesting
pre-lexical perception)





The state governors met with their respective legi*latures convening in the capital city.
It was fond that the *eel was on the shoe.
it was found that the *eel was on the orange.
It was found that the *eel was on the axle.
It was found that the *eel was on the table.
5. Controversy about whether or not we need to identify phonemes before recognizing
a word: research data suggest pre-lexical identification of a phoneme.