The phonetics of speech errors

1. The phonetics of speech errors Frisch, S. A. University of South Florida frisch@cas.usf.edu This work supported by NIH-NIDCD R03 06164

2. 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

3. 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’

4. 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

5. 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

6. 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

7. 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

8. 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?

9. 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

10. 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

11. Distribution of voicing

12. Current research • Speech errors studied using ultrasound • Ultrasound recordings give a means to directly measure articulation • Similar to Pouplier (2003) EMA studies

13. Participants • Four undergraduate students from the CSD department • Monolingual English speakers • No self-reported history of speech/hearing disorder

14. 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

15. 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)

16. Stimuli • Baseline recordings of productions of a speech sound by a participant e.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

17. Stimuli • Experimental recordings of stimuli with alternations e.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

18. 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)

19. 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)

20. “normal” alveolar

21. 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)

22. Representative participant Alveolar targets Velar targets

23. An apparent gradient error from /g/ gesture intrusion “normal” /t/ /t/ with dorsum raised

24. Representative participant Alveolar targets Velar targets Not a gradient error… a vowel error… produced /ge/

25. 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

26. 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

27. 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

28. 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

29. Representative participant Alveolar targets Velar targets

30. 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