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The phonetics of speech errors
Frisch, S. A.
University of South Florida
This work supported by NIH-NIDCD R03 06164
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Study of Speech Errors
• The study of how speech can go wrong in speech errors tells us something about how the speech production mechanism works
• Error patterns are not “random”
• Similar approach used in (non-clinical) aphasia research: The disordered brain tells us about normal brain function
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Phonological Segment Errors
• Many speech errors involve the apparent mis-production of a single phoneme
• For example,“Frisch fry” for ‘fish fry’
“like box” for ‘bike locks’
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One Model
• Dell (1986 inter alia) spreading activation model– Word nodes activate phonemes– Phonemes activate related words, creating
competition– The model is also noisy– Accidental over-activation of an incorrect
phoneme creates a speech error
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Support for the Model
• Increased speech error rate when phonemic context is shared in experiments that elicit errors
• For example, initial /b, m/ errors– Most common: make bake– Less common: made bake– Least common: mad bake
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More Support
• Errors that create words are more common than errors that don’t– For example, sip zap vs. sung zone
• Also, effect of word level can be influenced by processing time– Demand for a quick response results in less
of a lexical effect– Not enough time for competition to build
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Errors at the Gestural Level
• Sub-phonemic errors have not been studied much
• Mowrey & MacKay (1990) used electrodes to examine muscle activation in errors, and found evidence for frequent “gradient” errors
• Pouplier (2003) EMA study found gestural insertion common in errors
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Research Program
• Is the gestural level just another interactive layer in the connectionist model, or a separate component?
• Is gestural activation and competition like phonemic/lexical activation and competition?
• Can lexical influences on gestural errors be found?
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Frisch & Wright (2002)
• Acoustic study of speech errors between /s/ and /z/– Crucially differ in voicing (periodicity)– Less crucial differences in amplitude and
duration– However, some potential interdependence of
these differences
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Categorical Gestures
• Errors that switched all the way to the ‘norm’ of the other category were more common than extreme gradient errors
• Clearer to see for /s/ targets than for /z/ targets, as devoicing of /z/ is phonetically normal
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Distribution of voicing
/s/ /z/
0% 324 56
0-30% 50 71
30-60% 4 23
60-100% 6 33
100% 13 252
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Current research
• Speech errors studied using ultrasound
• Ultrasound recordings give a means to directly measure articulation
• Similar to Pouplier (2003) EMA studies
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Participants
• Four undergraduate students from the CSD department
• Monolingual English speakers
• No self-reported history of speech/hearing disorder
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Procedure
• Participant seated in head stabilizing apparatus
• Ultrasound probe held under chin by a cross bar
• Compressible acoustically transparent standoff between chin and probe
• Participant produces six repetitions of each tongue twister
• Stimuli read off of a printed sheet
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Stimuli
• Four word tongue twisters designed to elicit stop onset errors– Tongue twisters focusing on onset segments
used to increase error rate– Error patterns in tongue twisters similar to
error patterns in comparable spontaneous speech (Shattuck-Hufnagel 1992)
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Stimuli
• Baseline recordings of productions of a speech sound by a participante.g. “ta tae tae ta” – Determine normal patterns for /t, d, k, g/– 48 productions of each onset (2 stimuli with
4 onsets repeated 6 times)– Measure tongue blade and dorsum raising
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Stimuli
• Experimental recordings of stimuli with alternationse.g. “cop tab top cab”
e.g. “ka tae ta kae”– Six word stimuli, eight non-word stimuli– Measure tongue blade angle and dorsum
raising– Compare with normal patterns – Look for abnormalities
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Measures
• Dorsum raising measure– Direction of dorsum raising varies by vowel
(Wodzinski 2004)– Typical direction of dorsum raising
determined from baseline– Distance of dorsum raising along typical
direction measured for each stimulus (both velar and alveolar)
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Measures
• Tongue blade angle– Elevation of the tongue tip/blade measured
as an angle– Line segment drawn over last 1 cm of
visible tongue tip– Angle of elevation measured from proximal
point to distal point (0 is level, positive is inclined, negative is declined)
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“normal” alveolar
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Results so far
• Both categorical and gradient errors observed
• Small perturbations from baseline values commonly observed in tongue twisters
• Are perturbations gradient errors?– Ordinary coarticulation vs.– “Traces” of activation of intended target
(Goldrick 2004)
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7.0
7.5
8.0
8.5
9.0
9.5
-80 -60 -40 -20 0
Blade Angle (deg)
Do
rsu
m D
ista
nc
e (
cm
)
alv base
alv nonwd
alv wd
7.0
7.5
8.0
8.5
9.0
9.5
-80 -60 -40 -20 0
Blade Angle (deg)
Do
rsu
m D
ista
nc
e (
cm
)
vel base
vel nonwd
vel wd
Representative participant
Alveolar targets Velar targets
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“normal” /t/ /t/ with dorsum raised
An apparent gradient error from /g/ gesture intrusion
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7.0
7.5
8.0
8.5
9.0
9.5
-80 -60 -40 -20 0
Blade Angle (deg)
Do
rsu
m D
ista
nc
e (
cm
)
alv base
alv nonwd
alv wd
7.0
7.5
8.0
8.5
9.0
9.5
-80 -60 -40 -20 0
Blade Angle (deg)
Do
rsu
m D
ista
nc
e (
cm
)
vel base
vel nonwd
vel wd
Representative participant
Alveolar targets Velar targets
Not a gradient error… a vowel error… produced /ge/
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Discussion
• Gradient errors confirmed in a more natural production task than Pouplier (2003)
• Categorical errors appear to be much more common
• Error data difficult to quantify– “Normal” alveolars in alternating context
produced differently than in baseline– Difference not found in velars
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Gestural activation
• Findings are consistent with a model of error production as erroneous gestural activation– Competing articulators may be
simultaneously activated, producing an abnormal combination
– Activation can be partial and not total, and so not accounted for by a completely symbolic linguistic model
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Gestural activation
• Prevalence of categorical errors– For the most part, however, erroneous
activation of gestures falls into the normal phonetic categories
– Consistent with gestural level as another level of the hierarchy
– Activation of coordinated combinations is supported by segment and word level activation
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Lexical effects?
• Error rates higher in nonword case
• But no obvious tendency for more gradient or categorical errors in one case or the other
• Emphasizes need to quantify data
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7.0
7.5
8.0
8.5
9.0
9.5
-80 -60 -40 -20 0
Blade Angle (deg)
Do
rsu
m D
ista
nc
e (
cm
)
alv base
alv nonwd
alv wd
7.0
7.5
8.0
8.5
9.0
9.5
-80 -60 -40 -20 0
Blade Angle (deg)
Do
rsu
m D
ista
nc
e (
cm
)
vel base
vel nonwd
vel wd
Representative participant
Alveolar targets Velar targets
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Conclusions
• Making progress…– While this study does not address many of
the long-term questions of the research program, it is generating valuable basic data on speech errors
– Even this relatively simple study has illuminated many challenges to the study of gestural speech errors within the speech production system
